2008-05-30 Vladimir Makarov <vmakarov@redhat.com>

[official-gcc.git] / gcc / ada / checks.ads

------------------------------------------------------------------------------
--                                                                          --
--                         GNAT COMPILER COMPONENTS                         --
--                                                                          --
--                               C H E C K S                                --
--                                                                          --
--                                 S p e c                                  --
--                                                                          --
--          Copyright (C) 1992-2008, Free Software Foundation, Inc.         --
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-- GNAT is free software;  you can  redistribute it  and/or modify it under --
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-- GNAT was originally developed  by the GNAT team at  New York University. --
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------------------------------------------------------------------------------

--  Package containing routines used to deal with runtime checks. These
--  routines are used both by the semantics and by the expander. In some
--  cases, checks are enabled simply by setting flags for gigi, and in
--  other cases the code for the check is expanded.

--  The approach used for range and length checks, in regards to suppressed
--  checks, is to attempt to detect at compilation time that a constraint
--  error will occur. If this is detected a warning or error is issued and the
--  offending expression or statement replaced with a constraint error node.
--  This always occurs whether checks are suppressed or not.  Dynamic range
--  checks are, of course, not inserted if checks are suppressed.

with Namet;  use Namet;
with Table;
with Types;  use Types;
with Uintp;  use Uintp;

package Checks is

   procedure Initialize;
   --  Called for each new main source program, to initialize internal
   --  variables used in the package body of the Checks unit.

   function Access_Checks_Suppressed        (E : Entity_Id) return Boolean;
   function Accessibility_Checks_Suppressed (E : Entity_Id) return Boolean;
   function Alignment_Checks_Suppressed     (E : Entity_Id) return Boolean;
   function Discriminant_Checks_Suppressed  (E : Entity_Id) return Boolean;
   function Division_Checks_Suppressed      (E : Entity_Id) return Boolean;
   function Elaboration_Checks_Suppressed   (E : Entity_Id) return Boolean;
   function Index_Checks_Suppressed         (E : Entity_Id) return Boolean;
   function Length_Checks_Suppressed        (E : Entity_Id) return Boolean;
   function Overflow_Checks_Suppressed      (E : Entity_Id) return Boolean;
   function Range_Checks_Suppressed         (E : Entity_Id) return Boolean;
   function Storage_Checks_Suppressed       (E : Entity_Id) return Boolean;
   function Tag_Checks_Suppressed           (E : Entity_Id) return Boolean;
   function Validity_Checks_Suppressed      (E : Entity_Id) return Boolean;
   --  These functions check to see if the named check is suppressed, either
   --  by an active scope suppress setting, or because the check has been
   --  specifically suppressed for the given entity. If no entity is relevant
   --  for the current check, then Empty is used as an argument. Note: the
   --  reason we insist on specifying Empty is to force the caller to think
   --  about whether there is any relevant entity that should be checked.

   -------------------------------------------
   -- Procedures to Activate Checking Flags --
   -------------------------------------------

   procedure Activate_Division_Check (N : Node_Id);
   pragma Inline (Activate_Division_Check);
   --  Sets Do_Division_Check flag in node N, and handles possible local raise.
   --  Always call this routine rather than calling Set_Do_Division_Check to
   --  set an explicit value of True, to ensure handling the local raise case.

   procedure Activate_Overflow_Check (N : Node_Id);
   pragma Inline (Activate_Overflow_Check);
   --  Sets Do_Overflow_Check flag in node N, and handles possible local raise.
   --  Always call this routine rather than calling Set_Do_Overflow_Check to
   --  set an explicit value of True, to ensure handling the local raise case.

   procedure Activate_Range_Check (N : Node_Id);
   pragma Inline (Activate_Range_Check);
   --  Sets Do_Range_Check flag in node N, and handles possible local raise
   --  Always call this routine rather than calling Set_Do_Range_Check to
   --  set an explicit value of True, to ensure handling the local raise case.

   --------------------------------
   -- Procedures to Apply Checks --
   --------------------------------

   --  General note on following checks. These checks are always active if
   --  Expander_Active and not Inside_A_Generic. They are inactive and have
   --  no effect Inside_A_Generic. In the case where not Expander_Active
   --  and not Inside_A_Generic, most of them are inactive, but some of them
   --  operate anyway since they may generate useful compile time warnings.

   procedure Apply_Access_Check (N : Node_Id);
   --  Determines whether an expression node requires a runtime access
   --  check and if so inserts the appropriate run-time check.

   procedure Apply_Accessibility_Check (N : Node_Id; Typ : Entity_Id);
   --  Given a name N denoting an access parameter, emits a run-time
   --  accessibility check (if necessary), checking that the level of
   --  the object denoted by the access parameter is not deeper than the
   --  level of the type Typ. Program_Error is raised if the check fails.

   procedure Apply_Address_Clause_Check (E : Entity_Id; N : Node_Id);
   --  E is the entity for an object which has an address clause. If checks
   --  are enabled, then this procedure generates a check that the specified
   --  address has an alignment consistent with the alignment of the object,
   --  raising PE if this is not the case. The resulting check (if one is
   --  generated) is inserted before node N. check is also made for the case of
   --  a clear overlay situation that the size of the overlaying object is not
   --  larger than the overlaid object.

   procedure Apply_Arithmetic_Overflow_Check (N : Node_Id);
   --  Given a binary arithmetic operator (+ - *) expand a software integer
   --  overflow check using range checks on a larger checking type or a call
   --  to an appropriate runtime routine. This is used for all three operators
   --  for the signed integer case, and for +/- in the fixed-point case. The
   --  check is expanded only if Software_Overflow_Checking is enabled and
   --  Do_Overflow_Check is set on node N. Note that divide is handled
   --  separately using Apply_Arithmetic_Divide_Overflow_Check.

   procedure Apply_Constraint_Check
     (N          : Node_Id;
      Typ        : Entity_Id;
      No_Sliding : Boolean := False);
   --  Top-level procedure, calls all the others depending on the class of Typ.
   --  Checks that expression N verifies the constraint of type Typ. No_Sliding
   --  is only relevant for constrained array types, id set to true, it
   --  checks that indexes are in range.

   procedure Apply_Discriminant_Check
     (N   : Node_Id;
      Typ : Entity_Id;
      Lhs : Node_Id := Empty);
   --  Given an expression N of a discriminated type, or of an access type
   --  whose designated type is a discriminanted type, generates a check to
   --  ensure that the expression can be converted to the subtype given as
   --  the second parameter. Lhs is empty except in the case of assignments,
   --  where the target object may be needed to determine the subtype to
   --  check against (such as the cases of unconstrained formal parameters
   --  and unconstrained aliased objects). For the case of unconstrained
   --  formals, the check is peformed only if the corresponding actual is
   --  constrained, i.e., whether Lhs'Constrained is True.

   function Build_Discriminant_Checks
     (N     : Node_Id;
      T_Typ : Entity_Id)
      return  Node_Id;
   --  Subsidiary routine for Apply_Discriminant_Check. Builds the expression
   --  that compares discriminants of the expression with discriminants of the
   --  type. Also used directly for membership tests (see Exp_Ch4.Expand_N_In).

   procedure Apply_Divide_Check (N : Node_Id);
   --  The node kind is N_Op_Divide, N_Op_Mod, or N_Op_Rem. An appropriate
   --  check is generated to ensure that the right operand is non-zero. In
   --  the divide case, we also check that we do not have the annoying case
   --  of the largest negative number divided by minus one.

   procedure Apply_Type_Conversion_Checks (N : Node_Id);
   --  N is an N_Type_Conversion node. A type conversion actually involves
   --  two sorts of checks. The first check is the checks that ensures that
   --  the operand in the type conversion fits onto the base type of the
   --  subtype it is being converted to (see RM 4.6 (28)-(50)). The second
   --  check is there to ensure that once the operand has been converted to
   --  a value of the target type, this converted value meets the
   --  constraints imposed by the target subtype (see RM 4.6 (51)).

   procedure Apply_Universal_Integer_Attribute_Checks (N : Node_Id);
   --  The argument N is an attribute reference node intended for processing
   --  by gigi. The attribute is one that returns a universal integer, but
   --  the attribute reference node is currently typed with the expected
   --  result type. This routine deals with range and overflow checks needed
   --  to make sure that the universal result is in range.

   procedure Determine_Range
     (N  : Node_Id;
      OK : out Boolean;
      Lo : out Uint;
      Hi : out Uint);
   --  N is a node for a subexpression. If N is of a discrete type with no
   --  error indications, and no other peculiarities (e.g. missing type
   --  fields), then OK is True on return, and Lo and Hi are set to a
   --  conservative estimate of the possible range of values of N. Thus if OK
   --  is True on return, the value of the subexpression N is known to like in
   --  the range Lo .. Hi (inclusive). If the expression is not of a discrete
   --  type, or some kind of error condition is detected, then OK is False on
   --  exit, and Lo/Hi are set to No_Uint. Thus the significance of OK being
   --  False on return is that no useful information is available on the range
   --  of the expression.

   procedure Install_Null_Excluding_Check (N : Node_Id);
   --  Determines whether an access node requires a runtime access check and
   --  if so inserts the appropriate run-time check.

   -------------------------------------------------------
   -- Control and Optimization of Range/Overflow Checks --
   -------------------------------------------------------

   --  Range checks are controlled by the Do_Range_Check flag. The front end
   --  is responsible for setting this flag in relevant nodes. Originally
   --  the back end generated all corresponding range checks. But later on
   --  we decided to generate all range checks in the front end. We are now
   --  in the transitional phase where some of these checks are still done
   --  by the back end, but many are done by the front end.

   --  Overflow checks are similarly controlled by the Do_Overflow_Check flag.
   --  The difference here is that if back end overflow checks are inactive
   --  (Backend_Overflow_Checks_On_Target set False), then the actual overflow
   --  checks are generated by the front end, but if back end overflow checks
   --  are active (Backend_Overflow_Checks_On_Target set True), then the back
   --  end does generate the checks.

   --  The following two routines are used to set these flags, they allow
   --  for the possibility of eliminating checks. Checks can be eliminated
   --  if an identical check has already been performed.

   procedure Enable_Overflow_Check (N : Node_Id);
   --  First this routine determines if an overflow check is needed by doing
   --  an appropriate range check. If a check is not needed, then the call
   --  has no effect. If a check is needed then this routine sets the flag
   --  Set Do_Overflow_Check in node N to True, unless it can be determined
   --  that the check is not needed. The only condition under which this is
   --  the case is if there was an identical check earlier on.

   procedure Enable_Range_Check (N : Node_Id);
   --  Set Do_Range_Check flag in node N True, unless it can be determined
   --  that the check is not needed. The only condition under which this is
   --  the case is if there was an identical check earlier on. This routine
   --  is not responsible for doing range analysis to determine whether or
   --  not such a check is needed -- the caller is expected to do this. The
   --  one other case in which the request to set the flag is ignored is
   --  when Kill_Range_Check is set in an N_Unchecked_Conversion node.

   --  The following routines are used to keep track of processing sequences
   --  of statements (e.g. the THEN statements of an IF statement). A check
   --  that appears within such a sequence can eliminate an identical check
   --  within this sequence of statements. However, after the end of the
   --  sequence of statements, such a check is no longer of interest, since
   --  it may not have been executed.

   procedure Conditional_Statements_Begin;
   --  This call marks the start of processing of a sequence of statements.
   --  Every call to this procedure must be followed by a matching call to
   --  Conditional_Statements_End.

   procedure Conditional_Statements_End;
   --  This call removes from consideration all saved checks since the
   --  corresponding call to Conditional_Statements_Begin. These two
   --  procedures operate in a stack like manner.

   --  The mechanism for optimizing checks works by remembering checks
   --  that have already been made, but certain conditions, for example
   --  an assignment to a variable involved in a check, may mean that the
   --  remembered check is no longer valid, in the sense that if the same
   --  expression appears again, another check is required because the
   --  value may have changed.

   --  The following routines are used to note conditions which may render
   --  some or all of the stored and remembered checks to be invalidated.

   procedure Kill_Checks (V : Entity_Id);
   --  This procedure records an assignment or other condition that causes
   --  the value of the variable to be changed, invalidating any stored
   --  checks that reference the value. Note that all such checks must
   --  be discarded, even if they are not in the current statement range.

   procedure Kill_All_Checks;
   --  This procedure kills all remembered checks

   -----------------------------
   -- Length and Range Checks --
   -----------------------------

   --  In the following procedures, there are three arguments which have
   --  a common meaning as follows:

   --    Expr        The expression to be checked. If a check is required,
   --                the appropriate flag will be placed on this node. Whether
   --                this node is further examined depends on the setting of
   --                the parameter Source_Typ, as described below.

   --    ??? Apply_Length_Check and Apply_Range_Check do not have an Expr
   --        formal

   --    ??? Apply_Length_Check and Apply_Range_Check have a Ck_Node formal
   --        which is undocumented, is it the same as Expr?

   --    Target_Typ  The target type on which the check is to be based. For
   --                example, if we have a scalar range check, then the check
   --                is that we are in range of this type.

   --    Source_Typ  Normally Empty, but can be set to a type, in which case
   --                this type is used for the check, see below.

   --  The checks operate in one of two modes:

   --    If Source_Typ is Empty, then the node Expr is examined, at the very
   --    least to get the source subtype. In addition for some of the checks,
   --    the actual form of the node may be examined. For example, a node of
   --    type Integer whose actual form is an Integer conversion from a type
   --    with range 0 .. 3 can be determined to have a value in range 0 .. 3.

   --    If Source_Typ is given, then nothing can be assumed about the Expr,
   --    and indeed its contents are not examined. In this case the check is
   --    based on the assumption that Expr can be an arbitrary value of the
   --    given Source_Typ.

   --  Currently, the only case in which a Source_Typ is explicitly supplied
   --  is for the case of Out and In_Out parameters, where, for the conversion
   --  on return (the Out direction), the types must be reversed. This is
   --  handled by the caller.

   procedure Apply_Length_Check
     (Ck_Node    : Node_Id;
      Target_Typ : Entity_Id;
      Source_Typ : Entity_Id := Empty);
   --  This procedure builds a sequence of declarations to do a length check
   --  that checks if the lengths of the two arrays Target_Typ and source type
   --  are the same. The resulting actions are inserted at Node using a call
   --  to Insert_Actions.
   --
   --  For access types, the Directly_Designated_Type is retrieved and
   --  processing continues as enumerated above, with a guard against null
   --  values.
   --
   --  Note: calls to Apply_Length_Check currently never supply an explicit
   --  Source_Typ parameter, but Apply_Length_Check takes this parameter and
   --  processes it as described above for consistency with the other routines
   --  in this section.

   procedure Apply_Range_Check
     (Ck_Node    : Node_Id;
      Target_Typ : Entity_Id;
      Source_Typ : Entity_Id := Empty);
   --  For a Node of kind N_Range, constructs a range check action that tests
   --  first that the range is not null and then that the range is contained in
   --  the Target_Typ range.
   --
   --  For scalar types, constructs a range check action that first tests that
   --  the expression is contained in the Target_Typ range. The difference
   --  between this and Apply_Scalar_Range_Check is that the latter generates
   --  the actual checking code in gigi against the Etype of the expression.
   --
   --  For constrained array types, construct series of range check actions
   --  to check that each Expr range is properly contained in the range of
   --  Target_Typ.
   --
   --  For a type conversion to an unconstrained array type, constructs a range
   --  check action to check that the bounds of the source type are within the
   --  constraints imposed by the Target_Typ.
   --
   --  For access types, the Directly_Designated_Type is retrieved and
   --  processing continues as enumerated above, with a guard against null
   --  values.
   --
   --  The source type is used by type conversions to unconstrained array
   --  types to retrieve the corresponding bounds.

   procedure Apply_Static_Length_Check
     (Expr       : Node_Id;
      Target_Typ : Entity_Id;
      Source_Typ : Entity_Id := Empty);
   --  Tries to determine statically whether the two array types source type
   --  and Target_Typ have the same length. If it can be determined at compile
   --  time that they do not, then an N_Raise_Constraint_Error node replaces
   --  Expr, and a warning message is issued.

   procedure Apply_Scalar_Range_Check
     (Expr       : Node_Id;
      Target_Typ : Entity_Id;
      Source_Typ : Entity_Id := Empty;
      Fixed_Int  : Boolean   := False);
   --  For scalar types, determines whether an expression node should be
   --  flagged as needing a runtime range check. If the node requires such a
   --  check, the Do_Range_Check flag is turned on. The Fixed_Int flag if set
   --  causes any fixed-point values to be treated as though they were discrete
   --  values (i.e. the underlying integer value is used).

   type Check_Result is private;
   --  Type used to return result of Get_Range_Checks call, for later use in
   --  call to Insert_Range_Checks procedure.

   function Get_Range_Checks
     (Ck_Node    : Node_Id;
      Target_Typ : Entity_Id;
      Source_Typ : Entity_Id := Empty;
      Warn_Node  : Node_Id   := Empty) return Check_Result;
   --  Like Apply_Range_Check, except it does not modify anything. Instead
   --  it returns an encapsulated result of the check operations for later
   --  use in a call to Insert_Range_Checks. If Warn_Node is non-empty, its
   --  Sloc is used, in the static case, for the generated warning or error.
   --  Additionally, it is used rather than Expr (or Low/High_Bound of Expr)
   --  in constructing the check.

   procedure Append_Range_Checks
     (Checks       : Check_Result;
      Stmts        : List_Id;
      Suppress_Typ : Entity_Id;
      Static_Sloc  : Source_Ptr;
      Flag_Node    : Node_Id);
   --  Called to append range checks as returned by a call to Get_Range_Checks.
   --  Stmts is a list to which either the dynamic check is appended or the
   --  raise Constraint_Error statement is appended (for static checks).
   --  Static_Sloc is the Sloc at which the raise CE node points, Flag_Node is
   --  used as the node at which to set the Has_Dynamic_Check flag. Checks_On
   --  is a boolean value that says if range and index checking is on or not.

   procedure Insert_Range_Checks
     (Checks       : Check_Result;
      Node         : Node_Id;
      Suppress_Typ : Entity_Id;
      Static_Sloc  : Source_Ptr := No_Location;
      Flag_Node    : Node_Id    := Empty;
      Do_Before    : Boolean    := False);
   --  Called to insert range checks as returned by a call to Get_Range_Checks.
   --  Node is the node after which either the dynamic check is inserted or
   --  the raise Constraint_Error statement is inserted (for static checks).
   --  Suppress_Typ is the type to check to determine if checks are suppressed.
   --  Static_Sloc, if passed, is the Sloc at which the raise CE node points,
   --  otherwise Sloc (Node) is used. The Has_Dynamic_Check flag is normally
   --  set at Node. If Flag_Node is present, then this is used instead as the
   --  node at which to set the Has_Dynamic_Check flag. Normally the check is
   --  inserted after, if Do_Before is True, the check is inserted before
   --  Node.

   -----------------------
   -- Expander Routines --
   -----------------------

   --  Some of the earlier processing for checks results in temporarily setting
   --  the Do_Range_Check flag rather than actually generating checks. Now we
   --  are moving the generation of such checks into the front end for reasons
   --  of efficiency and simplicity (there were difficutlies in handling this
   --  in the back end when side effects were present in the expressions being
   --  checked).

   --  Probably we could eliminate the Do_Range_Check flag entirely and
   --  generate the checks earlier, but this is a delicate area and it
   --  seemed safer to implement the following routines, which are called
   --  late on in the expansion process. They check the Do_Range_Check flag
   --  and if it is set, generate the actual checks and reset the flag.

   procedure Generate_Range_Check
     (N           : Node_Id;
      Target_Type : Entity_Id;
      Reason      : RT_Exception_Code);
   --  This procedure is called to actually generate and insert a range check.
   --  A check is generated to ensure that the value of N lies within the range
   --  of the target type. Note that the base type of N may be different from
   --  the base type of the target type. This happens in the conversion case.
   --  The Reason parameter is the exception code to be used for the exception
   --  if raised.
   --
   --  Note on the relation of this routine to the Do_Range_Check flag. Mostly
   --  for historical reasons, we often set the Do_Range_Check flag and then
   --  later we call Generate_Range_Check if this flag is set. Most probably we
   --  could eliminate this intermediate setting of the flag (historically the
   --  back end dealt with range checks, using this flag to indicate if a check
   --  was required, then we moved checks into the front end).

   procedure Generate_Index_Checks (N : Node_Id);
   --  This procedure is called to generate index checks on the subscripts for
   --  the indexed component node N. Each subscript expression is examined, and
   --  if the Do_Range_Check flag is set, an appropriate index check is
   --  generated and the flag is reset.

   --  Similarly, we set the flag Do_Discriminant_Check in the semantic
   --  analysis to indicate that a discriminant check is required for selected
   --  component of a discriminated type. The following routine is called from
   --  the expander to actually generate the call.

   procedure Generate_Discriminant_Check (N : Node_Id);
   --  N is a selected component for which a discriminant check is required to
   --  make sure that the discriminants have appropriate values for the
   --  selection. This is done by calling the appropriate discriminant checking
   --  routine for the selector.

   -----------------------
   -- Validity Checking --
   -----------------------

   --  In (RM 13.9.1(9-11)) we have the following rules on invalid values

   --    If the representation of a scalar object does not represent value of
   --    the object's subtype (perhaps because the object was not initialized),
   --    the object is said to have an invalid representation. It is a bounded
   --    error to evaluate the value of such an object. If the error is
   --    detected, either Constraint_Error or Program_Error is raised.
   --    Otherwise, execution continues using the invalid representation. The
   --    rules of the language outside this subclause assume that all objects
   --    have valid representations. The semantics of operations on invalid
   --    representations are as follows:
   --
   --       10  If the representation of the object represents a value of the
   --           object's type, the value of the type is used.
   --
   --       11  If the representation of the object does not represent a value
   --           of the object's type, the semantics of operations on such
   --           representations is implementation-defined, but does not by
   --           itself lead to erroneous or unpredictable execution, or to
   --           other objects becoming abnormal.

   --  We quote the rules in full here since they are quite delicate. Most
   --  of the time, we can just compute away with wrong values, and get a
   --  possibly wrong result, which is well within the range of allowed
   --  implementation defined behavior. The two tricky cases are subscripted
   --  array assignments, where we don't want to do wild stores, and case
   --  statements where we don't want to do wild jumps.

   --  In GNAT, we control validity checking with a switch -gnatV that can take
   --  three parameters, n/d/f for None/Default/Full. These modes have the
   --  following meanings:

   --    None (no validity checking)

   --      In this mode, there is no specific checking for invalid values
   --      and the code generator assumes that all stored values are always
   --      within the bounds of the object subtype. The consequences are as
   --      follows:

   --        For case statements, an out of range invalid value will cause
   --        Constraint_Error to be raised, or an arbitrary one of the case
   --        alternatives will be executed. Wild jumps cannot result even
   --        in this mode, since we always do a range check

   --        For subscripted array assignments, wild stores will result in
   --        the expected manner when addresses are calculated using values
   --        of subscripts that are out of range.

   --      It could perhaps be argued that this mode is still conformant with
   --      the letter of the RM, since implementation defined is a rather
   --      broad category, but certainly it is not in the spirit of the
   --      RM requirement, since wild stores certainly seem to be a case of
   --      erroneous behavior.

   --    Default (default standard RM-compatible validity checking)

   --      In this mode, which is the default, minimal validity checking is
   --      performed to ensure no erroneous behavior as follows:

   --        For case statements, an out of range invalid value will cause
   --        Constraint_Error to be raised.

   --        For subscripted array assignments, invalid out of range
   --        subscript values will cause Constraint_Error to be raised.

   --    Full (Full validity checking)

   --      In this mode, the protections guaranteed by the standard mode are
   --      in place, and the following additional checks are made:

   --        For every assignment, the right side is checked for validity

   --        For every call, IN and IN OUT parameters are checked for validity

   --        For every subscripted array reference, both for stores and loads,
   --        all subscripts are checked for validity.

   --      These checks are not required by the RM, but will in practice
   --      improve the detection of uninitialized variables, particularly
   --      if used in conjunction with pragma Normalize_Scalars.

   --  In the above description, we talk about performing validity checks,
   --  but we don't actually generate a check in a case where the compiler
   --  can be sure that the value is valid. Note that this assurance must
   --  be achieved without assuming that any uninitialized value lies within
   --  the range of its type. The following are cases in which values are
   --  known to be valid. The flag Is_Known_Valid is used to keep track of
   --  some of these cases.

   --    If all possible stored values are valid, then any uninitialized
   --    value must be valid.

   --    Literals, including enumeration literals, are clearly always valid

   --    Constants are always assumed valid, with a validity check being
   --    performed on the initializing value where necessary to ensure that
   --    this is the case.

   --    For variables, the status is set to known valid if there is an
   --    initializing expression. Again a check is made on the initializing
   --    value if necessary to ensure that this assumption is valid. The
   --    status can change as a result of local assignments to a variable.
   --    If a known valid value is unconditionally assigned, then we mark
   --    the left side as known valid. If a value is assigned that is not
   --    known to be valid, then we mark the left side as invalid. This
   --    kind of processing does NOT apply to non-local variables since we
   --    are not following the flow graph (more properly the flow of actual
   --    processing only corresponds to the flow graph for local assignments).
   --    For non-local variables, we preserve the current setting, i.e. a
   --    validity check is performed when assigning to a knonwn valid global.

   --  Note: no validity checking is required if range checks are suppressed
   --  regardless of the setting of the validity checking mode.

   --  The following procedures are used in handling validity checking

   procedure Apply_Subscript_Validity_Checks (Expr : Node_Id);
   --  Expr is the node for an indexed component. If validity checking and
   --  range checking are enabled, all subscripts for this indexed component
   --  are checked for validity.

   procedure Check_Valid_Lvalue_Subscripts (Expr : Node_Id);
   --  Expr is a lvalue, i.e. an expression representing the target of an
   --  assignment. This procedure checks for this expression involving an
   --  assignment to an array value. We have to be sure that all the subscripts
   --  in such a case are valid, since according to the rules in (RM
   --  13.9.1(9-11)) such assignments are not permitted to result in erroneous
   --  behavior in the case of invalid subscript values.

   procedure Ensure_Valid (Expr : Node_Id; Holes_OK : Boolean := False);
   --  Ensure that Expr represents a valid value of its type. If this type
   --  is not a scalar type, then the call has no effect, since validity
   --  is only an issue for scalar types. The effect of this call is to
   --  check if the value is known valid, if so, nothing needs to be done.
   --  If this is not known, then either Expr is set to be range checked,
   --  or specific checking code is inserted so that an exception is raised
   --  if the value is not valid.
   --
   --  The optional argument Holes_OK indicates whether it is necessary to
   --  worry about enumeration types with non-standard representations leading
   --  to "holes" in the range of possible representations. If Holes_OK is
   --  True, then such values are assumed valid (this is used when the caller
   --  will make a separate check for this case anyway). If Holes_OK is False,
   --  then this case is checked, and code is inserted to ensure that Expr is
   --  valid, raising Constraint_Error if the value is not valid.

   function Expr_Known_Valid (Expr : Node_Id) return Boolean;
   --  This function tests it the value of Expr is known to be valid in the
   --  sense of RM 13.9.1(9-11). In the case of GNAT, it is only discrete types
   --  which are a concern, since for non-discrete types we simply continue
   --  computation with invalid values, which does not lead to erroneous
   --  behavior. Thus Expr_Known_Valid always returns True if the type of Expr
   --  is non-discrete. For discrete types the value returned is True only if
   --  it can be determined that the value is Valid. Otherwise False is
   --  returned.

   procedure Insert_Valid_Check (Expr : Node_Id);
   --  Inserts code that will check for the value of Expr being valid, in
   --  the sense of the 'Valid attribute returning True. Constraint_Error
   --  will be raised if the value is not valid.

   procedure Null_Exclusion_Static_Checks (N : Node_Id);
   --  Ada 2005 (AI-231): Check bad usages of the null-exclusion issue

   procedure Remove_Checks (Expr : Node_Id);
   --  Remove all checks from Expr except those that are only executed
   --  conditionally (on the right side of And Then/Or Else. This call
   --  removes only embedded checks (Do_Range_Check, Do_Overflow_Check).

   procedure Validity_Check_Range (N : Node_Id);
   --  If N is an N_Range node, then Ensure_Valid is called on its bounds,
   --  if validity checking of operands is enabled.

   -----------------------------
   -- Handling of Check Names --
   -----------------------------

   --  The following table contains Name_Id's for recognized checks. The first
   --  entries (corresponding to the values of the subtype Predefined_Check_Id)
   --  contain the Name_Id values for the checks that are predefined, including
   --  All_Checks (see Types). Remaining entries are those that are introduced
   --  by pragma Check_Names.

   package Check_Names is new Table.Table (
     Table_Component_Type => Name_Id,
     Table_Index_Type     => Check_Id,
     Table_Low_Bound      => 1,
     Table_Initial        => 30,
     Table_Increment      => 200,
     Table_Name           => "Name_Check_Names");

   function Get_Check_Id (N : Name_Id) return Check_Id;
   --  Function to search above table for matching name. If found returns the
   --  corresponding Check_Id value in the range 1 .. Check_Name.Last. If not
   --  found returns No_Check_Id.

private

   type Check_Result is array (Positive range 1 .. 2) of Node_Id;
   --  There are two cases for the result returned by Range_Check:
   --
   --    For the static case the result is one or two nodes that should cause
   --    a Constraint_Error. Typically these will include Expr itself or the
   --    direct descendents of Expr, such as Low/High_Bound (Expr)). It is the
   --    responsibility of the caller to rewrite and substitute the nodes with
   --    N_Raise_Constraint_Error nodes.
   --
   --    For the non-static case a single N_Raise_Constraint_Error node with a
   --    non-empty Condition field is returned.
   --
   --  Unused entries in Check_Result, if any, are simply set to Empty For
   --  external clients, the required processing on this result is achieved
   --  using the Insert_Range_Checks routine.

   pragma Inline (Apply_Length_Check);
   pragma Inline (Apply_Range_Check);
   pragma Inline (Apply_Static_Length_Check);
end Checks;