compiler: use correct init order for multi-value initialization

[official-gcc.git] / gcc / doc / generic.texi

@c Copyright (C) 2004-2022 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

@c ---------------------------------------------------------------------
@c GENERIC
@c ---------------------------------------------------------------------

@node GENERIC
@chapter GENERIC
@cindex GENERIC

The purpose of GENERIC is simply to provide a
language-independent way of representing an entire function in
trees.  To this end, it was necessary to add a few new tree codes
to the back end, but almost everything was already there.  If you
can express it with the codes in @code{gcc/tree.def}, it's
GENERIC@.

Early on, there was a great deal of debate about how to think
about statements in a tree IL@.  In GENERIC, a statement is
defined as any expression whose value, if any, is ignored.  A
statement will always have @code{TREE_SIDE_EFFECTS} set (or it
will be discarded), but a non-statement expression may also have
side effects.  A @code{CALL_EXPR}, for instance.

It would be possible for some local optimizations to work on the
GENERIC form of a function; indeed, the adapted tree inliner
works fine on GENERIC, but the current compiler performs inlining
after lowering to GIMPLE (a restricted form described in the next
section). Indeed, currently the frontends perform this lowering
before handing off to @code{tree_rest_of_compilation}, but this
seems inelegant.

@menu
* Deficiencies::                Topics not yet covered in this document.
* Tree overview::               All about @code{tree}s.
* Types::                       Fundamental and aggregate types.
* Declarations::                Type declarations and variables.
* Attributes::                  Declaration and type attributes.
* Expressions: Expression trees.            Operating on data.
* Statements::                  Control flow and related trees.
* Functions::                   Function bodies, linkage, and other aspects.
* Language-dependent trees::    Topics and trees specific to language front ends.
* C and C++ Trees::             Trees specific to C and C++.
@end menu

@c ---------------------------------------------------------------------
@c Deficiencies
@c ---------------------------------------------------------------------

@node Deficiencies
@section Deficiencies

@c The spelling of "incomplet" and "incorrekt" below is intentional.
There are many places in which this document is incomplet and incorrekt.
It is, as of yet, only @emph{preliminary} documentation.

@c ---------------------------------------------------------------------
@c Overview
@c ---------------------------------------------------------------------

@node Tree overview
@section Overview
@cindex tree
@findex TREE_CODE

The central data structure used by the internal representation is the
@code{tree}.  These nodes, while all of the C type @code{tree}, are of
many varieties.  A @code{tree} is a pointer type, but the object to
which it points may be of a variety of types.  From this point forward,
we will refer to trees in ordinary type, rather than in @code{this
font}, except when talking about the actual C type @code{tree}.

You can tell what kind of node a particular tree is by using the
@code{TREE_CODE} macro.  Many, many macros take trees as input and
return trees as output.  However, most macros require a certain kind of
tree node as input.  In other words, there is a type-system for trees,
but it is not reflected in the C type-system.

For safety, it is useful to configure GCC with @option{--enable-checking}.
Although this results in a significant performance penalty (since all
tree types are checked at run-time), and is therefore inappropriate in a
release version, it is extremely helpful during the development process.

Many macros behave as predicates.  Many, although not all, of these
predicates end in @samp{_P}.  Do not rely on the result type of these
macros being of any particular type.  You may, however, rely on the fact
that the type can be compared to @code{0}, so that statements like
@smallexample
if (TEST_P (t) && !TEST_P (y))
  x = 1;
@end smallexample
@noindent
and
@smallexample
int i = (TEST_P (t) != 0);
@end smallexample
@noindent
are legal.  Macros that return @code{int} values now may be changed to
return @code{tree} values, or other pointers in the future.  Even those
that continue to return @code{int} may return multiple nonzero codes
where previously they returned only zero and one.  Therefore, you should
not write code like
@smallexample
if (TEST_P (t) == 1)
@end smallexample
@noindent
as this code is not guaranteed to work correctly in the future.

You should not take the address of values returned by the macros or
functions described here.  In particular, no guarantee is given that the
values are lvalues.

In general, the names of macros are all in uppercase, while the names of
functions are entirely in lowercase.  There are rare exceptions to this
rule.  You should assume that any macro or function whose name is made
up entirely of uppercase letters may evaluate its arguments more than
once.  You may assume that a macro or function whose name is made up
entirely of lowercase letters will evaluate its arguments only once.

The @code{error_mark_node} is a special tree.  Its tree code is
@code{ERROR_MARK}, but since there is only ever one node with that code,
the usual practice is to compare the tree against
@code{error_mark_node}.  (This test is just a test for pointer
equality.)  If an error has occurred during front-end processing the
flag @code{errorcount} will be set.  If the front end has encountered
code it cannot handle, it will issue a message to the user and set
@code{sorrycount}.  When these flags are set, any macro or function
which normally returns a tree of a particular kind may instead return
the @code{error_mark_node}.  Thus, if you intend to do any processing of
erroneous code, you must be prepared to deal with the
@code{error_mark_node}.

Occasionally, a particular tree slot (like an operand to an expression,
or a particular field in a declaration) will be referred to as
``reserved for the back end''.  These slots are used to store RTL when
the tree is converted to RTL for use by the GCC back end.  However, if
that process is not taking place (e.g., if the front end is being hooked
up to an intelligent editor), then those slots may be used by the
back end presently in use.

If you encounter situations that do not match this documentation, such
as tree nodes of types not mentioned here, or macros documented to
return entities of a particular kind that instead return entities of
some different kind, you have found a bug, either in the front end or in
the documentation.  Please report these bugs as you would any other
bug.

@menu
* Macros and Functions::Macros and functions that can be used with all trees.
* Identifiers::         The names of things.
* Containers::          Lists and vectors.
@end menu

@c ---------------------------------------------------------------------
@c Trees
@c ---------------------------------------------------------------------

@node Macros and Functions
@subsection Trees
@cindex tree
@findex TREE_CHAIN
@findex TREE_TYPE

All GENERIC trees have two fields in common.  First, @code{TREE_CHAIN}
is a pointer that can be used as a singly-linked list to other trees.
The other is @code{TREE_TYPE}.  Many trees store the type of an
expression or declaration in this field.

These are some other functions for handling trees:

@ftable @code

@item tree_size
Return the number of bytes a tree takes.

@item build0
@itemx build1
@itemx build2
@itemx build3
@itemx build4
@itemx build5
@itemx build6

These functions build a tree and supply values to put in each
parameter.  The basic signature is @samp{@w{code, type, [operands]}}.
@code{code} is the @code{TREE_CODE}, and @code{type} is a tree
representing the @code{TREE_TYPE}.  These are followed by the
operands, each of which is also a tree.

@end ftable


@c ---------------------------------------------------------------------
@c Identifiers
@c ---------------------------------------------------------------------

@node Identifiers
@subsection Identifiers
@cindex identifier
@cindex name
@tindex IDENTIFIER_NODE

An @code{IDENTIFIER_NODE} represents a slightly more general concept
than the standard C or C++ concept of identifier.  In particular, an
@code{IDENTIFIER_NODE} may contain a @samp{$}, or other extraordinary
characters.

There are never two distinct @code{IDENTIFIER_NODE}s representing the
same identifier.  Therefore, you may use pointer equality to compare
@code{IDENTIFIER_NODE}s, rather than using a routine like
@code{strcmp}.  Use @code{get_identifier} to obtain the unique
@code{IDENTIFIER_NODE} for a supplied string.

You can use the following macros to access identifiers:
@ftable @code
@item IDENTIFIER_POINTER
The string represented by the identifier, represented as a
@code{char*}.  This string is always @code{NUL}-terminated, and contains
no embedded @code{NUL} characters.

@item IDENTIFIER_LENGTH
The length of the string returned by @code{IDENTIFIER_POINTER}, not
including the trailing @code{NUL}.  This value of
@code{IDENTIFIER_LENGTH (x)} is always the same as @code{strlen
(IDENTIFIER_POINTER (x))}.

@item IDENTIFIER_OPNAME_P
This predicate holds if the identifier represents the name of an
overloaded operator.  In this case, you should not depend on the
contents of either the @code{IDENTIFIER_POINTER} or the
@code{IDENTIFIER_LENGTH}.

@item IDENTIFIER_TYPENAME_P
This predicate holds if the identifier represents the name of a
user-defined conversion operator.  In this case, the @code{TREE_TYPE} of
the @code{IDENTIFIER_NODE} holds the type to which the conversion
operator converts.

@end ftable

@c ---------------------------------------------------------------------
@c Containers
@c ---------------------------------------------------------------------

@node Containers
@subsection Containers
@cindex container
@cindex list
@cindex vector
@tindex TREE_LIST
@tindex TREE_VEC
@findex TREE_PURPOSE
@findex TREE_VALUE
@findex TREE_VEC_LENGTH
@findex TREE_VEC_ELT

Two common container data structures can be represented directly with
tree nodes.  A @code{TREE_LIST} is a singly linked list containing two
trees per node.  These are the @code{TREE_PURPOSE} and @code{TREE_VALUE}
of each node.  (Often, the @code{TREE_PURPOSE} contains some kind of
tag, or additional information, while the @code{TREE_VALUE} contains the
majority of the payload.  In other cases, the @code{TREE_PURPOSE} is
simply @code{NULL_TREE}, while in still others both the
@code{TREE_PURPOSE} and @code{TREE_VALUE} are of equal stature.)  Given
one @code{TREE_LIST} node, the next node is found by following the
@code{TREE_CHAIN}.  If the @code{TREE_CHAIN} is @code{NULL_TREE}, then
you have reached the end of the list.

A @code{TREE_VEC} is a simple vector.  The @code{TREE_VEC_LENGTH} is an
integer (not a tree) giving the number of nodes in the vector.  The
nodes themselves are accessed using the @code{TREE_VEC_ELT} macro, which
takes two arguments.  The first is the @code{TREE_VEC} in question; the
second is an integer indicating which element in the vector is desired.
The elements are indexed from zero.

@c ---------------------------------------------------------------------
@c Types
@c ---------------------------------------------------------------------

@node Types
@section Types
@cindex type
@cindex pointer
@cindex reference
@cindex fundamental type
@cindex array
@tindex VOID_TYPE
@tindex INTEGER_TYPE
@tindex TYPE_MIN_VALUE
@tindex TYPE_MAX_VALUE
@tindex REAL_TYPE
@tindex FIXED_POINT_TYPE
@tindex COMPLEX_TYPE
@tindex ENUMERAL_TYPE
@tindex BOOLEAN_TYPE
@tindex POINTER_TYPE
@tindex REFERENCE_TYPE
@tindex FUNCTION_TYPE
@tindex METHOD_TYPE
@tindex ARRAY_TYPE
@tindex RECORD_TYPE
@tindex UNION_TYPE
@tindex OPAQUE_TYPE
@tindex UNKNOWN_TYPE
@tindex OFFSET_TYPE
@findex TYPE_UNQUALIFIED
@findex TYPE_QUAL_CONST
@findex TYPE_QUAL_VOLATILE
@findex TYPE_QUAL_RESTRICT
@findex TYPE_MAIN_VARIANT
@cindex qualified type
@findex TYPE_SIZE
@findex TYPE_ALIGN
@findex TYPE_PRECISION
@findex TYPE_ARG_TYPES
@findex TYPE_METHOD_BASETYPE
@findex TYPE_OFFSET_BASETYPE
@findex TREE_TYPE
@findex TYPE_CONTEXT
@findex TYPE_NAME
@findex TYPENAME_TYPE_FULLNAME
@findex TYPE_FIELDS
@findex TYPE_CANONICAL
@findex TYPE_STRUCTURAL_EQUALITY_P
@findex SET_TYPE_STRUCTURAL_EQUALITY

All types have corresponding tree nodes.  However, you should not assume
that there is exactly one tree node corresponding to each type.  There
are often multiple nodes corresponding to the same type.

For the most part, different kinds of types have different tree codes.
(For example, pointer types use a @code{POINTER_TYPE} code while arrays
use an @code{ARRAY_TYPE} code.)  However, pointers to member functions
use the @code{RECORD_TYPE} code.  Therefore, when writing a
@code{switch} statement that depends on the code associated with a
particular type, you should take care to handle pointers to member
functions under the @code{RECORD_TYPE} case label.

The following functions and macros deal with cv-qualification of types:
@ftable @code
@item TYPE_MAIN_VARIANT
This macro returns the unqualified version of a type.  It may be applied
to an unqualified type, but it is not always the identity function in
that case.
@end ftable

A few other macros and functions are usable with all types:
@ftable @code
@item TYPE_SIZE
The number of bits required to represent the type, represented as an
@code{INTEGER_CST}.  For an incomplete type, @code{TYPE_SIZE} will be
@code{NULL_TREE}.

@item TYPE_ALIGN
The alignment of the type, in bits, represented as an @code{int}.

@item TYPE_NAME
This macro returns a declaration (in the form of a @code{TYPE_DECL}) for
the type.  (Note this macro does @emph{not} return an
@code{IDENTIFIER_NODE}, as you might expect, given its name!)  You can
look at the @code{DECL_NAME} of the @code{TYPE_DECL} to obtain the
actual name of the type.  The @code{TYPE_NAME} will be @code{NULL_TREE}
for a type that is not a built-in type, the result of a typedef, or a
named class type.

@item TYPE_CANONICAL
This macro returns the ``canonical'' type for the given type
node. Canonical types are used to improve performance in the C++ and
Objective-C++ front ends by allowing efficient comparison between two
type nodes in @code{same_type_p}: if the @code{TYPE_CANONICAL} values
of the types are equal, the types are equivalent; otherwise, the types
are not equivalent. The notion of equivalence for canonical types is
the same as the notion of type equivalence in the language itself. For
instance,

When @code{TYPE_CANONICAL} is @code{NULL_TREE}, there is no canonical
type for the given type node. In this case, comparison between this
type and any other type requires the compiler to perform a deep,
``structural'' comparison to see if the two type nodes have the same
form and properties.

The canonical type for a node is always the most fundamental type in
the equivalence class of types. For instance, @code{int} is its own
canonical type. A typedef @code{I} of @code{int} will have @code{int}
as its canonical type. Similarly, @code{I*}@ and a typedef @code{IP}@
(defined to @code{I*}) will has @code{int*} as their canonical
type. When building a new type node, be sure to set
@code{TYPE_CANONICAL} to the appropriate canonical type. If the new
type is a compound type (built from other types), and any of those
other types require structural equality, use
@code{SET_TYPE_STRUCTURAL_EQUALITY} to ensure that the new type also
requires structural equality. Finally, if for some reason you cannot
guarantee that @code{TYPE_CANONICAL} will point to the canonical type,
use @code{SET_TYPE_STRUCTURAL_EQUALITY} to make sure that the new
type--and any type constructed based on it--requires structural
equality. If you suspect that the canonical type system is
miscomparing types, pass @code{--param verify-canonical-types=1} to
the compiler or configure with @code{--enable-checking} to force the
compiler to verify its canonical-type comparisons against the
structural comparisons; the compiler will then print any warnings if
the canonical types miscompare.

@item TYPE_STRUCTURAL_EQUALITY_P
This predicate holds when the node requires structural equality
checks, e.g., when @code{TYPE_CANONICAL} is @code{NULL_TREE}.

@item SET_TYPE_STRUCTURAL_EQUALITY
This macro states that the type node it is given requires structural
equality checks, e.g., it sets @code{TYPE_CANONICAL} to
@code{NULL_TREE}.

@item same_type_p
This predicate takes two types as input, and holds if they are the same
type.  For example, if one type is a @code{typedef} for the other, or
both are @code{typedef}s for the same type.  This predicate also holds if
the two trees given as input are simply copies of one another; i.e.,
there is no difference between them at the source level, but, for
whatever reason, a duplicate has been made in the representation.  You
should never use @code{==} (pointer equality) to compare types; always
use @code{same_type_p} instead.
@end ftable

Detailed below are the various kinds of types, and the macros that can
be used to access them.  Although other kinds of types are used
elsewhere in G++, the types described here are the only ones that you
will encounter while examining the intermediate representation.

@table @code
@item VOID_TYPE
Used to represent the @code{void} type.

@item INTEGER_TYPE
Used to represent the various integral types, including @code{char},
@code{short}, @code{int}, @code{long}, and @code{long long}.  This code
is not used for enumeration types, nor for the @code{bool} type.
The @code{TYPE_PRECISION} is the number of bits used in
the representation, represented as an @code{unsigned int}.  (Note that
in the general case this is not the same value as @code{TYPE_SIZE};
suppose that there were a 24-bit integer type, but that alignment
requirements for the ABI required 32-bit alignment.  Then,
@code{TYPE_SIZE} would be an @code{INTEGER_CST} for 32, while
@code{TYPE_PRECISION} would be 24.)  The integer type is unsigned if
@code{TYPE_UNSIGNED} holds; otherwise, it is signed.

The @code{TYPE_MIN_VALUE} is an @code{INTEGER_CST} for the smallest
integer that may be represented by this type.  Similarly, the
@code{TYPE_MAX_VALUE} is an @code{INTEGER_CST} for the largest integer
that may be represented by this type.

@item REAL_TYPE
Used to represent the @code{float}, @code{double}, and @code{long
double} types.  The number of bits in the floating-point representation
is given by @code{TYPE_PRECISION}, as in the @code{INTEGER_TYPE} case.

@item FIXED_POINT_TYPE
Used to represent the @code{short _Fract}, @code{_Fract}, @code{long
_Fract}, @code{long long _Fract}, @code{short _Accum}, @code{_Accum},
@code{long _Accum}, and @code{long long _Accum} types.  The number of bits
in the fixed-point representation is given by @code{TYPE_PRECISION},
as in the @code{INTEGER_TYPE} case.  There may be padding bits, fractional
bits and integral bits.  The number of fractional bits is given by
@code{TYPE_FBIT}, and the number of integral bits is given by @code{TYPE_IBIT}.
The fixed-point type is unsigned if @code{TYPE_UNSIGNED} holds; otherwise,
it is signed.
The fixed-point type is saturating if @code{TYPE_SATURATING} holds; otherwise,
it is not saturating.

@item COMPLEX_TYPE
Used to represent GCC built-in @code{__complex__} data types.  The
@code{TREE_TYPE} is the type of the real and imaginary parts.

@item ENUMERAL_TYPE
Used to represent an enumeration type.  The @code{TYPE_PRECISION} gives
(as an @code{int}), the number of bits used to represent the type.  If
there are no negative enumeration constants, @code{TYPE_UNSIGNED} will
hold.  The minimum and maximum enumeration constants may be obtained
with @code{TYPE_MIN_VALUE} and @code{TYPE_MAX_VALUE}, respectively; each
of these macros returns an @code{INTEGER_CST}.

The actual enumeration constants themselves may be obtained by looking
at the @code{TYPE_VALUES}.  This macro will return a @code{TREE_LIST},
containing the constants.  The @code{TREE_PURPOSE} of each node will be
an @code{IDENTIFIER_NODE} giving the name of the constant; the
@code{TREE_VALUE} will be an @code{INTEGER_CST} giving the value
assigned to that constant.  These constants will appear in the order in
which they were declared.  The @code{TREE_TYPE} of each of these
constants will be the type of enumeration type itself.

@item OPAQUE_TYPE
Used for things that have a @code{MODE_OPAQUE} mode class in the
backend. Opaque types have a size and precision, and can be held in
memory or registers. They are used when we do not want the compiler to
make assumptions about the availability of other operations as would
happen with integer types.

@item BOOLEAN_TYPE
Used to represent the @code{bool} type.

@item POINTER_TYPE
Used to represent pointer types, and pointer to data member types.  The
@code{TREE_TYPE} gives the type to which this type points.

@item REFERENCE_TYPE
Used to represent reference types.  The @code{TREE_TYPE} gives the type
to which this type refers.

@item FUNCTION_TYPE
Used to represent the type of non-member functions and of static member
functions.  The @code{TREE_TYPE} gives the return type of the function.
The @code{TYPE_ARG_TYPES} are a @code{TREE_LIST} of the argument types.
The @code{TREE_VALUE} of each node in this list is the type of the
corresponding argument; the @code{TREE_PURPOSE} is an expression for the
default argument value, if any.  If the last node in the list is
@code{void_list_node} (a @code{TREE_LIST} node whose @code{TREE_VALUE}
is the @code{void_type_node}), then functions of this type do not take
variable arguments.  Otherwise, they do take a variable number of
arguments.

Note that in C (but not in C++) a function declared like @code{void f()}
is an unprototyped function taking a variable number of arguments; the
@code{TYPE_ARG_TYPES} of such a function will be @code{NULL}.

@item METHOD_TYPE
Used to represent the type of a non-static member function.  Like a
@code{FUNCTION_TYPE}, the return type is given by the @code{TREE_TYPE}.
The type of @code{*this}, i.e., the class of which functions of this
type are a member, is given by the @code{TYPE_METHOD_BASETYPE}.  The
@code{TYPE_ARG_TYPES} is the parameter list, as for a
@code{FUNCTION_TYPE}, and includes the @code{this} argument.

@item ARRAY_TYPE
Used to represent array types.  The @code{TREE_TYPE} gives the type of
the elements in the array.  If the array-bound is present in the type,
the @code{TYPE_DOMAIN} is an @code{INTEGER_TYPE} whose
@code{TYPE_MIN_VALUE} and @code{TYPE_MAX_VALUE} will be the lower and
upper bounds of the array, respectively.  The @code{TYPE_MIN_VALUE} will
always be an @code{INTEGER_CST} for zero, while the
@code{TYPE_MAX_VALUE} will be one less than the number of elements in
the array, i.e., the highest value which may be used to index an element
in the array.

@item RECORD_TYPE
Used to represent @code{struct} and @code{class} types, as well as
pointers to member functions and similar constructs in other languages.
@code{TYPE_FIELDS} contains the items contained in this type, each of
which can be a @code{FIELD_DECL}, @code{VAR_DECL}, @code{CONST_DECL}, or
@code{TYPE_DECL}.  You may not make any assumptions about the ordering
of the fields in the type or whether one or more of them overlap.

@item UNION_TYPE
Used to represent @code{union} types.  Similar to @code{RECORD_TYPE}
except that all @code{FIELD_DECL} nodes in @code{TYPE_FIELD} start at
bit position zero.

@item QUAL_UNION_TYPE
Used to represent part of a variant record in Ada.  Similar to
@code{UNION_TYPE} except that each @code{FIELD_DECL} has a
@code{DECL_QUALIFIER} field, which contains a boolean expression that
indicates whether the field is present in the object.  The type will only
have one field, so each field's @code{DECL_QUALIFIER} is only evaluated
if none of the expressions in the previous fields in @code{TYPE_FIELDS}
are nonzero.  Normally these expressions will reference a field in the
outer object using a @code{PLACEHOLDER_EXPR}.

@item LANG_TYPE
This node is used to represent a language-specific type.  The front
end must handle it.

@item OFFSET_TYPE
This node is used to represent a pointer-to-data member.  For a data
member @code{X::m} the @code{TYPE_OFFSET_BASETYPE} is @code{X} and the
@code{TREE_TYPE} is the type of @code{m}.

@end table

There are variables whose values represent some of the basic types.
These include:
@table @code
@item void_type_node
A node for @code{void}.

@item integer_type_node
A node for @code{int}.

@item unsigned_type_node.
A node for @code{unsigned int}.

@item char_type_node.
A node for @code{char}.
@end table
@noindent
It may sometimes be useful to compare one of these variables with a type
in hand, using @code{same_type_p}.

@c ---------------------------------------------------------------------
@c Declarations
@c ---------------------------------------------------------------------

@node Declarations
@section Declarations
@cindex declaration
@cindex variable
@cindex type declaration
@tindex LABEL_DECL
@tindex CONST_DECL
@tindex TYPE_DECL
@tindex VAR_DECL
@tindex PARM_DECL
@tindex DEBUG_EXPR_DECL
@tindex FIELD_DECL
@tindex NAMESPACE_DECL
@tindex RESULT_DECL
@tindex TEMPLATE_DECL
@tindex THUNK_DECL
@findex THUNK_DELTA
@findex DECL_INITIAL
@findex DECL_SIZE
@findex DECL_ALIGN
@findex DECL_EXTERNAL

This section covers the various kinds of declarations that appear in the
internal representation, except for declarations of functions
(represented by @code{FUNCTION_DECL} nodes), which are described in
@ref{Functions}.

@menu
* Working with declarations::  Macros and functions that work on
declarations.
* Internal structure:: How declaration nodes are represented.
@end menu

@node Working with declarations
@subsection Working with declarations

Some macros can be used with any kind of declaration.  These include:
@ftable @code
@item DECL_NAME
This macro returns an @code{IDENTIFIER_NODE} giving the name of the
entity.

@item TREE_TYPE
This macro returns the type of the entity declared.

@item EXPR_FILENAME
This macro returns the name of the file in which the entity was
declared, as a @code{char*}.  For an entity declared implicitly by the
compiler (like @code{__builtin_memcpy}), this will be the string
@code{"<internal>"}.

@item EXPR_LINENO
This macro returns the line number at which the entity was declared, as
an @code{int}.

@item DECL_ARTIFICIAL
This predicate holds if the declaration was implicitly generated by the
compiler.  For example, this predicate will hold of an implicitly
declared member function, or of the @code{TYPE_DECL} implicitly
generated for a class type.  Recall that in C++ code like:
@smallexample
struct S @{@};
@end smallexample
@noindent
is roughly equivalent to C code like:
@smallexample
struct S @{@};
typedef struct S S;
@end smallexample
The implicitly generated @code{typedef} declaration is represented by a
@code{TYPE_DECL} for which @code{DECL_ARTIFICIAL} holds.

@end ftable

The various kinds of declarations include:
@table @code
@item LABEL_DECL
These nodes are used to represent labels in function bodies.  For more
information, see @ref{Functions}.  These nodes only appear in block
scopes.

@item CONST_DECL
These nodes are used to represent enumeration constants.  The value of
the constant is given by @code{DECL_INITIAL} which will be an
@code{INTEGER_CST} with the same type as the @code{TREE_TYPE} of the
@code{CONST_DECL}, i.e., an @code{ENUMERAL_TYPE}.

@item RESULT_DECL
These nodes represent the value returned by a function.  When a value is
assigned to a @code{RESULT_DECL}, that indicates that the value should
be returned, via bitwise copy, by the function.  You can use
@code{DECL_SIZE} and @code{DECL_ALIGN} on a @code{RESULT_DECL}, just as
with a @code{VAR_DECL}.

@item TYPE_DECL
These nodes represent @code{typedef} declarations.  The @code{TREE_TYPE}
is the type declared to have the name given by @code{DECL_NAME}.  In
some cases, there is no associated name.

@item VAR_DECL
These nodes represent variables with namespace or block scope, as well
as static data members.  The @code{DECL_SIZE} and @code{DECL_ALIGN} are
analogous to @code{TYPE_SIZE} and @code{TYPE_ALIGN}.  For a declaration,
you should always use the @code{DECL_SIZE} and @code{DECL_ALIGN} rather
than the @code{TYPE_SIZE} and @code{TYPE_ALIGN} given by the
@code{TREE_TYPE}, since special attributes may have been applied to the
variable to give it a particular size and alignment.  You may use the
predicates @code{DECL_THIS_STATIC} or @code{DECL_THIS_EXTERN} to test
whether the storage class specifiers @code{static} or @code{extern} were
used to declare a variable.

If this variable is initialized (but does not require a constructor),
the @code{DECL_INITIAL} will be an expression for the initializer.  The
initializer should be evaluated, and a bitwise copy into the variable
performed.  If the @code{DECL_INITIAL} is the @code{error_mark_node},
there is an initializer, but it is given by an explicit statement later
in the code; no bitwise copy is required.

GCC provides an extension that allows either automatic variables, or
global variables, to be placed in particular registers.  This extension
is being used for a particular @code{VAR_DECL} if @code{DECL_REGISTER}
holds for the @code{VAR_DECL}, and if @code{DECL_ASSEMBLER_NAME} is not
equal to @code{DECL_NAME}.  In that case, @code{DECL_ASSEMBLER_NAME} is
the name of the register into which the variable will be placed.

@item PARM_DECL
Used to represent a parameter to a function.  Treat these nodes
similarly to @code{VAR_DECL} nodes.  These nodes only appear in the
@code{DECL_ARGUMENTS} for a @code{FUNCTION_DECL}.

The @code{DECL_ARG_TYPE} for a @code{PARM_DECL} is the type that will
actually be used when a value is passed to this function.  It may be a
wider type than the @code{TREE_TYPE} of the parameter; for example, the
ordinary type might be @code{short} while the @code{DECL_ARG_TYPE} is
@code{int}.

@item DEBUG_EXPR_DECL
Used to represent an anonymous debug-information temporary created to
hold an expression as it is optimized away, so that its value can be
referenced in debug bind statements.

@item FIELD_DECL
These nodes represent non-static data members.  The @code{DECL_SIZE} and
@code{DECL_ALIGN} behave as for @code{VAR_DECL} nodes.
The position of the field within the parent record is specified by a
combination of three attributes.  @code{DECL_FIELD_OFFSET} is the position,
counting in bytes, of the @code{DECL_OFFSET_ALIGN}-bit sized word containing
the bit of the field closest to the beginning of the structure.
@code{DECL_FIELD_BIT_OFFSET} is the bit offset of the first bit of the field
within this word; this may be nonzero even for fields that are not bit-fields,
since @code{DECL_OFFSET_ALIGN} may be greater than the natural alignment
of the field's type.

If @code{DECL_C_BIT_FIELD} holds, this field is a bit-field.  In a bit-field,
@code{DECL_BIT_FIELD_TYPE} also contains the type that was originally
specified for it, while DECL_TYPE may be a modified type with lesser precision,
according to the size of the bit field.

@item NAMESPACE_DECL
Namespaces provide a name hierarchy for other declarations.  They
appear in the @code{DECL_CONTEXT} of other @code{_DECL} nodes.

@end table

@node Internal structure
@subsection Internal structure

@code{DECL} nodes are represented internally as a hierarchy of
structures.

@menu
* Current structure hierarchy::  The current DECL node structure
hierarchy.
* Adding new DECL node types:: How to add a new DECL node to a
frontend.
@end menu

@node Current structure hierarchy
@subsubsection Current structure hierarchy

@table @code

@item struct tree_decl_minimal
This is the minimal structure to inherit from in order for common
@code{DECL} macros to work.  The fields it contains are a unique ID,
source location, context, and name.

@item struct tree_decl_common
This structure inherits from @code{struct tree_decl_minimal}.  It
contains fields that most @code{DECL} nodes need, such as a field to
store alignment, machine mode, size, and attributes.

@item struct tree_field_decl
This structure inherits from @code{struct tree_decl_common}.  It is
used to represent @code{FIELD_DECL}.

@item struct tree_label_decl
This structure inherits from @code{struct tree_decl_common}.  It is
used to represent @code{LABEL_DECL}.

@item struct tree_translation_unit_decl
This structure inherits from @code{struct tree_decl_common}.  It is
used to represent @code{TRANSLATION_UNIT_DECL}.

@item struct tree_decl_with_rtl
This structure inherits from @code{struct tree_decl_common}.  It
contains a field to store the low-level RTL associated with a
@code{DECL} node.

@item struct tree_result_decl
This structure inherits from @code{struct tree_decl_with_rtl}.  It is
used to represent @code{RESULT_DECL}.

@item struct tree_const_decl
This structure inherits from @code{struct tree_decl_with_rtl}.  It is
used to represent @code{CONST_DECL}.

@item struct tree_parm_decl
This structure inherits from @code{struct tree_decl_with_rtl}.  It is
used to represent @code{PARM_DECL}.

@item struct tree_decl_with_vis
This structure inherits from @code{struct tree_decl_with_rtl}.  It
contains fields necessary to store visibility information, as well as
a section name and assembler name.

@item struct tree_var_decl
This structure inherits from @code{struct tree_decl_with_vis}.  It is
used to represent @code{VAR_DECL}.

@item struct tree_function_decl
This structure inherits from @code{struct tree_decl_with_vis}.  It is
used to represent @code{FUNCTION_DECL}.

@end table
@node Adding new DECL node types
@subsubsection Adding new DECL node types

Adding a new @code{DECL} tree consists of the following steps

@table @asis

@item Add a new tree code for the @code{DECL} node
For language specific @code{DECL} nodes, there is a @file{.def} file
in each frontend directory where the tree code should be added.
For @code{DECL} nodes that are part of the middle-end, the code should
be added to @file{tree.def}.

@item Create a new structure type for the @code{DECL} node
These structures should inherit from one of the existing structures in
the language hierarchy by using that structure as the first member.

@smallexample
struct tree_foo_decl
@{
   struct tree_decl_with_vis common;
@}
@end smallexample

Would create a structure name @code{tree_foo_decl} that inherits from
@code{struct tree_decl_with_vis}.

For language specific @code{DECL} nodes, this new structure type
should go in the appropriate @file{.h} file.
For @code{DECL} nodes that are part of the middle-end, the structure
type should go in @file{tree.h}.

@item Add a member to the tree structure enumerator for the node
For garbage collection and dynamic checking purposes, each @code{DECL}
node structure type is required to have a unique enumerator value
specified with it.
For language specific @code{DECL} nodes, this new enumerator value
should go in the appropriate @file{.def} file.
For @code{DECL} nodes that are part of the middle-end, the enumerator
values are specified in @file{treestruct.def}.

@item Update @code{union tree_node}
In order to make your new structure type usable, it must be added to
@code{union tree_node}.
For language specific @code{DECL} nodes, a new entry should be added
to the appropriate @file{.h} file of the form
@smallexample
  struct tree_foo_decl GTY ((tag ("TS_VAR_DECL"))) foo_decl;
@end smallexample
For @code{DECL} nodes that are part of the middle-end, the additional
member goes directly into @code{union tree_node} in @file{tree.h}.

@item Update dynamic checking info
In order to be able to check whether accessing a named portion of
@code{union tree_node} is legal, and whether a certain @code{DECL} node
contains one of the enumerated @code{DECL} node structures in the
hierarchy, a simple lookup table is used.
This lookup table needs to be kept up to date with the tree structure
hierarchy, or else checking and containment macros will fail
inappropriately.

For language specific @code{DECL} nodes, there is an @code{init_ts}
function in an appropriate @file{.c} file, which initializes the lookup
table.
Code setting up the table for new @code{DECL} nodes should be added
there.
For each @code{DECL} tree code and enumerator value representing a
member of the inheritance  hierarchy, the table should contain 1 if
that tree code inherits (directly or indirectly) from that member.
Thus, a @code{FOO_DECL} node derived from @code{struct decl_with_rtl},
and enumerator value @code{TS_FOO_DECL}, would be set up as follows
@smallexample
tree_contains_struct[FOO_DECL][TS_FOO_DECL] = 1;
tree_contains_struct[FOO_DECL][TS_DECL_WRTL] = 1;
tree_contains_struct[FOO_DECL][TS_DECL_COMMON] = 1;
tree_contains_struct[FOO_DECL][TS_DECL_MINIMAL] = 1;
@end smallexample

For @code{DECL} nodes that are part of the middle-end, the setup code
goes into @file{tree.cc}.

@item Add macros to access any new fields and flags

Each added field or flag should have a macro that is used to access
it, that performs appropriate checking to ensure only the right type of
@code{DECL} nodes access the field.

These macros generally take the following form
@smallexample
#define FOO_DECL_FIELDNAME(NODE) FOO_DECL_CHECK(NODE)->foo_decl.fieldname
@end smallexample
However, if the structure is simply a base class for further
structures, something like the following should be used
@smallexample
#define BASE_STRUCT_CHECK(T) CONTAINS_STRUCT_CHECK(T, TS_BASE_STRUCT)
#define BASE_STRUCT_FIELDNAME(NODE) \
   (BASE_STRUCT_CHECK(NODE)->base_struct.fieldname
@end smallexample

Reading them from the generated @file{all-tree.def} file (which in
turn includes all the @file{tree.def} files), @file{gencheck.cc} is
used during GCC's build to generate the @code{*_CHECK} macros for all
tree codes.

@end table


@c ---------------------------------------------------------------------
@c Attributes
@c ---------------------------------------------------------------------
@node Attributes
@section Attributes in trees
@cindex attributes

Attributes, as specified using the @code{__attribute__} keyword, are
represented internally as a @code{TREE_LIST}.  The @code{TREE_PURPOSE}
is the name of the attribute, as an @code{IDENTIFIER_NODE}.  The
@code{TREE_VALUE} is a @code{TREE_LIST} of the arguments of the
attribute, if any, or @code{NULL_TREE} if there are no arguments; the
arguments are stored as the @code{TREE_VALUE} of successive entries in
the list, and may be identifiers or expressions.  The @code{TREE_CHAIN}
of the attribute is the next attribute in a list of attributes applying
to the same declaration or type, or @code{NULL_TREE} if there are no
further attributes in the list.

Attributes may be attached to declarations and to types; these
attributes may be accessed with the following macros.  All attributes
are stored in this way, and many also cause other changes to the
declaration or type or to other internal compiler data structures.

@deftypefn {Tree Macro} tree DECL_ATTRIBUTES (tree @var{decl})
This macro returns the attributes on the declaration @var{decl}.
@end deftypefn

@deftypefn {Tree Macro} tree TYPE_ATTRIBUTES (tree @var{type})
This macro returns the attributes on the type @var{type}.
@end deftypefn


@c ---------------------------------------------------------------------
@c Expressions
@c ---------------------------------------------------------------------

@node Expression trees
@section Expressions
@cindex expression
@findex TREE_TYPE
@findex TREE_OPERAND

The internal representation for expressions is for the most part quite
straightforward.  However, there are a few facts that one must bear in
mind.  In particular, the expression ``tree'' is actually a directed
acyclic graph.  (For example there may be many references to the integer
constant zero throughout the source program; many of these will be
represented by the same expression node.)  You should not rely on
certain kinds of node being shared, nor should you rely on certain kinds of
nodes being unshared.

The following macros can be used with all expression nodes:

@ftable @code
@item TREE_TYPE
Returns the type of the expression.  This value may not be precisely the
same type that would be given the expression in the original program.
@end ftable

In what follows, some nodes that one might expect to always have type
@code{bool} are documented to have either integral or boolean type.  At
some point in the future, the C front end may also make use of this same
intermediate representation, and at this point these nodes will
certainly have integral type.  The previous sentence is not meant to
imply that the C++ front end does not or will not give these nodes
integral type.

Below, we list the various kinds of expression nodes.  Except where
noted otherwise, the operands to an expression are accessed using the
@code{TREE_OPERAND} macro.  For example, to access the first operand to
a binary plus expression @code{expr}, use:

@smallexample
TREE_OPERAND (expr, 0)
@end smallexample
@noindent

As this example indicates, the operands are zero-indexed.


@menu
* Constants: Constant expressions.
* Storage References::
* Unary and Binary Expressions::
* Vectors::
@end menu

@node Constant expressions
@subsection Constant expressions
@tindex INTEGER_CST
@findex tree_int_cst_lt
@findex tree_int_cst_equal
@tindex tree_fits_uhwi_p
@tindex tree_fits_shwi_p
@tindex tree_to_uhwi
@tindex tree_to_shwi
@tindex TREE_INT_CST_NUNITS
@tindex TREE_INT_CST_ELT
@tindex TREE_INT_CST_LOW
@tindex REAL_CST
@tindex FIXED_CST
@tindex COMPLEX_CST
@tindex VECTOR_CST
@tindex STRING_CST
@tindex POLY_INT_CST
@findex TREE_STRING_LENGTH
@findex TREE_STRING_POINTER

The table below begins with constants, moves on to unary expressions,
then proceeds to binary expressions, and concludes with various other
kinds of expressions:

@table @code
@item INTEGER_CST
These nodes represent integer constants.  Note that the type of these
constants is obtained with @code{TREE_TYPE}; they are not always of type
@code{int}.  In particular, @code{char} constants are represented with
@code{INTEGER_CST} nodes.  The value of the integer constant @code{e} is
represented in an array of HOST_WIDE_INT.   There are enough elements
in the array to represent the value without taking extra elements for
redundant 0s or -1.  The number of elements used to represent @code{e}
is available via @code{TREE_INT_CST_NUNITS}. Element @code{i} can be
extracted by using @code{TREE_INT_CST_ELT (e, i)}.
@code{TREE_INT_CST_LOW} is a shorthand for @code{TREE_INT_CST_ELT (e, 0)}.

The functions @code{tree_fits_shwi_p} and @code{tree_fits_uhwi_p}
can be used to tell if the value is small enough to fit in a
signed HOST_WIDE_INT or an unsigned HOST_WIDE_INT respectively.
The value can then be extracted using @code{tree_to_shwi} and
@code{tree_to_uhwi}.

@item REAL_CST

FIXME: Talk about how to obtain representations of this constant, do
comparisons, and so forth.

@item FIXED_CST

These nodes represent fixed-point constants.  The type of these constants
is obtained with @code{TREE_TYPE}.  @code{TREE_FIXED_CST_PTR} points to
a @code{struct fixed_value};  @code{TREE_FIXED_CST} returns the structure
itself.  @code{struct fixed_value} contains @code{data} with the size of two
@code{HOST_BITS_PER_WIDE_INT} and @code{mode} as the associated fixed-point
machine mode for @code{data}.

@item COMPLEX_CST
These nodes are used to represent complex number constants, that is a
@code{__complex__} whose parts are constant nodes.  The
@code{TREE_REALPART} and @code{TREE_IMAGPART} return the real and the
imaginary parts respectively.

@item VECTOR_CST
These nodes are used to represent vector constants.  Each vector
constant @var{v} is treated as a specific instance of an arbitrary-length
sequence that itself contains @samp{VECTOR_CST_NPATTERNS (@var{v})}
interleaved patterns.  Each pattern has the form:

@smallexample
@{ @var{base0}, @var{base1}, @var{base1} + @var{step}, @var{base1} + @var{step} * 2, @dots{} @}
@end smallexample

The first three elements in each pattern are enough to determine the
values of the other elements.  However, if all @var{step}s are zero,
only the first two elements are needed.  If in addition each @var{base1}
is equal to the corresponding @var{base0}, only the first element in
each pattern is needed.  The number of encoded elements per pattern
is given by @samp{VECTOR_CST_NELTS_PER_PATTERN (@var{v})}.

For example, the constant:

@smallexample
@{ 0, 1, 2, 6, 3, 8, 4, 10, 5, 12, 6, 14, 7, 16, 8, 18 @}
@end smallexample

is interpreted as an interleaving of the sequences:

@smallexample
@{ 0, 2, 3, 4, 5, 6, 7, 8 @}
@{ 1, 6, 8, 10, 12, 14, 16, 18 @}
@end smallexample

where the sequences are represented by the following patterns:

@smallexample
@var{base0} == 0, @var{base1} == 2, @var{step} == 1
@var{base0} == 1, @var{base1} == 6, @var{step} == 2
@end smallexample

In this case:

@smallexample
VECTOR_CST_NPATTERNS (@var{v}) == 2
VECTOR_CST_NELTS_PER_PATTERN (@var{v}) == 3
@end smallexample

The vector is therefore encoded using the first 6 elements
(@samp{@{ 0, 1, 2, 6, 3, 8 @}}), with the remaining 10 elements
being implicit extensions of them.

Sometimes this scheme can create two possible encodings of the same
vector.  For example @{ 0, 1 @} could be seen as two patterns with
one element each or one pattern with two elements (@var{base0} and
@var{base1}).  The canonical encoding is always the one with the
fewest patterns or (if both encodings have the same number of
petterns) the one with the fewest encoded elements.

@samp{vector_cst_encoding_nelts (@var{v})} gives the total number of
encoded elements in @var{v}, which is 6 in the example above.
@code{VECTOR_CST_ENCODED_ELTS (@var{v})} gives a pointer to the elements
encoded in @var{v} and @code{VECTOR_CST_ENCODED_ELT (@var{v}, @var{i})}
accesses the value of encoded element @var{i}.

@samp{VECTOR_CST_DUPLICATE_P (@var{v})} is true if @var{v} simply contains
repeated instances of @samp{VECTOR_CST_NPATTERNS (@var{v})} values.  This is
a shorthand for testing @samp{VECTOR_CST_NELTS_PER_PATTERN (@var{v}) == 1}.

@samp{VECTOR_CST_STEPPED_P (@var{v})} is true if at least one
pattern in @var{v} has a nonzero step.  This is a shorthand for
testing @samp{VECTOR_CST_NELTS_PER_PATTERN (@var{v}) == 3}.

The utility function @code{vector_cst_elt} gives the value of an
arbitrary index as a @code{tree}.  @code{vector_cst_int_elt} gives
the same value as a @code{wide_int}.

@item STRING_CST
These nodes represent string-constants.  The @code{TREE_STRING_LENGTH}
returns the length of the string, as an @code{int}.  The
@code{TREE_STRING_POINTER} is a @code{char*} containing the string
itself.  The string may not be @code{NUL}-terminated, and it may contain
embedded @code{NUL} characters.  Therefore, the
@code{TREE_STRING_LENGTH} includes the trailing @code{NUL} if it is
present.

For wide string constants, the @code{TREE_STRING_LENGTH} is the number
of bytes in the string, and the @code{TREE_STRING_POINTER}
points to an array of the bytes of the string, as represented on the
target system (that is, as integers in the target endianness).  Wide and
non-wide string constants are distinguished only by the @code{TREE_TYPE}
of the @code{STRING_CST}.

FIXME: The formats of string constants are not well-defined when the
target system bytes are not the same width as host system bytes.

@item POLY_INT_CST
These nodes represent invariants that depend on some target-specific
runtime parameters.  They consist of @code{NUM_POLY_INT_COEFFS}
coefficients, with the first coefficient being the constant term and
the others being multipliers that are applied to the runtime parameters.

@code{POLY_INT_CST_ELT (@var{x}, @var{i})} references coefficient number
@var{i} of @code{POLY_INT_CST} node @var{x}.  Each coefficient is an
@code{INTEGER_CST}.

@end table

@node Storage References
@subsection References to storage
@tindex ADDR_EXPR
@tindex INDIRECT_REF
@tindex MEM_REF
@tindex ARRAY_REF
@tindex ARRAY_RANGE_REF
@tindex TARGET_MEM_REF
@tindex COMPONENT_REF

@table @code
@item ARRAY_REF
These nodes represent array accesses.  The first operand is the array;
the second is the index.  To calculate the address of the memory
accessed, you must scale the index by the size of the type of the array
elements.  The type of these expressions must be the type of a component of
the array.  The third and fourth operands are used after gimplification
to represent the lower bound and component size but should not be used
directly; call @code{array_ref_low_bound} and @code{array_ref_element_size}
instead.

@item ARRAY_RANGE_REF
These nodes represent access to a range (or ``slice'') of an array.  The
operands are the same as that for @code{ARRAY_REF} and have the same
meanings.  The type of these expressions must be an array whose component
type is the same as that of the first operand.  The range of that array
type determines the amount of data these expressions access.

@item COMPONENT_REF
These nodes represent non-static data member accesses.  The first
operand is the object (rather than a pointer to it); the second operand
is the @code{FIELD_DECL} for the data member.  The third operand represents
the byte offset of the field, but should not be used directly; call
@code{component_ref_field_offset} instead.

@item ADDR_EXPR
These nodes are used to represent the address of an object.  (These
expressions will always have pointer or reference type.)  The operand may
be another expression, or it may be a declaration.

As an extension, GCC allows users to take the address of a label.  In
this case, the operand of the @code{ADDR_EXPR} will be a
@code{LABEL_DECL}.  The type of such an expression is @code{void*}.

If the object addressed is not an lvalue, a temporary is created, and
the address of the temporary is used.

@item INDIRECT_REF
These nodes are used to represent the object pointed to by a pointer.
The operand is the pointer being dereferenced; it will always have
pointer or reference type.

@item MEM_REF
These nodes are used to represent the object pointed to by a pointer
offset by a constant.
The first operand is the pointer being dereferenced; it will always have
pointer or reference type.  The second operand is a pointer constant
serving as constant offset applied to the pointer being dereferenced
with its type specifying the type to be used for type-based alias analysis.
The type of the node specifies the alignment of the access.

@item TARGET_MEM_REF
These nodes represent memory accesses whose address directly map to
an addressing mode of the target architecture.  The first argument
is @code{TMR_BASE} and is a pointer to the object being accessed.
The second argument is @code{TMR_OFFSET} which is a pointer constant
with dual purpose serving both as constant offset and holder of
the type used for type-based alias analysis.  The first two operands
have exactly the same semantics as @code{MEM_REF}.  The third
and fourth operand are @code{TMR_INDEX} and @code{TMR_STEP} where
the former is an integer and the latter an integer constant.  The
fifth and last operand is @code{TMR_INDEX2} which is an alternate
non-constant offset.  Any of the third to last operands may be
@code{NULL} if the corresponding component does not appear in
the address, but @code{TMR_INDEX} and @code{TMR_STEP} shall be
always supplied in pair.  The Address of the @code{TARGET_MEM_REF}
is determined in the following way.

@smallexample
TMR_BASE + TMR_OFFSET + TMR_INDEX * TMR_STEP + TMR_INDEX2
@end smallexample

The type of the node specifies the alignment of the access.

@end table

@node Unary and Binary Expressions
@subsection Unary and Binary Expressions
@tindex NEGATE_EXPR
@tindex ABS_EXPR
@tindex ABSU_EXPR
@tindex BIT_NOT_EXPR
@tindex TRUTH_NOT_EXPR
@tindex PREDECREMENT_EXPR
@tindex PREINCREMENT_EXPR
@tindex POSTDECREMENT_EXPR
@tindex POSTINCREMENT_EXPR
@tindex FIX_TRUNC_EXPR
@tindex FLOAT_EXPR
@tindex COMPLEX_EXPR
@tindex CONJ_EXPR
@tindex REALPART_EXPR
@tindex IMAGPART_EXPR
@tindex NON_LVALUE_EXPR
@tindex NOP_EXPR
@tindex CONVERT_EXPR
@tindex FIXED_CONVERT_EXPR
@tindex THROW_EXPR
@tindex LSHIFT_EXPR
@tindex RSHIFT_EXPR
@tindex BIT_IOR_EXPR
@tindex BIT_XOR_EXPR
@tindex BIT_AND_EXPR
@tindex TRUTH_ANDIF_EXPR
@tindex TRUTH_ORIF_EXPR
@tindex TRUTH_AND_EXPR
@tindex TRUTH_OR_EXPR
@tindex TRUTH_XOR_EXPR
@tindex POINTER_PLUS_EXPR
@tindex POINTER_DIFF_EXPR
@tindex PLUS_EXPR
@tindex MINUS_EXPR
@tindex MULT_EXPR
@tindex WIDEN_MULT_EXPR
@tindex MULT_HIGHPART_EXPR
@tindex RDIV_EXPR
@tindex TRUNC_DIV_EXPR
@tindex FLOOR_DIV_EXPR
@tindex CEIL_DIV_EXPR
@tindex ROUND_DIV_EXPR
@tindex TRUNC_MOD_EXPR
@tindex FLOOR_MOD_EXPR
@tindex CEIL_MOD_EXPR
@tindex ROUND_MOD_EXPR
@tindex EXACT_DIV_EXPR
@tindex LT_EXPR
@tindex LE_EXPR
@tindex GT_EXPR
@tindex GE_EXPR
@tindex EQ_EXPR
@tindex NE_EXPR
@tindex ORDERED_EXPR
@tindex UNORDERED_EXPR
@tindex UNLT_EXPR
@tindex UNLE_EXPR
@tindex UNGT_EXPR
@tindex UNGE_EXPR
@tindex UNEQ_EXPR
@tindex LTGT_EXPR
@tindex MODIFY_EXPR
@tindex INIT_EXPR
@tindex COMPOUND_EXPR
@tindex COND_EXPR
@tindex CALL_EXPR
@tindex STMT_EXPR
@tindex BIND_EXPR
@tindex LOOP_EXPR
@tindex EXIT_EXPR
@tindex CLEANUP_POINT_EXPR
@tindex CONSTRUCTOR
@tindex COMPOUND_LITERAL_EXPR
@tindex SAVE_EXPR
@tindex TARGET_EXPR
@tindex VA_ARG_EXPR
@tindex ANNOTATE_EXPR

@table @code
@item NEGATE_EXPR
These nodes represent unary negation of the single operand, for both
integer and floating-point types.  The type of negation can be
determined by looking at the type of the expression.

The behavior of this operation on signed arithmetic overflow is
controlled by the @code{flag_wrapv} and @code{flag_trapv} variables.

@item ABS_EXPR
These nodes represent the absolute value of the single operand, for
both integer and floating-point types.  This is typically used to
implement the @code{abs}, @code{labs} and @code{llabs} builtins for
integer types, and the @code{fabs}, @code{fabsf} and @code{fabsl}
builtins for floating point types.  The type of abs operation can
be determined by looking at the type of the expression.

This node is not used for complex types.  To represent the modulus
or complex abs of a complex value, use the @code{BUILT_IN_CABS},
@code{BUILT_IN_CABSF} or @code{BUILT_IN_CABSL} builtins, as used
to implement the C99 @code{cabs}, @code{cabsf} and @code{cabsl}
built-in functions.

@item ABSU_EXPR
These nodes represent the absolute value of the single operand in
equivalent unsigned type such that @code{ABSU_EXPR} of @code{TYPE_MIN}
is well defined.

@item BIT_NOT_EXPR
These nodes represent bitwise complement, and will always have integral
type.  The only operand is the value to be complemented.

@item TRUTH_NOT_EXPR
These nodes represent logical negation, and will always have integral
(or boolean) type.  The operand is the value being negated.  The type
of the operand and that of the result are always of @code{BOOLEAN_TYPE}
or @code{INTEGER_TYPE}.

@item PREDECREMENT_EXPR
@itemx PREINCREMENT_EXPR
@itemx POSTDECREMENT_EXPR
@itemx POSTINCREMENT_EXPR
These nodes represent increment and decrement expressions.  The value of
the single operand is computed, and the operand incremented or
decremented.  In the case of @code{PREDECREMENT_EXPR} and
@code{PREINCREMENT_EXPR}, the value of the expression is the value
resulting after the increment or decrement; in the case of
@code{POSTDECREMENT_EXPR} and @code{POSTINCREMENT_EXPR} is the value
before the increment or decrement occurs.  The type of the operand, like
that of the result, will be either integral, boolean, or floating-point.

@item FIX_TRUNC_EXPR
These nodes represent conversion of a floating-point value to an
integer.  The single operand will have a floating-point type, while
the complete expression will have an integral (or boolean) type.  The
operand is rounded towards zero.

@item FLOAT_EXPR
These nodes represent conversion of an integral (or boolean) value to a
floating-point value.  The single operand will have integral type, while
the complete expression will have a floating-point type.

FIXME: How is the operand supposed to be rounded?  Is this dependent on
@option{-mieee}?

@item COMPLEX_EXPR
These nodes are used to represent complex numbers constructed from two
expressions of the same (integer or real) type.  The first operand is the
real part and the second operand is the imaginary part.

@item CONJ_EXPR
These nodes represent the conjugate of their operand.

@item REALPART_EXPR
@itemx IMAGPART_EXPR
These nodes represent respectively the real and the imaginary parts
of complex numbers (their sole argument).

@item NON_LVALUE_EXPR
These nodes indicate that their one and only operand is not an lvalue.
A back end can treat these identically to the single operand.

@item NOP_EXPR
These nodes are used to represent conversions that do not require any
code-generation.  For example, conversion of a @code{char*} to an
@code{int*} does not require any code be generated; such a conversion is
represented by a @code{NOP_EXPR}.  The single operand is the expression
to be converted.  The conversion from a pointer to a reference is also
represented with a @code{NOP_EXPR}.

@item CONVERT_EXPR
These nodes are similar to @code{NOP_EXPR}s, but are used in those
situations where code may need to be generated.  For example, if an
@code{int*} is converted to an @code{int} code may need to be generated
on some platforms.  These nodes are never used for C++-specific
conversions, like conversions between pointers to different classes in
an inheritance hierarchy.  Any adjustments that need to be made in such
cases are always indicated explicitly.  Similarly, a user-defined
conversion is never represented by a @code{CONVERT_EXPR}; instead, the
function calls are made explicit.

@item FIXED_CONVERT_EXPR
These nodes are used to represent conversions that involve fixed-point
values.  For example, from a fixed-point value to another fixed-point value,
from an integer to a fixed-point value, from a fixed-point value to an
integer, from a floating-point value to a fixed-point value, or from
a fixed-point value to a floating-point value.

@item LSHIFT_EXPR
@itemx RSHIFT_EXPR
These nodes represent left and right shifts, respectively.  The first
operand is the value to shift; it will always be of integral type.  The
second operand is an expression for the number of bits by which to
shift.  Right shift should be treated as arithmetic, i.e., the
high-order bits should be zero-filled when the expression has unsigned
type and filled with the sign bit when the expression has signed type.
Note that the result is undefined if the second operand is larger
than or equal to the first operand's type size. Unlike most nodes, these
can have a vector as first operand and a scalar as second operand.


@item BIT_IOR_EXPR
@itemx BIT_XOR_EXPR
@itemx BIT_AND_EXPR
These nodes represent bitwise inclusive or, bitwise exclusive or, and
bitwise and, respectively.  Both operands will always have integral
type.

@item TRUTH_ANDIF_EXPR
@itemx TRUTH_ORIF_EXPR
These nodes represent logical ``and'' and logical ``or'', respectively.
These operators are not strict; i.e., the second operand is evaluated
only if the value of the expression is not determined by evaluation of
the first operand.  The type of the operands and that of the result are
always of @code{BOOLEAN_TYPE} or @code{INTEGER_TYPE}.

@item TRUTH_AND_EXPR
@itemx TRUTH_OR_EXPR
@itemx TRUTH_XOR_EXPR
These nodes represent logical and, logical or, and logical exclusive or.
They are strict; both arguments are always evaluated.  There are no
corresponding operators in C or C++, but the front end will sometimes
generate these expressions anyhow, if it can tell that strictness does
not matter.  The type of the operands and that of the result are
always of @code{BOOLEAN_TYPE} or @code{INTEGER_TYPE}.

@item POINTER_PLUS_EXPR
This node represents pointer arithmetic.  The first operand is always
a pointer/reference type.  The second operand is always an unsigned
integer type compatible with sizetype.  This and POINTER_DIFF_EXPR are
the only binary arithmetic operators that can operate on pointer types.

@item POINTER_DIFF_EXPR
This node represents pointer subtraction.  The two operands always
have pointer/reference type.  It returns a signed integer of the same
precision as the pointers.  The behavior is undefined if the difference
of the two pointers, seen as infinite precision non-negative integers,
does not fit in the result type.  The result does not depend on the
pointer type, it is not divided by the size of the pointed-to type.

@item PLUS_EXPR
@itemx MINUS_EXPR
@itemx MULT_EXPR
These nodes represent various binary arithmetic operations.
Respectively, these operations are addition, subtraction (of the second
operand from the first) and multiplication.  Their operands may have
either integral or floating type, but there will never be case in which
one operand is of floating type and the other is of integral type.

The behavior of these operations on signed arithmetic overflow is
controlled by the @code{flag_wrapv} and @code{flag_trapv} variables.

@item WIDEN_MULT_EXPR
This node represents a widening multiplication.  The operands have
integral types with same @var{b} bits of precision, producing an
integral type result with at least @math{2@var{b}} bits of precision.
The behaviour is equivalent to extending both operands, possibly of
different signedness, to the result type, then multiplying them.

@item MULT_HIGHPART_EXPR
This node represents the ``high-part'' of a widening multiplication.
For an integral type with @var{b} bits of precision, the result is
the most significant @var{b} bits of the full @math{2@var{b}} product.
Both operands must have the same precision and same signedness.

@item RDIV_EXPR
This node represents a floating point division operation.

@item TRUNC_DIV_EXPR
@itemx FLOOR_DIV_EXPR
@itemx CEIL_DIV_EXPR
@itemx ROUND_DIV_EXPR
These nodes represent integer division operations that return an integer
result.  @code{TRUNC_DIV_EXPR} rounds towards zero, @code{FLOOR_DIV_EXPR}
rounds towards negative infinity, @code{CEIL_DIV_EXPR} rounds towards
positive infinity and @code{ROUND_DIV_EXPR} rounds to the closest integer.
Integer division in C and C++ is truncating, i.e.@: @code{TRUNC_DIV_EXPR}.

The behavior of these operations on signed arithmetic overflow, when
dividing the minimum signed integer by minus one, is controlled by the
@code{flag_wrapv} and @code{flag_trapv} variables.

@item TRUNC_MOD_EXPR
@itemx FLOOR_MOD_EXPR
@itemx CEIL_MOD_EXPR
@itemx ROUND_MOD_EXPR
These nodes represent the integer remainder or modulus operation.
The integer modulus of two operands @code{a} and @code{b} is
defined as @code{a - (a/b)*b} where the division calculated using
the corresponding division operator.  Hence for @code{TRUNC_MOD_EXPR}
this definition assumes division using truncation towards zero, i.e.@:
@code{TRUNC_DIV_EXPR}.  Integer remainder in C and C++ uses truncating
division, i.e.@: @code{TRUNC_MOD_EXPR}.

@item EXACT_DIV_EXPR
The @code{EXACT_DIV_EXPR} code is used to represent integer divisions where
the numerator is known to be an exact multiple of the denominator.  This
allows the backend to choose between the faster of @code{TRUNC_DIV_EXPR},
@code{CEIL_DIV_EXPR} and @code{FLOOR_DIV_EXPR} for the current target.

@item LT_EXPR
@itemx LE_EXPR
@itemx GT_EXPR
@itemx GE_EXPR
@itemx LTGT_EXPR
@itemx EQ_EXPR
@itemx NE_EXPR
These nodes represent the less than, less than or equal to, greater than,
greater than or equal to, less or greater than, equal, and not equal
comparison operators.  The first and second operands will either be both
of integral type, both of floating type or both of vector type, except for
LTGT_EXPR where they will only be both of floating type.  The result type
of these expressions will always be of integral, boolean or signed integral
vector type.  These operations return the result type's zero value for false,
the result type's one value for true, and a vector whose elements are zero
(false) or minus one (true) for vectors.

For floating point comparisons, if we honor IEEE NaNs and either operand
is NaN, then @code{NE_EXPR} always returns true and the remaining operators
always return false.  On some targets, comparisons against an IEEE NaN,
other than equality and inequality, may generate a floating-point exception.

@item ORDERED_EXPR
@itemx UNORDERED_EXPR
These nodes represent non-trapping ordered and unordered comparison
operators.  These operations take two floating point operands and
determine whether they are ordered or unordered relative to each other.
If either operand is an IEEE NaN, their comparison is defined to be
unordered, otherwise the comparison is defined to be ordered.  The
result type of these expressions will always be of integral or boolean
type.  These operations return the result type's zero value for false,
and the result type's one value for true.

@item UNLT_EXPR
@itemx UNLE_EXPR
@itemx UNGT_EXPR
@itemx UNGE_EXPR
@itemx UNEQ_EXPR
These nodes represent the unordered comparison operators.
These operations take two floating point operands and determine whether
the operands are unordered or are less than, less than or equal to,
greater than, greater than or equal to, or equal respectively.  For
example, @code{UNLT_EXPR} returns true if either operand is an IEEE
NaN or the first operand is less than the second.  All these operations
are guaranteed not to generate a floating point exception.  The result
type of these expressions will always be of integral or boolean type.
These operations return the result type's zero value for false,
and the result type's one value for true.

@item MODIFY_EXPR
These nodes represent assignment.  The left-hand side is the first
operand; the right-hand side is the second operand.  The left-hand side
will be a @code{VAR_DECL}, @code{INDIRECT_REF}, @code{COMPONENT_REF}, or
other lvalue.

These nodes are used to represent not only assignment with @samp{=} but
also compound assignments (like @samp{+=}), by reduction to @samp{=}
assignment.  In other words, the representation for @samp{i += 3} looks
just like that for @samp{i = i + 3}.

@item INIT_EXPR
These nodes are just like @code{MODIFY_EXPR}, but are used only when a
variable is initialized, rather than assigned to subsequently.  This
means that we can assume that the target of the initialization is not
used in computing its own value; any reference to the lhs in computing
the rhs is undefined.

@item COMPOUND_EXPR
These nodes represent comma-expressions.  The first operand is an
expression whose value is computed and thrown away prior to the
evaluation of the second operand.  The value of the entire expression is
the value of the second operand.

@item COND_EXPR
These nodes represent @code{?:} expressions.  The first operand
is of boolean or integral type.  If it evaluates to a nonzero value,
the second operand should be evaluated, and returned as the value of the
expression.  Otherwise, the third operand is evaluated, and returned as
the value of the expression.

The second operand must have the same type as the entire expression,
unless it unconditionally throws an exception or calls a noreturn
function, in which case it should have void type.  The same constraints
apply to the third operand.  This allows array bounds checks to be
represented conveniently as @code{(i >= 0 && i < 10) ? i : abort()}.

As a GNU extension, the C language front-ends allow the second
operand of the @code{?:} operator may be omitted in the source.
For example, @code{x ? : 3} is equivalent to @code{x ? x : 3},
assuming that @code{x} is an expression without side effects.
In the tree representation, however, the second operand is always
present, possibly protected by @code{SAVE_EXPR} if the first
argument does cause side effects.

@item CALL_EXPR
These nodes are used to represent calls to functions, including
non-static member functions.  @code{CALL_EXPR}s are implemented as
expression nodes with a variable number of operands.  Rather than using
@code{TREE_OPERAND} to extract them, it is preferable to use the
specialized accessor macros and functions that operate specifically on
@code{CALL_EXPR} nodes.

@code{CALL_EXPR_FN} returns a pointer to the
function to call; it is always an expression whose type is a
@code{POINTER_TYPE}.

The number of arguments to the call is returned by @code{call_expr_nargs},
while the arguments themselves can be accessed with the @code{CALL_EXPR_ARG}
macro.  The arguments are zero-indexed and numbered left-to-right.
You can iterate over the arguments using @code{FOR_EACH_CALL_EXPR_ARG}, as in:

@smallexample
tree call, arg;
call_expr_arg_iterator iter;
FOR_EACH_CALL_EXPR_ARG (arg, iter, call)
  /* arg is bound to successive arguments of call.  */
  @dots{};
@end smallexample

For non-static
member functions, there will be an operand corresponding to the
@code{this} pointer.  There will always be expressions corresponding to
all of the arguments, even if the function is declared with default
arguments and some arguments are not explicitly provided at the call
sites.

@code{CALL_EXPR}s also have a @code{CALL_EXPR_STATIC_CHAIN} operand that
is used to implement nested functions.  This operand is otherwise null.

@item CLEANUP_POINT_EXPR
These nodes represent full-expressions.  The single operand is an
expression to evaluate.  Any destructor calls engendered by the creation
of temporaries during the evaluation of that expression should be
performed immediately after the expression is evaluated.

@item CONSTRUCTOR
These nodes represent the brace-enclosed initializers for a structure or an
array.  They contain a sequence of component values made out of a vector of
constructor_elt, which is a (@code{INDEX}, @code{VALUE}) pair.

If the @code{TREE_TYPE} of the @code{CONSTRUCTOR} is a @code{RECORD_TYPE},
@code{UNION_TYPE} or @code{QUAL_UNION_TYPE} then the @code{INDEX} of each
node in the sequence will be a @code{FIELD_DECL} and the @code{VALUE} will
be the expression used to initialize that field.

If the @code{TREE_TYPE} of the @code{CONSTRUCTOR} is an @code{ARRAY_TYPE},
then the @code{INDEX} of each node in the sequence will be an
@code{INTEGER_CST} or a @code{RANGE_EXPR} of two @code{INTEGER_CST}s.
A single @code{INTEGER_CST} indicates which element of the array is being
assigned to.  A @code{RANGE_EXPR} indicates an inclusive range of elements
to initialize.  In both cases the @code{VALUE} is the corresponding
initializer.  It is re-evaluated for each element of a
@code{RANGE_EXPR}.  If the @code{INDEX} is @code{NULL_TREE}, then
the initializer is for the next available array element.

In the front end, you should not depend on the fields appearing in any
particular order.  However, in the middle end, fields must appear in
declaration order.  You should not assume that all fields will be
represented.  Unrepresented fields will be cleared (zeroed), unless the
CONSTRUCTOR_NO_CLEARING flag is set, in which case their value becomes
undefined.

@item COMPOUND_LITERAL_EXPR
@findex COMPOUND_LITERAL_EXPR_DECL_EXPR
@findex COMPOUND_LITERAL_EXPR_DECL
These nodes represent ISO C99 compound literals.  The
@code{COMPOUND_LITERAL_EXPR_DECL_EXPR} is a @code{DECL_EXPR}
containing an anonymous @code{VAR_DECL} for
the unnamed object represented by the compound literal; the
@code{DECL_INITIAL} of that @code{VAR_DECL} is a @code{CONSTRUCTOR}
representing the brace-enclosed list of initializers in the compound
literal.  That anonymous @code{VAR_DECL} can also be accessed directly
by the @code{COMPOUND_LITERAL_EXPR_DECL} macro.

@item SAVE_EXPR

A @code{SAVE_EXPR} represents an expression (possibly involving
side effects) that is used more than once.  The side effects should
occur only the first time the expression is evaluated.  Subsequent uses
should just reuse the computed value.  The first operand to the
@code{SAVE_EXPR} is the expression to evaluate.  The side effects should
be executed where the @code{SAVE_EXPR} is first encountered in a
depth-first preorder traversal of the expression tree.

@item TARGET_EXPR
A @code{TARGET_EXPR} represents a temporary object.  The first operand
is a @code{VAR_DECL} for the temporary variable.  The second operand is
the initializer for the temporary.  The initializer is evaluated and,
if non-void, copied (bitwise) into the temporary.  If the initializer
is void, that means that it will perform the initialization itself.

Often, a @code{TARGET_EXPR} occurs on the right-hand side of an
assignment, or as the second operand to a comma-expression which is
itself the right-hand side of an assignment, etc.  In this case, we say
that the @code{TARGET_EXPR} is ``normal''; otherwise, we say it is
``orphaned''.  For a normal @code{TARGET_EXPR} the temporary variable
should be treated as an alias for the left-hand side of the assignment,
rather than as a new temporary variable.

The third operand to the @code{TARGET_EXPR}, if present, is a
cleanup-expression (i.e., destructor call) for the temporary.  If this
expression is orphaned, then this expression must be executed when the
statement containing this expression is complete.  These cleanups must
always be executed in the order opposite to that in which they were
encountered.  Note that if a temporary is created on one branch of a
conditional operator (i.e., in the second or third operand to a
@code{COND_EXPR}), the cleanup must be run only if that branch is
actually executed.

@item VA_ARG_EXPR
This node is used to implement support for the C/C++ variable argument-list
mechanism.  It represents expressions like @code{va_arg (ap, type)}.
Its @code{TREE_TYPE} yields the tree representation for @code{type} and
its sole argument yields the representation for @code{ap}.

@item ANNOTATE_EXPR
This node is used to attach markers to an expression. The first operand
is the annotated expression, the second is an @code{INTEGER_CST} with
a value from @code{enum annot_expr_kind}, the third is an @code{INTEGER_CST}.
@end table


@node Vectors
@subsection Vectors
@tindex VEC_DUPLICATE_EXPR
@tindex VEC_SERIES_EXPR
@tindex VEC_LSHIFT_EXPR
@tindex VEC_RSHIFT_EXPR
@tindex VEC_WIDEN_MULT_HI_EXPR
@tindex VEC_WIDEN_MULT_LO_EXPR
@tindex VEC_WIDEN_PLUS_HI_EXPR
@tindex VEC_WIDEN_PLUS_LO_EXPR
@tindex VEC_WIDEN_MINUS_HI_EXPR
@tindex VEC_WIDEN_MINUS_LO_EXPR
@tindex VEC_UNPACK_HI_EXPR
@tindex VEC_UNPACK_LO_EXPR
@tindex VEC_UNPACK_FLOAT_HI_EXPR
@tindex VEC_UNPACK_FLOAT_LO_EXPR
@tindex VEC_UNPACK_FIX_TRUNC_HI_EXPR
@tindex VEC_UNPACK_FIX_TRUNC_LO_EXPR
@tindex VEC_PACK_TRUNC_EXPR
@tindex VEC_PACK_SAT_EXPR
@tindex VEC_PACK_FIX_TRUNC_EXPR
@tindex VEC_PACK_FLOAT_EXPR
@tindex VEC_COND_EXPR
@tindex SAD_EXPR

@table @code
@item VEC_DUPLICATE_EXPR
This node has a single operand and represents a vector in which every
element is equal to that operand.

@item VEC_SERIES_EXPR
This node represents a vector formed from a scalar base and step,
given as the first and second operands respectively.  Element @var{i}
of the result is equal to @samp{@var{base} + @var{i}*@var{step}}.

This node is restricted to integral types, in order to avoid
specifying the rounding behavior for floating-point types.

@item VEC_LSHIFT_EXPR
@itemx VEC_RSHIFT_EXPR
These nodes represent whole vector left and right shifts, respectively.
The first operand is the vector to shift; it will always be of vector type.
The second operand is an expression for the number of bits by which to
shift.  Note that the result is undefined if the second operand is larger
than or equal to the first operand's type size.

@item VEC_WIDEN_MULT_HI_EXPR
@itemx VEC_WIDEN_MULT_LO_EXPR
These nodes represent widening vector multiplication of the high and low
parts of the two input vectors, respectively.  Their operands are vectors
that contain the same number of elements (@code{N}) of the same integral type.
The result is a vector that contains half as many elements, of an integral type
whose size is twice as wide.  In the case of @code{VEC_WIDEN_MULT_HI_EXPR} the
high @code{N/2} elements of the two vector are multiplied to produce the
vector of @code{N/2} products. In the case of @code{VEC_WIDEN_MULT_LO_EXPR} the
low @code{N/2} elements of the two vector are multiplied to produce the
vector of @code{N/2} products.

@item VEC_WIDEN_PLUS_HI_EXPR
@itemx VEC_WIDEN_PLUS_LO_EXPR
These nodes represent widening vector addition of the high and low parts of
the two input vectors, respectively.  Their operands are vectors that contain
the same number of elements (@code{N}) of the same integral type. The result
is a vector that contains half as many elements, of an integral type whose size
is twice as wide.  In the case of @code{VEC_WIDEN_PLUS_HI_EXPR} the high
@code{N/2} elements of the two vectors are added to produce the vector of
@code{N/2} products.  In the case of @code{VEC_WIDEN_PLUS_LO_EXPR} the low
@code{N/2} elements of the two vectors are added to produce the vector of
@code{N/2} products.

@item VEC_WIDEN_MINUS_HI_EXPR
@itemx VEC_WIDEN_MINUS_LO_EXPR
These nodes represent widening vector subtraction of the high and low parts of
the two input vectors, respectively.  Their operands are vectors that contain
the same number of elements (@code{N}) of the same integral type. The high/low
elements of the second vector are subtracted from the high/low elements of the
first. The result is a vector that contains half as many elements, of an
integral type whose size is twice as wide.  In the case of
@code{VEC_WIDEN_MINUS_HI_EXPR} the high @code{N/2} elements of the second
vector are subtracted from the high @code{N/2} of the first to produce the
vector of @code{N/2} products.  In the case of
@code{VEC_WIDEN_MINUS_LO_EXPR} the low @code{N/2} elements of the second
vector are subtracted from the low @code{N/2} of the first to produce the
vector of @code{N/2} products.

@item VEC_UNPACK_HI_EXPR
@itemx VEC_UNPACK_LO_EXPR
These nodes represent unpacking of the high and low parts of the input vector,
respectively.  The single operand is a vector that contains @code{N} elements
of the same integral or floating point type.  The result is a vector
that contains half as many elements, of an integral or floating point type
whose size is twice as wide.  In the case of @code{VEC_UNPACK_HI_EXPR} the
high @code{N/2} elements of the vector are extracted and widened (promoted).
In the case of @code{VEC_UNPACK_LO_EXPR} the low @code{N/2} elements of the
vector are extracted and widened (promoted).

@item VEC_UNPACK_FLOAT_HI_EXPR
@itemx VEC_UNPACK_FLOAT_LO_EXPR
These nodes represent unpacking of the high and low parts of the input vector,
where the values are converted from fixed point to floating point.  The
single operand is a vector that contains @code{N} elements of the same
integral type.  The result is a vector that contains half as many elements
of a floating point type whose size is twice as wide.  In the case of
@code{VEC_UNPACK_FLOAT_HI_EXPR} the high @code{N/2} elements of the vector are
extracted, converted and widened.  In the case of @code{VEC_UNPACK_FLOAT_LO_EXPR}
the low @code{N/2} elements of the vector are extracted, converted and widened.

@item VEC_UNPACK_FIX_TRUNC_HI_EXPR
@itemx VEC_UNPACK_FIX_TRUNC_LO_EXPR
These nodes represent unpacking of the high and low parts of the input vector,
where the values are truncated from floating point to fixed point.  The
single operand is a vector that contains @code{N} elements of the same
floating point type.  The result is a vector that contains half as many
elements of an integral type whose size is twice as wide.  In the case of
@code{VEC_UNPACK_FIX_TRUNC_HI_EXPR} the high @code{N/2} elements of the
vector are extracted and converted with truncation.  In the case of
@code{VEC_UNPACK_FIX_TRUNC_LO_EXPR} the low @code{N/2} elements of the
vector are extracted and converted with truncation.

@item VEC_PACK_TRUNC_EXPR
This node represents packing of truncated elements of the two input vectors
into the output vector.  Input operands are vectors that contain the same
number of elements of the same integral or floating point type.  The result
is a vector that contains twice as many elements of an integral or floating
point type whose size is half as wide. The elements of the two vectors are
demoted and merged (concatenated) to form the output vector.

@item VEC_PACK_SAT_EXPR
This node represents packing of elements of the two input vectors into the
output vector using saturation.  Input operands are vectors that contain
the same number of elements of the same integral type.  The result is a
vector that contains twice as many elements of an integral type whose size
is half as wide.  The elements of the two vectors are demoted and merged
(concatenated) to form the output vector.

@item VEC_PACK_FIX_TRUNC_EXPR
This node represents packing of elements of the two input vectors into the
output vector, where the values are converted from floating point
to fixed point.  Input operands are vectors that contain the same number
of elements of a floating point type.  The result is a vector that contains
twice as many elements of an integral type whose size is half as wide.  The
elements of the two vectors are merged (concatenated) to form the output
vector.

@item VEC_PACK_FLOAT_EXPR
This node represents packing of elements of the two input vectors into the
output vector, where the values are converted from fixed point to floating
point.  Input operands are vectors that contain the same number of elements
of an integral type.  The result is a vector that contains twice as many
elements of floating point type whose size is half as wide.  The elements of
the two vectors are merged (concatenated) to form the output vector.

@item VEC_COND_EXPR
These nodes represent @code{?:} expressions.  The three operands must be
vectors of the same size and number of elements.  The second and third
operands must have the same type as the entire expression.  The first
operand is of signed integral vector type.  If an element of the first
operand evaluates to a zero value, the corresponding element of the
result is taken from the third operand. If it evaluates to a minus one
value, it is taken from the second operand. It should never evaluate to
any other value currently, but optimizations should not rely on that
property. In contrast with a @code{COND_EXPR}, all operands are always
evaluated.

@item SAD_EXPR
This node represents the Sum of Absolute Differences operation.  The three
operands must be vectors of integral types.  The first and second operand
must have the same type.  The size of the vector element of the third
operand must be at lease twice of the size of the vector element of the
first and second one.  The SAD is calculated between the first and second
operands, added to the third operand, and returned.

@end table


@c ---------------------------------------------------------------------
@c Statements
@c ---------------------------------------------------------------------

@node Statements
@section Statements
@cindex Statements

Most statements in GIMPLE are assignment statements, represented by
@code{GIMPLE_ASSIGN}.  No other C expressions can appear at statement level;
a reference to a volatile object is converted into a
@code{GIMPLE_ASSIGN}.

There are also several varieties of complex statements.

@menu
* Basic Statements::
* Blocks::
* Statement Sequences::
* Empty Statements::
* Jumps::
* Cleanups::
* OpenMP::
* OpenACC::
@end menu

@node Basic Statements
@subsection Basic Statements
@cindex Basic Statements

@table @code
@item ASM_EXPR

Used to represent an inline assembly statement.  For an inline assembly
statement like:
@smallexample
asm ("mov x, y");
@end smallexample
The @code{ASM_STRING} macro will return a @code{STRING_CST} node for
@code{"mov x, y"}.  If the original statement made use of the
extended-assembly syntax, then @code{ASM_OUTPUTS},
@code{ASM_INPUTS}, and @code{ASM_CLOBBERS} will be the outputs, inputs,
and clobbers for the statement, represented as @code{STRING_CST} nodes.
The extended-assembly syntax looks like:
@smallexample
asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));
@end smallexample
The first string is the @code{ASM_STRING}, containing the instruction
template.  The next two strings are the output and inputs, respectively;
this statement has no clobbers.  As this example indicates, ``plain''
assembly statements are merely a special case of extended assembly
statements; they have no cv-qualifiers, outputs, inputs, or clobbers.
All of the strings will be @code{NUL}-terminated, and will contain no
embedded @code{NUL}-characters.

If the assembly statement is declared @code{volatile}, or if the
statement was not an extended assembly statement, and is therefore
implicitly volatile, then the predicate @code{ASM_VOLATILE_P} will hold
of the @code{ASM_EXPR}.

@item DECL_EXPR

Used to represent a local declaration.  The @code{DECL_EXPR_DECL} macro
can be used to obtain the entity declared.  This declaration may be a
@code{LABEL_DECL}, indicating that the label declared is a local label.
(As an extension, GCC allows the declaration of labels with scope.)  In
C, this declaration may be a @code{FUNCTION_DECL}, indicating the
use of the GCC nested function extension.  For more information,
@pxref{Functions}.

@item LABEL_EXPR

Used to represent a label.  The @code{LABEL_DECL} declared by this
statement can be obtained with the @code{LABEL_EXPR_LABEL} macro.  The
@code{IDENTIFIER_NODE} giving the name of the label can be obtained from
the @code{LABEL_DECL} with @code{DECL_NAME}.

@item GOTO_EXPR

Used to represent a @code{goto} statement.  The @code{GOTO_DESTINATION} will
usually be a @code{LABEL_DECL}.  However, if the ``computed goto'' extension
has been used, the @code{GOTO_DESTINATION} will be an arbitrary expression
indicating the destination.  This expression will always have pointer type.

@item RETURN_EXPR

Used to represent a @code{return} statement.  Operand 0 represents the
value to return.  It should either be the @code{RESULT_DECL} for the
containing function, or a @code{MODIFY_EXPR} or @code{INIT_EXPR}
setting the function's @code{RESULT_DECL}.  It will be
@code{NULL_TREE} if the statement was just
@smallexample
return;
@end smallexample

@item LOOP_EXPR
These nodes represent ``infinite'' loops.  The @code{LOOP_EXPR_BODY}
represents the body of the loop.  It should be executed forever, unless
an @code{EXIT_EXPR} is encountered.

@item EXIT_EXPR
These nodes represent conditional exits from the nearest enclosing
@code{LOOP_EXPR}.  The single operand is the condition; if it is
nonzero, then the loop should be exited.  An @code{EXIT_EXPR} will only
appear within a @code{LOOP_EXPR}.

@item SWITCH_EXPR

Used to represent a @code{switch} statement.  The @code{SWITCH_COND}
is the expression on which the switch is occurring.  The
@code{SWITCH_BODY} is the body of the switch statement.
@code{SWITCH_ALL_CASES_P} is true if the switch includes a default
label or the case label ranges cover all possible values of the
condition expression.

Note that @code{TREE_TYPE} for a @code{SWITCH_EXPR} represents the
original type of switch expression as given in the source, before any
compiler conversions, instead of the type of the switch expression
itself (which is not meaningful).

@item CASE_LABEL_EXPR

Use to represent a @code{case} label, range of @code{case} labels, or a
@code{default} label.  If @code{CASE_LOW} is @code{NULL_TREE}, then this is a
@code{default} label.  Otherwise, if @code{CASE_HIGH} is @code{NULL_TREE}, then
this is an ordinary @code{case} label.  In this case, @code{CASE_LOW} is
an expression giving the value of the label.  Both @code{CASE_LOW} and
@code{CASE_HIGH} are @code{INTEGER_CST} nodes.  These values will have
the same type as the condition expression in the switch statement.

Otherwise, if both @code{CASE_LOW} and @code{CASE_HIGH} are defined, the
statement is a range of case labels.  Such statements originate with the
extension that allows users to write things of the form:
@smallexample
case 2 ... 5:
@end smallexample
The first value will be @code{CASE_LOW}, while the second will be
@code{CASE_HIGH}.

@item DEBUG_BEGIN_STMT

Marks the beginning of a source statement, for purposes of debug
information generation.

@end table


@node Blocks
@subsection Blocks
@cindex Blocks

Block scopes and the variables they declare in GENERIC are
expressed using the @code{BIND_EXPR} code, which in previous
versions of GCC was primarily used for the C statement-expression
extension.

Variables in a block are collected into @code{BIND_EXPR_VARS} in
declaration order through their @code{TREE_CHAIN} field.  Any runtime
initialization is moved out of @code{DECL_INITIAL} and into a
statement in the controlled block.  When gimplifying from C or C++,
this initialization replaces the @code{DECL_STMT}.  These variables
will never require cleanups.  The scope of these variables is just the
body

Variable-length arrays (VLAs) complicate this process, as their size
often refers to variables initialized earlier in the block and their
initialization involves an explicit stack allocation.  To handle this,
we add an indirection and replace them with a pointer to stack space
allocated by means of @code{alloca}.  In most cases, we also arrange
for this space to be reclaimed when the enclosing @code{BIND_EXPR} is
exited, the exception to this being when there is an explicit call to
@code{alloca} in the source code, in which case the stack is left
depressed on exit of the @code{BIND_EXPR}.

A C++ program will usually contain more @code{BIND_EXPR}s than
there are syntactic blocks in the source code, since several C++
constructs have implicit scopes associated with them.  On the
other hand, although the C++ front end uses pseudo-scopes to
handle cleanups for objects with destructors, these don't
translate into the GIMPLE form; multiple declarations at the same
level use the same @code{BIND_EXPR}.

@node Statement Sequences
@subsection Statement Sequences
@cindex Statement Sequences

Multiple statements at the same nesting level are collected into
a @code{STATEMENT_LIST}.  Statement lists are modified and
traversed using the interface in @samp{tree-iterator.h}.

@node Empty Statements
@subsection Empty Statements
@cindex Empty Statements

Whenever possible, statements with no effect are discarded.  But
if they are nested within another construct which cannot be
discarded for some reason, they are instead replaced with an
empty statement, generated by @code{build_empty_stmt}.
Initially, all empty statements were shared, after the pattern of
the Java front end, but this caused a lot of trouble in practice.

An empty statement is represented as @code{(void)0}.

@node Jumps
@subsection Jumps
@cindex Jumps

Other jumps are expressed by either @code{GOTO_EXPR} or
@code{RETURN_EXPR}.

The operand of a @code{GOTO_EXPR} must be either a label or a
variable containing the address to jump to.

The operand of a @code{RETURN_EXPR} is either @code{NULL_TREE},
@code{RESULT_DECL}, or a @code{MODIFY_EXPR} which sets the return
value.  It would be nice to move the @code{MODIFY_EXPR} into a
separate statement, but the special return semantics in
@code{expand_return} make that difficult.  It may still happen in
the future, perhaps by moving most of that logic into
@code{expand_assignment}.

@node Cleanups
@subsection Cleanups
@cindex Cleanups

Destructors for local C++ objects and similar dynamic cleanups are
represented in GIMPLE by a @code{TRY_FINALLY_EXPR}.
@code{TRY_FINALLY_EXPR} has two operands, both of which are a sequence
of statements to execute.  The first sequence is executed.  When it
completes the second sequence is executed.

The first sequence may complete in the following ways:

@enumerate

@item Execute the last statement in the sequence and fall off the
end.

@item Execute a goto statement (@code{GOTO_EXPR}) to an ordinary
label outside the sequence.

@item Execute a return statement (@code{RETURN_EXPR}).

@item Throw an exception.  This is currently not explicitly represented in
GIMPLE.

@end enumerate

The second sequence is not executed if the first sequence completes by
calling @code{setjmp} or @code{exit} or any other function that does
not return.  The second sequence is also not executed if the first
sequence completes via a non-local goto or a computed goto (in general
the compiler does not know whether such a goto statement exits the
first sequence or not, so we assume that it doesn't).

After the second sequence is executed, if it completes normally by
falling off the end, execution continues wherever the first sequence
would have continued, by falling off the end, or doing a goto, etc.

If the second sequence is an @code{EH_ELSE_EXPR} selector, then the
sequence in its first operand is used when the first sequence completes
normally, and that in its second operand is used for exceptional
cleanups, i.e., when an exception propagates out of the first sequence.

@code{TRY_FINALLY_EXPR} complicates the flow graph, since the cleanup
needs to appear on every edge out of the controlled block; this
reduces the freedom to move code across these edges.  Therefore, the
EH lowering pass which runs before most of the optimization passes
eliminates these expressions by explicitly adding the cleanup to each
edge.  Rethrowing the exception is represented using @code{RESX_EXPR}.

@node OpenMP
@subsection OpenMP
@tindex OMP_PARALLEL
@tindex OMP_FOR
@tindex OMP_SECTIONS
@tindex OMP_SINGLE
@tindex OMP_SECTION
@tindex OMP_MASTER
@tindex OMP_ORDERED
@tindex OMP_CRITICAL
@tindex OMP_RETURN
@tindex OMP_CONTINUE
@tindex OMP_ATOMIC
@tindex OMP_CLAUSE

All the statements starting with @code{OMP_} represent directives and
clauses used by the OpenMP API @w{@uref{https://www.openmp.org}}.

@table @code
@item OMP_PARALLEL

Represents @code{#pragma omp parallel [clause1 @dots{} clauseN]}. It
has four operands:

Operand @code{OMP_PARALLEL_BODY} is valid while in GENERIC and
High GIMPLE forms.  It contains the body of code to be executed
by all the threads.  During GIMPLE lowering, this operand becomes
@code{NULL} and the body is emitted linearly after
@code{OMP_PARALLEL}.

Operand @code{OMP_PARALLEL_CLAUSES} is the list of clauses
associated with the directive.

Operand @code{OMP_PARALLEL_FN} is created by
@code{pass_lower_omp}, it contains the @code{FUNCTION_DECL}
for the function that will contain the body of the parallel
region.

Operand @code{OMP_PARALLEL_DATA_ARG} is also created by
@code{pass_lower_omp}. If there are shared variables to be
communicated to the children threads, this operand will contain
the @code{VAR_DECL} that contains all the shared values and
variables.

@item OMP_FOR

Represents @code{#pragma omp for [clause1 @dots{} clauseN]}.  It has
six operands:

Operand @code{OMP_FOR_BODY} contains the loop body.

Operand @code{OMP_FOR_CLAUSES} is the list of clauses
associated with the directive.

Operand @code{OMP_FOR_INIT} is the loop initialization code of
the form @code{VAR = N1}.

Operand @code{OMP_FOR_COND} is the loop conditional expression
of the form @code{VAR @{<,>,<=,>=@} N2}.

Operand @code{OMP_FOR_INCR} is the loop index increment of the
form @code{VAR @{+=,-=@} INCR}.

Operand @code{OMP_FOR_PRE_BODY} contains side effect code from
operands @code{OMP_FOR_INIT}, @code{OMP_FOR_COND} and
@code{OMP_FOR_INC}.  These side effects are part of the
@code{OMP_FOR} block but must be evaluated before the start of
loop body.

The loop index variable @code{VAR} must be a signed integer variable,
which is implicitly private to each thread.  Bounds
@code{N1} and @code{N2} and the increment expression
@code{INCR} are required to be loop invariant integer
expressions that are evaluated without any synchronization. The
evaluation order, frequency of evaluation and side effects are
unspecified by the standard.

@item OMP_SECTIONS

Represents @code{#pragma omp sections [clause1 @dots{} clauseN]}.

Operand @code{OMP_SECTIONS_BODY} contains the sections body,
which in turn contains a set of @code{OMP_SECTION} nodes for
each of the concurrent sections delimited by @code{#pragma omp
section}.

Operand @code{OMP_SECTIONS_CLAUSES} is the list of clauses
associated with the directive.

@item OMP_SECTION

Section delimiter for @code{OMP_SECTIONS}.

@item OMP_SINGLE

Represents @code{#pragma omp single}.

Operand @code{OMP_SINGLE_BODY} contains the body of code to be
executed by a single thread.

Operand @code{OMP_SINGLE_CLAUSES} is the list of clauses
associated with the directive.

@item OMP_MASTER

Represents @code{#pragma omp master}.

Operand @code{OMP_MASTER_BODY} contains the body of code to be
executed by the master thread.

@item OMP_ORDERED

Represents @code{#pragma omp ordered}.

Operand @code{OMP_ORDERED_BODY} contains the body of code to be
executed in the sequential order dictated by the loop index
variable.

@item OMP_CRITICAL

Represents @code{#pragma omp critical [name]}.

Operand @code{OMP_CRITICAL_BODY} is the critical section.

Operand @code{OMP_CRITICAL_NAME} is an optional identifier to
label the critical section.

@item OMP_RETURN

This does not represent any OpenMP directive, it is an artificial
marker to indicate the end of the body of an OpenMP@. It is used
by the flow graph (@code{tree-cfg.cc}) and OpenMP region
building code (@code{omp-low.cc}).

@item OMP_CONTINUE

Similarly, this instruction does not represent an OpenMP
directive, it is used by @code{OMP_FOR} (and similar codes) as well as
@code{OMP_SECTIONS} to mark the place where the code needs to
loop to the next iteration, or the next section, respectively.

In some cases, @code{OMP_CONTINUE} is placed right before
@code{OMP_RETURN}.  But if there are cleanups that need to
occur right after the looping body, it will be emitted between
@code{OMP_CONTINUE} and @code{OMP_RETURN}.

@item OMP_ATOMIC

Represents @code{#pragma omp atomic}.

Operand 0 is the address at which the atomic operation is to be
performed.

Operand 1 is the expression to evaluate.  The gimplifier tries
three alternative code generation strategies.  Whenever possible,
an atomic update built-in is used.  If that fails, a
compare-and-swap loop is attempted.  If that also fails, a
regular critical section around the expression is used.

@item OMP_CLAUSE

Represents clauses associated with one of the @code{OMP_} directives.
Clauses are represented by separate subcodes defined in
@file{tree.h}.  Clauses codes can be one of:
@code{OMP_CLAUSE_PRIVATE}, @code{OMP_CLAUSE_SHARED},
@code{OMP_CLAUSE_FIRSTPRIVATE},
@code{OMP_CLAUSE_LASTPRIVATE}, @code{OMP_CLAUSE_COPYIN},
@code{OMP_CLAUSE_COPYPRIVATE}, @code{OMP_CLAUSE_IF},
@code{OMP_CLAUSE_NUM_THREADS}, @code{OMP_CLAUSE_SCHEDULE},
@code{OMP_CLAUSE_NOWAIT}, @code{OMP_CLAUSE_ORDERED},
@code{OMP_CLAUSE_DEFAULT}, @code{OMP_CLAUSE_REDUCTION},
@code{OMP_CLAUSE_COLLAPSE}, @code{OMP_CLAUSE_UNTIED},
@code{OMP_CLAUSE_FINAL}, and @code{OMP_CLAUSE_MERGEABLE}.  Each code
represents the corresponding OpenMP clause.

Clauses associated with the same directive are chained together
via @code{OMP_CLAUSE_CHAIN}. Those clauses that accept a list
of variables are restricted to exactly one, accessed with
@code{OMP_CLAUSE_VAR}.  Therefore, multiple variables under the
same clause @code{C} need to be represented as multiple @code{C} clauses
chained together.  This facilitates adding new clauses during
compilation.

@end table

@node OpenACC
@subsection OpenACC
@tindex OACC_CACHE
@tindex OACC_DATA
@tindex OACC_DECLARE
@tindex OACC_ENTER_DATA
@tindex OACC_EXIT_DATA
@tindex OACC_HOST_DATA
@tindex OACC_KERNELS
@tindex OACC_LOOP
@tindex OACC_PARALLEL
@tindex OACC_SERIAL
@tindex OACC_UPDATE

All the statements starting with @code{OACC_} represent directives and
clauses used by the OpenACC API @w{@uref{https://www.openacc.org}}.

@table @code
@item OACC_CACHE

Represents @code{#pragma acc cache (var @dots{})}.

@item OACC_DATA

Represents @code{#pragma acc data [clause1 @dots{} clauseN]}.

@item OACC_DECLARE

Represents @code{#pragma acc declare [clause1 @dots{} clauseN]}.

@item OACC_ENTER_DATA

Represents @code{#pragma acc enter data [clause1 @dots{} clauseN]}.

@item OACC_EXIT_DATA

Represents @code{#pragma acc exit data [clause1 @dots{} clauseN]}.

@item OACC_HOST_DATA

Represents @code{#pragma acc host_data [clause1 @dots{} clauseN]}.

@item OACC_KERNELS

Represents @code{#pragma acc kernels [clause1 @dots{} clauseN]}.

@item OACC_LOOP

Represents @code{#pragma acc loop [clause1 @dots{} clauseN]}.

See the description of the @code{OMP_FOR} code.

@item OACC_PARALLEL

Represents @code{#pragma acc parallel [clause1 @dots{} clauseN]}.

@item OACC_SERIAL

Represents @code{#pragma acc serial [clause1 @dots{} clauseN]}.

@item OACC_UPDATE

Represents @code{#pragma acc update [clause1 @dots{} clauseN]}.

@end table

@c ---------------------------------------------------------------------
@c Functions
@c ---------------------------------------------------------------------

@node Functions
@section Functions
@cindex function
@tindex FUNCTION_DECL

A function is represented by a @code{FUNCTION_DECL} node.  It stores
the basic pieces of the function such as body, parameters, and return
type as well as information on the surrounding context, visibility,
and linkage.

@menu
* Function Basics::     Function names, body, and parameters.
* Function Properties:: Context, linkage, etc.
@end menu

@c ---------------------------------------------------------------------
@c Function Basics
@c ---------------------------------------------------------------------

@node Function Basics
@subsection Function Basics
@findex DECL_NAME
@findex DECL_ASSEMBLER_NAME
@findex TREE_PUBLIC
@findex DECL_ARTIFICIAL
@findex DECL_FUNCTION_SPECIFIC_TARGET
@findex DECL_FUNCTION_SPECIFIC_OPTIMIZATION

A function has four core parts: the name, the parameters, the result,
and the body.  The following macros and functions access these parts
of a @code{FUNCTION_DECL} as well as other basic features:
@ftable @code
@item DECL_NAME
This macro returns the unqualified name of the function, as an
@code{IDENTIFIER_NODE}.  For an instantiation of a function template,
the @code{DECL_NAME} is the unqualified name of the template, not
something like @code{f<int>}.  The value of @code{DECL_NAME} is
undefined when used on a constructor, destructor, overloaded operator,
or type-conversion operator, or any function that is implicitly
generated by the compiler.  See below for macros that can be used to
distinguish these cases.

@item DECL_ASSEMBLER_NAME
This macro returns the mangled name of the function, also an
@code{IDENTIFIER_NODE}.  This name does not contain leading underscores
on systems that prefix all identifiers with underscores.  The mangled
name is computed in the same way on all platforms; if special processing
is required to deal with the object file format used on a particular
platform, it is the responsibility of the back end to perform those
modifications.  (Of course, the back end should not modify
@code{DECL_ASSEMBLER_NAME} itself.)

Using @code{DECL_ASSEMBLER_NAME} will cause additional memory to be
allocated (for the mangled name of the entity) so it should be used
only when emitting assembly code.  It should not be used within the
optimizers to determine whether or not two declarations are the same,
even though some of the existing optimizers do use it in that way.
These uses will be removed over time.

@item DECL_ARGUMENTS
This macro returns the @code{PARM_DECL} for the first argument to the
function.  Subsequent @code{PARM_DECL} nodes can be obtained by
following the @code{TREE_CHAIN} links.

@item DECL_RESULT
This macro returns the @code{RESULT_DECL} for the function.

@item DECL_SAVED_TREE
This macro returns the complete body of the function.

@item TREE_TYPE
This macro returns the @code{FUNCTION_TYPE} or @code{METHOD_TYPE} for
the function.

@item DECL_INITIAL
A function that has a definition in the current translation unit will
have a non-@code{NULL} @code{DECL_INITIAL}.  However, back ends should not make
use of the particular value given by @code{DECL_INITIAL}.

It should contain a tree of @code{BLOCK} nodes that mirrors the scopes
that variables are bound in the function.  Each block contains a list
of decls declared in a basic block, a pointer to a chain of blocks at
the next lower scope level, then a pointer to the next block at the
same level and a backpointer to the parent @code{BLOCK} or
@code{FUNCTION_DECL}.  So given a function as follows:

@smallexample
void foo()
@{
  int a;
  @{
    int b;
  @}
  int c;
@}
@end smallexample

you would get the following:

@smallexample
tree foo = FUNCTION_DECL;
tree decl_a = VAR_DECL;
tree decl_b = VAR_DECL;
tree decl_c = VAR_DECL;
tree block_a = BLOCK;
tree block_b = BLOCK;
tree block_c = BLOCK;
BLOCK_VARS(block_a) = decl_a;
BLOCK_SUBBLOCKS(block_a) = block_b;
BLOCK_CHAIN(block_a) = block_c;
BLOCK_SUPERCONTEXT(block_a) = foo;
BLOCK_VARS(block_b) = decl_b;
BLOCK_SUPERCONTEXT(block_b) = block_a;
BLOCK_VARS(block_c) = decl_c;
BLOCK_SUPERCONTEXT(block_c) = foo;
DECL_INITIAL(foo) = block_a;
@end smallexample

@end ftable

@c ---------------------------------------------------------------------
@c Function Properties
@c ---------------------------------------------------------------------

@node Function Properties
@subsection Function Properties
@cindex function properties
@cindex statements

To determine the scope of a function, you can use the
@code{DECL_CONTEXT} macro.  This macro will return the class
(either a @code{RECORD_TYPE} or a @code{UNION_TYPE}) or namespace (a
@code{NAMESPACE_DECL}) of which the function is a member.  For a virtual
function, this macro returns the class in which the function was
actually defined, not the base class in which the virtual declaration
occurred.

In C, the @code{DECL_CONTEXT} for a function maybe another function.
This representation indicates that the GNU nested function extension
is in use.  For details on the semantics of nested functions, see the
GCC Manual.  The nested function can refer to local variables in its
containing function.  Such references are not explicitly marked in the
tree structure; back ends must look at the @code{DECL_CONTEXT} for the
referenced @code{VAR_DECL}.  If the @code{DECL_CONTEXT} for the
referenced @code{VAR_DECL} is not the same as the function currently
being processed, and neither @code{DECL_EXTERNAL} nor
@code{TREE_STATIC} hold, then the reference is to a local variable in
a containing function, and the back end must take appropriate action.

@ftable @code
@item DECL_EXTERNAL
This predicate holds if the function is undefined.

@item TREE_PUBLIC
This predicate holds if the function has external linkage.

@item TREE_STATIC
This predicate holds if the function has been defined.

@item TREE_THIS_VOLATILE
This predicate holds if the function does not return normally.

@item TREE_READONLY
This predicate holds if the function can only read its arguments.

@item DECL_PURE_P
This predicate holds if the function can only read its arguments, but
may also read global memory.

@item DECL_VIRTUAL_P
This predicate holds if the function is virtual.

@item DECL_ARTIFICIAL
This macro holds if the function was implicitly generated by the
compiler, rather than explicitly declared.  In addition to implicitly
generated class member functions, this macro holds for the special
functions created to implement static initialization and destruction, to
compute run-time type information, and so forth.

@item DECL_FUNCTION_SPECIFIC_TARGET
This macro returns a tree node that holds the target options that are
to be used to compile this particular function or @code{NULL_TREE} if
the function is to be compiled with the target options specified on
the command line.

@item DECL_FUNCTION_SPECIFIC_OPTIMIZATION
This macro returns a tree node that holds the optimization options
that are to be used to compile this particular function or
@code{NULL_TREE} if the function is to be compiled with the
optimization options specified on the command line.

@end ftable

@c ---------------------------------------------------------------------
@c Language-dependent trees
@c ---------------------------------------------------------------------

@node Language-dependent trees
@section Language-dependent trees
@cindex language-dependent trees

Front ends may wish to keep some state associated with various GENERIC
trees while parsing.  To support this, trees provide a set of flags
that may be used by the front end.  They are accessed using
@code{TREE_LANG_FLAG_n} where @samp{n} is currently 0 through 6.

If necessary, a front end can use some language-dependent tree
codes in its GENERIC representation, so long as it provides a
hook for converting them to GIMPLE and doesn't expect them to
work with any (hypothetical) optimizers that run before the
conversion to GIMPLE@. The intermediate representation used while
parsing C and C++ looks very little like GENERIC, but the C and
C++ gimplifier hooks are perfectly happy to take it as input and
spit out GIMPLE@.



@node C and C++ Trees
@section C and C++ Trees

This section documents the internal representation used by GCC to
represent C and C++ source programs.  When presented with a C or C++
source program, GCC parses the program, performs semantic analysis
(including the generation of error messages), and then produces the
internal representation described here.  This representation contains a
complete representation for the entire translation unit provided as
input to the front end.  This representation is then typically processed
by a code-generator in order to produce machine code, but could also be
used in the creation of source browsers, intelligent editors, automatic
documentation generators, interpreters, and any other programs needing
the ability to process C or C++ code.

This section explains the internal representation.  In particular, it
documents the internal representation for C and C++ source
constructs, and the macros, functions, and variables that can be used to
access these constructs.  The C++ representation is largely a superset
of the representation used in the C front end.  There is only one
construct used in C that does not appear in the C++ front end and that
is the GNU ``nested function'' extension.  Many of the macros documented
here do not apply in C because the corresponding language constructs do
not appear in C@.

The C and C++ front ends generate a mix of GENERIC trees and ones
specific to C and C++.  These language-specific trees are higher-level
constructs than the ones in GENERIC to make the parser's job easier.
This section describes those trees that aren't part of GENERIC as well
as aspects of GENERIC trees that are treated in a language-specific
manner.

If you are developing a ``back end'', be it is a code-generator or some
other tool, that uses this representation, you may occasionally find
that you need to ask questions not easily answered by the functions and
macros available here.  If that situation occurs, it is quite likely
that GCC already supports the functionality you desire, but that the
interface is simply not documented here.  In that case, you should ask
the GCC maintainers (via mail to @email{gcc@@gcc.gnu.org}) about
documenting the functionality you require.  Similarly, if you find
yourself writing functions that do not deal directly with your back end,
but instead might be useful to other people using the GCC front end, you
should submit your patches for inclusion in GCC@.

@menu
* Types for C++::               Fundamental and aggregate types.
* Namespaces::                  Namespaces.
* Classes::                     Classes.
* Functions for C++::           Overloading and accessors for C++.
* Statements for C and C++::    Statements specific to C and C++.
* C++ Expressions::    From @code{typeid} to @code{throw}.
@end menu

@node Types for C++
@subsection Types for C++
@tindex UNKNOWN_TYPE
@tindex TYPENAME_TYPE
@tindex TYPEOF_TYPE
@findex cp_type_quals
@findex TYPE_UNQUALIFIED
@findex TYPE_QUAL_CONST
@findex TYPE_QUAL_VOLATILE
@findex TYPE_QUAL_RESTRICT
@findex TYPE_MAIN_VARIANT
@cindex qualified type
@findex TYPE_SIZE
@findex TYPE_ALIGN
@findex TYPE_PRECISION
@findex TYPE_ARG_TYPES
@findex TYPE_METHOD_BASETYPE
@findex TYPE_PTRDATAMEM_P
@findex TYPE_OFFSET_BASETYPE
@findex TREE_TYPE
@findex TYPE_CONTEXT
@findex TYPE_NAME
@findex TYPENAME_TYPE_FULLNAME
@findex TYPE_FIELDS
@findex TYPE_PTROBV_P

In C++, an array type is not qualified; rather the type of the array
elements is qualified.  This situation is reflected in the intermediate
representation.  The macros described here will always examine the
qualification of the underlying element type when applied to an array
type.  (If the element type is itself an array, then the recursion
continues until a non-array type is found, and the qualification of this
type is examined.)  So, for example, @code{CP_TYPE_CONST_P} will hold of
the type @code{const int ()[7]}, denoting an array of seven @code{int}s.

The following functions and macros deal with cv-qualification of types:
@ftable @code
@item cp_type_quals
This function returns the set of type qualifiers applied to this type.
This value is @code{TYPE_UNQUALIFIED} if no qualifiers have been
applied.  The @code{TYPE_QUAL_CONST} bit is set if the type is
@code{const}-qualified.  The @code{TYPE_QUAL_VOLATILE} bit is set if the
type is @code{volatile}-qualified.  The @code{TYPE_QUAL_RESTRICT} bit is
set if the type is @code{restrict}-qualified.

@item CP_TYPE_CONST_P
This macro holds if the type is @code{const}-qualified.

@item CP_TYPE_VOLATILE_P
This macro holds if the type is @code{volatile}-qualified.

@item CP_TYPE_RESTRICT_P
This macro holds if the type is @code{restrict}-qualified.

@item CP_TYPE_CONST_NON_VOLATILE_P
This predicate holds for a type that is @code{const}-qualified, but
@emph{not} @code{volatile}-qualified; other cv-qualifiers are ignored as
well: only the @code{const}-ness is tested.

@end ftable

A few other macros and functions are usable with all types:
@ftable @code
@item TYPE_SIZE
The number of bits required to represent the type, represented as an
@code{INTEGER_CST}.  For an incomplete type, @code{TYPE_SIZE} will be
@code{NULL_TREE}.

@item TYPE_ALIGN
The alignment of the type, in bits, represented as an @code{int}.

@item TYPE_NAME
This macro returns a declaration (in the form of a @code{TYPE_DECL}) for
the type.  (Note this macro does @emph{not} return an
@code{IDENTIFIER_NODE}, as you might expect, given its name!)  You can
look at the @code{DECL_NAME} of the @code{TYPE_DECL} to obtain the
actual name of the type.  The @code{TYPE_NAME} will be @code{NULL_TREE}
for a type that is not a built-in type, the result of a typedef, or a
named class type.

@item CP_INTEGRAL_TYPE
This predicate holds if the type is an integral type.  Notice that in
C++, enumerations are @emph{not} integral types.

@item ARITHMETIC_TYPE_P
This predicate holds if the type is an integral type (in the C++ sense)
or a floating point type.

@item CLASS_TYPE_P
This predicate holds for a class-type.

@item TYPE_BUILT_IN
This predicate holds for a built-in type.

@item TYPE_PTRDATAMEM_P
This predicate holds if the type is a pointer to data member.

@item TYPE_PTR_P
This predicate holds if the type is a pointer type, and the pointee is
not a data member.

@item TYPE_PTRFN_P
This predicate holds for a pointer to function type.

@item TYPE_PTROB_P
This predicate holds for a pointer to object type.  Note however that it
does not hold for the generic pointer to object type @code{void *}.  You
may use @code{TYPE_PTROBV_P} to test for a pointer to object type as
well as @code{void *}.

@end ftable

The table below describes types specific to C and C++ as well as
language-dependent info about GENERIC types.

@table @code

@item POINTER_TYPE
Used to represent pointer types, and pointer to data member types.  If
@code{TREE_TYPE}
is a pointer to data member type, then @code{TYPE_PTRDATAMEM_P} will hold.
For a pointer to data member type of the form @samp{T X::*},
@code{TYPE_PTRMEM_CLASS_TYPE} will be the type @code{X}, while
@code{TYPE_PTRMEM_POINTED_TO_TYPE} will be the type @code{T}.

@item RECORD_TYPE
Used to represent @code{struct} and @code{class} types in C and C++.  If
@code{TYPE_PTRMEMFUNC_P} holds, then this type is a pointer-to-member
type.  In that case, the @code{TYPE_PTRMEMFUNC_FN_TYPE} is a
@code{POINTER_TYPE} pointing to a @code{METHOD_TYPE}.  The
@code{METHOD_TYPE} is the type of a function pointed to by the
pointer-to-member function.  If @code{TYPE_PTRMEMFUNC_P} does not hold,
this type is a class type.  For more information, @pxref{Classes}.

@item UNKNOWN_TYPE
This node is used to represent a type the knowledge of which is
insufficient for a sound processing.

@item TYPENAME_TYPE
Used to represent a construct of the form @code{typename T::A}.  The
@code{TYPE_CONTEXT} is @code{T}; the @code{TYPE_NAME} is an
@code{IDENTIFIER_NODE} for @code{A}.  If the type is specified via a
template-id, then @code{TYPENAME_TYPE_FULLNAME} yields a
@code{TEMPLATE_ID_EXPR}.  The @code{TREE_TYPE} is non-@code{NULL} if the
node is implicitly generated in support for the implicit typename
extension; in which case the @code{TREE_TYPE} is a type node for the
base-class.

@item TYPEOF_TYPE
Used to represent the @code{__typeof__} extension.  The
@code{TYPE_FIELDS} is the expression the type of which is being
represented.

@end table


@c ---------------------------------------------------------------------
@c Namespaces
@c ---------------------------------------------------------------------

@node Namespaces
@subsection Namespaces
@cindex namespace, scope
@tindex NAMESPACE_DECL

The root of the entire intermediate representation is the variable
@code{global_namespace}.  This is the namespace specified with @code{::}
in C++ source code.  All other namespaces, types, variables, functions,
and so forth can be found starting with this namespace.

However, except for the fact that it is distinguished as the root of the
representation, the global namespace is no different from any other
namespace.  Thus, in what follows, we describe namespaces generally,
rather than the global namespace in particular.

A namespace is represented by a @code{NAMESPACE_DECL} node.

The following macros and functions can be used on a @code{NAMESPACE_DECL}:

@ftable @code
@item DECL_NAME
This macro is used to obtain the @code{IDENTIFIER_NODE} corresponding to
the unqualified name of the name of the namespace (@pxref{Identifiers}).
The name of the global namespace is @samp{::}, even though in C++ the
global namespace is unnamed.  However, you should use comparison with
@code{global_namespace}, rather than @code{DECL_NAME} to determine
whether or not a namespace is the global one.  An unnamed namespace
will have a @code{DECL_NAME} equal to @code{anonymous_namespace_name}.
Within a single translation unit, all unnamed namespaces will have the
same name.

@item DECL_CONTEXT
This macro returns the enclosing namespace.  The @code{DECL_CONTEXT} for
the @code{global_namespace} is @code{NULL_TREE}.

@item DECL_NAMESPACE_ALIAS
If this declaration is for a namespace alias, then
@code{DECL_NAMESPACE_ALIAS} is the namespace for which this one is an
alias.

Do not attempt to use @code{cp_namespace_decls} for a namespace which is
an alias.  Instead, follow @code{DECL_NAMESPACE_ALIAS} links until you
reach an ordinary, non-alias, namespace, and call
@code{cp_namespace_decls} there.

@item DECL_NAMESPACE_STD_P
This predicate holds if the namespace is the special @code{::std}
namespace.

@item cp_namespace_decls
This function will return the declarations contained in the namespace,
including types, overloaded functions, other namespaces, and so forth.
If there are no declarations, this function will return
@code{NULL_TREE}.  The declarations are connected through their
@code{TREE_CHAIN} fields.

Although most entries on this list will be declarations,
@code{TREE_LIST} nodes may also appear.  In this case, the
@code{TREE_VALUE} will be an @code{OVERLOAD}.  The value of the
@code{TREE_PURPOSE} is unspecified; back ends should ignore this value.
As with the other kinds of declarations returned by
@code{cp_namespace_decls}, the @code{TREE_CHAIN} will point to the next
declaration in this list.

For more information on the kinds of declarations that can occur on this
list, @xref{Declarations}.  Some declarations will not appear on this
list.  In particular, no @code{FIELD_DECL}, @code{LABEL_DECL}, or
@code{PARM_DECL} nodes will appear here.

This function cannot be used with namespaces that have
@code{DECL_NAMESPACE_ALIAS} set.

@end ftable

@c ---------------------------------------------------------------------
@c Classes
@c ---------------------------------------------------------------------

@node Classes
@subsection Classes
@cindex class, scope
@tindex RECORD_TYPE
@tindex UNION_TYPE
@findex CLASSTYPE_DECLARED_CLASS
@findex TYPE_BINFO
@findex BINFO_TYPE
@findex TYPE_FIELDS
@findex TYPE_VFIELD

Besides namespaces, the other high-level scoping construct in C++ is the
class.  (Throughout this manual the term @dfn{class} is used to mean the
types referred to in the ANSI/ISO C++ Standard as classes; these include
types defined with the @code{class}, @code{struct}, and @code{union}
keywords.)

A class type is represented by either a @code{RECORD_TYPE} or a
@code{UNION_TYPE}.  A class declared with the @code{union} tag is
represented by a @code{UNION_TYPE}, while classes declared with either
the @code{struct} or the @code{class} tag are represented by
@code{RECORD_TYPE}s.  You can use the @code{CLASSTYPE_DECLARED_CLASS}
macro to discern whether or not a particular type is a @code{class} as
opposed to a @code{struct}.  This macro will be true only for classes
declared with the @code{class} tag.

Almost all members are available on the @code{TYPE_FIELDS}
list.  Given one member, the next can be found by following the
@code{TREE_CHAIN}.  You should not depend in any way on the order in
which fields appear on this list.  All nodes on this list will be
@samp{DECL} nodes.  A @code{FIELD_DECL} is used to represent a non-static
data member, a @code{VAR_DECL} is used to represent a static data
member, and a @code{TYPE_DECL} is used to represent a type.  Note that
the @code{CONST_DECL} for an enumeration constant will appear on this
list, if the enumeration type was declared in the class.  (Of course,
the @code{TYPE_DECL} for the enumeration type will appear here as well.)
There are no entries for base classes on this list.  In particular,
there is no @code{FIELD_DECL} for the ``base-class portion'' of an
object.  If a function member is overloaded, each of the overloaded
functions appears; no @code{OVERLOAD} nodes appear on the @code{TYPE_FIELDS}
list.  Implicitly declared functions (including default constructors,
copy constructors, assignment operators, and destructors) will appear on
this list as well.

The @code{TYPE_VFIELD} is a compiler-generated field used to point to
virtual function tables.  It may or may not appear on the
@code{TYPE_FIELDS} list.  However, back ends should handle the
@code{TYPE_VFIELD} just like all the entries on the @code{TYPE_FIELDS}
list.

Every class has an associated @dfn{binfo}, which can be obtained with
@code{TYPE_BINFO}.  Binfos are used to represent base-classes.  The
binfo given by @code{TYPE_BINFO} is the degenerate case, whereby every
class is considered to be its own base-class.  The base binfos for a
particular binfo are held in a vector, whose length is obtained with
@code{BINFO_N_BASE_BINFOS}.  The base binfos themselves are obtained
with @code{BINFO_BASE_BINFO} and @code{BINFO_BASE_ITERATE}.  To add a
new binfo, use @code{BINFO_BASE_APPEND}.  The vector of base binfos can
be obtained with @code{BINFO_BASE_BINFOS}, but normally you do not need
to use that.  The class type associated with a binfo is given by
@code{BINFO_TYPE}.  It is not always the case that @code{BINFO_TYPE
(TYPE_BINFO (x))}, because of typedefs and qualified types.  Neither is
it the case that @code{TYPE_BINFO (BINFO_TYPE (y))} is the same binfo as
@code{y}.  The reason is that if @code{y} is a binfo representing a
base-class @code{B} of a derived class @code{D}, then @code{BINFO_TYPE
(y)} will be @code{B}, and @code{TYPE_BINFO (BINFO_TYPE (y))} will be
@code{B} as its own base-class, rather than as a base-class of @code{D}.

The access to a base type can be found with @code{BINFO_BASE_ACCESS}.
This will produce @code{access_public_node}, @code{access_private_node}
or @code{access_protected_node}.  If bases are always public,
@code{BINFO_BASE_ACCESSES} may be @code{NULL}.

@code{BINFO_VIRTUAL_P} is used to specify whether the binfo is inherited
virtually or not.  The other flags, @code{BINFO_FLAG_0} to
@code{BINFO_FLAG_6}, can be used for language specific use.

The following macros can be used on a tree node representing a class-type.

@ftable @code
@item LOCAL_CLASS_P
This predicate holds if the class is local class @emph{i.e.}@: declared
inside a function body.

@item TYPE_POLYMORPHIC_P
This predicate holds if the class has at least one virtual function
(declared or inherited).

@item TYPE_HAS_DEFAULT_CONSTRUCTOR
This predicate holds whenever its argument represents a class-type with
default constructor.

@item CLASSTYPE_HAS_MUTABLE
@itemx TYPE_HAS_MUTABLE_P
These predicates hold for a class-type having a mutable data member.

@item CLASSTYPE_NON_POD_P
This predicate holds only for class-types that are not PODs.

@item TYPE_HAS_NEW_OPERATOR
This predicate holds for a class-type that defines
@code{operator new}.

@item TYPE_HAS_ARRAY_NEW_OPERATOR
This predicate holds for a class-type for which
@code{operator new[]} is defined.

@item TYPE_OVERLOADS_CALL_EXPR
This predicate holds for class-type for which the function call
@code{operator()} is overloaded.

@item TYPE_OVERLOADS_ARRAY_REF
This predicate holds for a class-type that overloads
@code{operator[]}

@item TYPE_OVERLOADS_ARROW
This predicate holds for a class-type for which @code{operator->} is
overloaded.

@end ftable

@node Functions for C++
@subsection Functions for C++
@cindex function
@tindex FUNCTION_DECL
@tindex OVERLOAD
@findex OVL_CURRENT
@findex OVL_NEXT

A function is represented by a @code{FUNCTION_DECL} node.  A set of
overloaded functions is sometimes represented by an @code{OVERLOAD} node.

An @code{OVERLOAD} node is not a declaration, so none of the
@samp{DECL_} macros should be used on an @code{OVERLOAD}.  An
@code{OVERLOAD} node is similar to a @code{TREE_LIST}.  Use
@code{OVL_CURRENT} to get the function associated with an
@code{OVERLOAD} node; use @code{OVL_NEXT} to get the next
@code{OVERLOAD} node in the list of overloaded functions.  The macros
@code{OVL_CURRENT} and @code{OVL_NEXT} are actually polymorphic; you can
use them to work with @code{FUNCTION_DECL} nodes as well as with
overloads.  In the case of a @code{FUNCTION_DECL}, @code{OVL_CURRENT}
will always return the function itself, and @code{OVL_NEXT} will always
be @code{NULL_TREE}.

To determine the scope of a function, you can use the
@code{DECL_CONTEXT} macro.  This macro will return the class
(either a @code{RECORD_TYPE} or a @code{UNION_TYPE}) or namespace (a
@code{NAMESPACE_DECL}) of which the function is a member.  For a virtual
function, this macro returns the class in which the function was
actually defined, not the base class in which the virtual declaration
occurred.

If a friend function is defined in a class scope, the
@code{DECL_FRIEND_CONTEXT} macro can be used to determine the class in
which it was defined.  For example, in
@smallexample
class C @{ friend void f() @{@} @};
@end smallexample
@noindent
the @code{DECL_CONTEXT} for @code{f} will be the
@code{global_namespace}, but the @code{DECL_FRIEND_CONTEXT} will be the
@code{RECORD_TYPE} for @code{C}.


The following macros and functions can be used on a @code{FUNCTION_DECL}:
@ftable @code
@item DECL_MAIN_P
This predicate holds for a function that is the program entry point
@code{::code}.

@item DECL_LOCAL_FUNCTION_P
This predicate holds if the function was declared at block scope, even
though it has a global scope.

@item DECL_ANTICIPATED
This predicate holds if the function is a built-in function but its
prototype is not yet explicitly declared.

@item DECL_EXTERN_C_FUNCTION_P
This predicate holds if the function is declared as an
`@code{extern "C"}' function.

@item DECL_LINKONCE_P
This macro holds if multiple copies of this function may be emitted in
various translation units.  It is the responsibility of the linker to
merge the various copies.  Template instantiations are the most common
example of functions for which @code{DECL_LINKONCE_P} holds; G++
instantiates needed templates in all translation units which require them,
and then relies on the linker to remove duplicate instantiations.

FIXME: This macro is not yet implemented.

@item DECL_FUNCTION_MEMBER_P
This macro holds if the function is a member of a class, rather than a
member of a namespace.

@item DECL_STATIC_FUNCTION_P
This predicate holds if the function a static member function.

@item DECL_NONSTATIC_MEMBER_FUNCTION_P
This macro holds for a non-static member function.

@item DECL_CONST_MEMFUNC_P
This predicate holds for a @code{const}-member function.

@item DECL_VOLATILE_MEMFUNC_P
This predicate holds for a @code{volatile}-member function.

@item DECL_CONSTRUCTOR_P
This macro holds if the function is a constructor.

@item DECL_NONCONVERTING_P
This predicate holds if the constructor is a non-converting constructor.

@item DECL_COMPLETE_CONSTRUCTOR_P
This predicate holds for a function which is a constructor for an object
of a complete type.

@item DECL_BASE_CONSTRUCTOR_P
This predicate holds for a function which is a constructor for a base
class sub-object.

@item DECL_COPY_CONSTRUCTOR_P
This predicate holds for a function which is a copy-constructor.

@item DECL_DESTRUCTOR_P
This macro holds if the function is a destructor.

@item DECL_COMPLETE_DESTRUCTOR_P
This predicate holds if the function is the destructor for an object a
complete type.

@item DECL_OVERLOADED_OPERATOR_P
This macro holds if the function is an overloaded operator.

@item DECL_CONV_FN_P
This macro holds if the function is a type-conversion operator.

@item DECL_GLOBAL_CTOR_P
This predicate holds if the function is a file-scope initialization
function.

@item DECL_GLOBAL_DTOR_P
This predicate holds if the function is a file-scope finalization
function.

@item DECL_THUNK_P
This predicate holds if the function is a thunk.

These functions represent stub code that adjusts the @code{this} pointer
and then jumps to another function.  When the jumped-to function
returns, control is transferred directly to the caller, without
returning to the thunk.  The first parameter to the thunk is always the
@code{this} pointer; the thunk should add @code{THUNK_DELTA} to this
value.  (The @code{THUNK_DELTA} is an @code{int}, not an
@code{INTEGER_CST}.)

Then, if @code{THUNK_VCALL_OFFSET} (an @code{INTEGER_CST}) is nonzero
the adjusted @code{this} pointer must be adjusted again.  The complete
calculation is given by the following pseudo-code:

@smallexample
this += THUNK_DELTA
if (THUNK_VCALL_OFFSET)
  this += (*((ptrdiff_t **) this))[THUNK_VCALL_OFFSET]
@end smallexample

Finally, the thunk should jump to the location given
by @code{DECL_INITIAL}; this will always be an expression for the
address of a function.

@item DECL_NON_THUNK_FUNCTION_P
This predicate holds if the function is @emph{not} a thunk function.

@item GLOBAL_INIT_PRIORITY
If either @code{DECL_GLOBAL_CTOR_P} or @code{DECL_GLOBAL_DTOR_P} holds,
then this gives the initialization priority for the function.  The
linker will arrange that all functions for which
@code{DECL_GLOBAL_CTOR_P} holds are run in increasing order of priority
before @code{main} is called.  When the program exits, all functions for
which @code{DECL_GLOBAL_DTOR_P} holds are run in the reverse order.

@item TYPE_RAISES_EXCEPTIONS
This macro returns the list of exceptions that a (member-)function can
raise.  The returned list, if non @code{NULL}, is comprised of nodes
whose @code{TREE_VALUE} represents a type.

@item TYPE_NOTHROW_P
This predicate holds when the exception-specification of its arguments
is of the form `@code{()}'.

@item DECL_ARRAY_DELETE_OPERATOR_P
This predicate holds if the function an overloaded
@code{operator delete[]}.

@end ftable

@c ---------------------------------------------------------------------
@c Function Bodies
@c ---------------------------------------------------------------------

@node Statements for C and C++
@subsection Statements for C and C++
@cindex statements
@tindex BREAK_STMT
@tindex CLEANUP_STMT
@findex CLEANUP_DECL
@findex CLEANUP_EXPR
@tindex CONTINUE_STMT
@tindex DECL_STMT
@findex DECL_STMT_DECL
@tindex DO_STMT
@findex DO_BODY
@findex DO_COND
@tindex EMPTY_CLASS_EXPR
@tindex EXPR_STMT
@findex EXPR_STMT_EXPR
@tindex FOR_STMT
@findex FOR_INIT_STMT
@findex FOR_COND
@findex FOR_EXPR
@findex FOR_BODY
@tindex HANDLER
@tindex IF_STMT
@findex IF_COND
@findex THEN_CLAUSE
@findex ELSE_CLAUSE
@tindex RETURN_STMT
@findex RETURN_EXPR
@tindex SUBOBJECT
@findex SUBOBJECT_CLEANUP
@tindex SWITCH_STMT
@findex SWITCH_COND
@findex SWITCH_BODY
@tindex TRY_BLOCK
@findex TRY_STMTS
@findex TRY_HANDLERS
@findex HANDLER_PARMS
@findex HANDLER_BODY
@findex USING_STMT
@tindex WHILE_STMT
@findex WHILE_BODY
@findex WHILE_COND

A function that has a definition in the current translation unit has
a non-@code{NULL} @code{DECL_INITIAL}.  However, back ends should not make
use of the particular value given by @code{DECL_INITIAL}.

The @code{DECL_SAVED_TREE} gives the complete body of the
function.

There are tree nodes corresponding to all of the source-level
statement constructs, used within the C and C++ frontends.  These are
enumerated here, together with a list of the various macros that can
be used to obtain information about them.  There are a few macros that
can be used with all statements:

@ftable @code
@item STMT_IS_FULL_EXPR_P
In C++, statements normally constitute ``full expressions''; temporaries
created during a statement are destroyed when the statement is complete.
However, G++ sometimes represents expressions by statements; these
statements will not have @code{STMT_IS_FULL_EXPR_P} set.  Temporaries
created during such statements should be destroyed when the innermost
enclosing statement with @code{STMT_IS_FULL_EXPR_P} set is exited.

@end ftable

Here is the list of the various statement nodes, and the macros used to
access them.  This documentation describes the use of these nodes in
non-template functions (including instantiations of template functions).
In template functions, the same nodes are used, but sometimes in
slightly different ways.

Many of the statements have substatements.  For example, a @code{while}
loop has a body, which is itself a statement.  If the substatement
is @code{NULL_TREE}, it is considered equivalent to a statement
consisting of a single @code{;}, i.e., an expression statement in which
the expression has been omitted.  A substatement may in fact be a list
of statements, connected via their @code{TREE_CHAIN}s.  So, you should
always process the statement tree by looping over substatements, like
this:
@smallexample
void process_stmt (stmt)
     tree stmt;
@{
  while (stmt)
    @{
      switch (TREE_CODE (stmt))
        @{
        case IF_STMT:
          process_stmt (THEN_CLAUSE (stmt));
          /* @r{More processing here.}  */
          break;

        @dots{}
        @}

      stmt = TREE_CHAIN (stmt);
    @}
@}
@end smallexample
In other words, while the @code{then} clause of an @code{if} statement
in C++ can be only one statement (although that one statement may be a
compound statement), the intermediate representation sometimes uses
several statements chained together.

@table @code
@item BREAK_STMT

Used to represent a @code{break} statement.  There are no additional
fields.

@item CLEANUP_STMT

Used to represent an action that should take place upon exit from the
enclosing scope.  Typically, these actions are calls to destructors for
local objects, but back ends cannot rely on this fact.  If these nodes
are in fact representing such destructors, @code{CLEANUP_DECL} will be
the @code{VAR_DECL} destroyed.  Otherwise, @code{CLEANUP_DECL} will be
@code{NULL_TREE}.  In any case, the @code{CLEANUP_EXPR} is the
expression to execute.  The cleanups executed on exit from a scope
should be run in the reverse order of the order in which the associated
@code{CLEANUP_STMT}s were encountered.

@item CONTINUE_STMT

Used to represent a @code{continue} statement.  There are no additional
fields.

@item CTOR_STMT

Used to mark the beginning (if @code{CTOR_BEGIN_P} holds) or end (if
@code{CTOR_END_P} holds of the main body of a constructor.  See also
@code{SUBOBJECT} for more information on how to use these nodes.

@item DO_STMT

Used to represent a @code{do} loop.  The body of the loop is given by
@code{DO_BODY} while the termination condition for the loop is given by
@code{DO_COND}.  The condition for a @code{do}-statement is always an
expression.

@item EMPTY_CLASS_EXPR

Used to represent a temporary object of a class with no data whose
address is never taken.  (All such objects are interchangeable.)  The
@code{TREE_TYPE} represents the type of the object.

@item EXPR_STMT

Used to represent an expression statement.  Use @code{EXPR_STMT_EXPR} to
obtain the expression.

@item FOR_STMT

Used to represent a @code{for} statement.  The @code{FOR_INIT_STMT} is
the initialization statement for the loop.  The @code{FOR_COND} is the
termination condition.  The @code{FOR_EXPR} is the expression executed
right before the @code{FOR_COND} on each loop iteration; often, this
expression increments a counter.  The body of the loop is given by
@code{FOR_BODY}.  @code{FOR_SCOPE} holds the scope of the @code{for}
statement (used in the C++ front end only).  Note that
@code{FOR_INIT_STMT} and @code{FOR_BODY} return statements, while
@code{FOR_COND} and @code{FOR_EXPR} return expressions.

@item HANDLER

Used to represent a C++ @code{catch} block.  The @code{HANDLER_TYPE}
is the type of exception that will be caught by this handler; it is
equal (by pointer equality) to @code{NULL} if this handler is for all
types.  @code{HANDLER_PARMS} is the @code{DECL_STMT} for the catch
parameter, and @code{HANDLER_BODY} is the code for the block itself.

@item IF_STMT

Used to represent an @code{if} statement.  The @code{IF_COND} is the
expression.

If the condition is a @code{TREE_LIST}, then the @code{TREE_PURPOSE} is
a statement (usually a @code{DECL_STMT}).  Each time the condition is
evaluated, the statement should be executed.  Then, the
@code{TREE_VALUE} should be used as the conditional expression itself.
This representation is used to handle C++ code like this:

@smallexample
if (int i = 7) @dots{}
@end smallexample

where there is a new local variable (or variables) declared within the
condition.

The @code{THEN_CLAUSE} represents the statement given by the @code{then}
condition, while the @code{ELSE_CLAUSE} represents the statement given
by the @code{else} condition.

C++ distinguishes between this and @code{COND_EXPR} for handling templates.

@item SUBOBJECT

In a constructor, these nodes are used to mark the point at which a
subobject of @code{this} is fully constructed.  If, after this point, an
exception is thrown before a @code{CTOR_STMT} with @code{CTOR_END_P} set
is encountered, the @code{SUBOBJECT_CLEANUP} must be executed.  The
cleanups must be executed in the reverse order in which they appear.

@item SWITCH_STMT

Used to represent a @code{switch} statement.  The @code{SWITCH_STMT_COND}
is the expression on which the switch is occurring.  See the documentation
for an @code{IF_STMT} for more information on the representation used
for the condition.  The @code{SWITCH_STMT_BODY} is the body of the switch
statement.   The @code{SWITCH_STMT_TYPE} is the original type of switch
expression as given in the source, before any compiler conversions.
The @code{SWITCH_STMT_SCOPE} is the statement scope (used in the
C++ front end only).

There are also two boolean flags used with @code{SWITCH_STMT}.
@code{SWITCH_STMT_ALL_CASES_P} is true if the switch includes a default label
or the case label ranges cover all possible values of the condition
expression.  @code{SWITCH_STMT_NO_BREAK_P} is true if there are no
@code{break} statements in the switch.

@item TRY_BLOCK
Used to represent a @code{try} block.  The body of the try block is
given by @code{TRY_STMTS}.  Each of the catch blocks is a @code{HANDLER}
node.  The first handler is given by @code{TRY_HANDLERS}.  Subsequent
handlers are obtained by following the @code{TREE_CHAIN} link from one
handler to the next.  The body of the handler is given by
@code{HANDLER_BODY}.

If @code{CLEANUP_P} holds of the @code{TRY_BLOCK}, then the
@code{TRY_HANDLERS} will not be a @code{HANDLER} node.  Instead, it will
be an expression that should be executed if an exception is thrown in
the try block.  It must rethrow the exception after executing that code.
And, if an exception is thrown while the expression is executing,
@code{terminate} must be called.

@item USING_STMT
Used to represent a @code{using} directive.  The namespace is given by
@code{USING_STMT_NAMESPACE}, which will be a NAMESPACE_DECL@.  This node
is needed inside template functions, to implement using directives
during instantiation.

@item WHILE_STMT

Used to represent a @code{while} loop.  The @code{WHILE_COND} is the
termination condition for the loop.  See the documentation for an
@code{IF_STMT} for more information on the representation used for the
condition.

The @code{WHILE_BODY} is the body of the loop.

@end table

@node C++ Expressions
@subsection C++ Expressions

This section describes expressions specific to the C and C++ front
ends.

@table @code
@item TYPEID_EXPR

Used to represent a @code{typeid} expression.

@item NEW_EXPR
@itemx VEC_NEW_EXPR

Used to represent a call to @code{new} and @code{new[]} respectively.

@item DELETE_EXPR
@itemx VEC_DELETE_EXPR

Used to represent a call to @code{delete} and @code{delete[]} respectively.

@item MEMBER_REF

Represents a reference to a member of a class.

@item THROW_EXPR

Represents an instance of @code{throw} in the program.  Operand 0,
which is the expression to throw, may be @code{NULL_TREE}.


@item AGGR_INIT_EXPR
An @code{AGGR_INIT_EXPR} represents the initialization as the return
value of a function call, or as the result of a constructor.  An
@code{AGGR_INIT_EXPR} will only appear as a full-expression, or as the
second operand of a @code{TARGET_EXPR}.  @code{AGGR_INIT_EXPR}s have
a representation similar to that of @code{CALL_EXPR}s.  You can use
the @code{AGGR_INIT_EXPR_FN} and @code{AGGR_INIT_EXPR_ARG} macros to access
the function to call and the arguments to pass.

If @code{AGGR_INIT_VIA_CTOR_P} holds of the @code{AGGR_INIT_EXPR}, then
the initialization is via a constructor call.  The address of the
@code{AGGR_INIT_EXPR_SLOT} operand, which is always a @code{VAR_DECL},
is taken, and this value replaces the first argument in the argument
list.

In either case, the expression is void.


@end table