fix unused/uninitalized warnings

[tinycc.git] / tcc-doc.texi

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@c %**start of header
@setfilename tcc-doc.info
@settitle Tiny C Compiler Reference Documentation
@c %**end of header

@include config.texi

@iftex
@titlepage
@afourpaper
@sp 7
@center @titlefont{Tiny C Compiler Reference Documentation}
@sp 3
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@headings double
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@contents

@node Top, Introduction, (dir), (dir)
@top Tiny C Compiler Reference Documentation

This manual documents version @value{VERSION} of the Tiny C Compiler.

@menu
* Introduction::                Introduction to tcc.
* Invoke::                      Invocation of tcc (command line, options).
* Clang::                       ANSI C and extensions.
* asm::                         Assembler syntax.
* linker::                      Output file generation and supported targets.
* Bounds::                      Automatic bounds-checking of C code.
* Libtcc::                      The libtcc library.
* devel::                       Guide for Developers.
@end menu


@node Introduction
@chapter Introduction

TinyCC (aka TCC) is a small but hyper fast C compiler. Unlike other C
compilers, it is meant to be self-relying: you do not need an
external assembler or linker because TCC does that for you.

TCC compiles so @emph{fast} that even for big projects @code{Makefile}s may
not be necessary.

TCC not only supports ANSI C, but also most of the new ISO C99
standard and many GNUC extensions including inline assembly.

TCC can also be used to make @emph{C scripts}, i.e. pieces of C source
that you run as a Perl or Python script. Compilation is so fast that
your script will be as fast as if it was an executable.

TCC can also automatically generate memory and bound checks
(@pxref{Bounds}) while allowing all C pointers operations. TCC can do
these checks even if non patched libraries are used.

With @code{libtcc}, you can use TCC as a backend for dynamic code
generation (@pxref{Libtcc}).

TCC mainly supports the i386 target on Linux and Windows. There are alpha
ports for the ARM (@code{arm-tcc}) and the TMS320C67xx targets
(@code{c67-tcc}). More information about the ARM port is available at
@url{http://lists.gnu.org/archive/html/tinycc-devel/2003-10/msg00044.html}.

For usage on Windows, see also tcc-win32.txt.

@node Invoke
@chapter Command line invocation

@section Quick start

@example
@c man begin SYNOPSIS
usage: tcc [options] [@var{infile1} @var{infile2}@dots{}] [@option{-run} @var{infile} @var{args}@dots{}]
@c man end
@end example

@noindent
@c man begin DESCRIPTION
TCC options are a very much like gcc options. The main difference is that TCC
can also execute directly the resulting program and give it runtime
arguments.

Here are some examples to understand the logic:

@table @code
@item @samp{tcc -run a.c}
Compile @file{a.c} and execute it directly

@item @samp{tcc -run a.c arg1}
Compile a.c and execute it directly. arg1 is given as first argument to
the @code{main()} of a.c.

@item @samp{tcc a.c -run b.c arg1}
Compile @file{a.c} and @file{b.c}, link them together and execute them. arg1 is given
as first argument to the @code{main()} of the resulting program. 
@ignore 
Because multiple C files are specified, @option{--} are necessary to clearly 
separate the program arguments from the TCC options.
@end ignore

@item @samp{tcc -o myprog a.c b.c}
Compile @file{a.c} and @file{b.c}, link them and generate the executable @file{myprog}.

@item @samp{tcc -o myprog a.o b.o}
link @file{a.o} and @file{b.o} together and generate the executable @file{myprog}.

@item @samp{tcc -c a.c}
Compile @file{a.c} and generate object file @file{a.o}.

@item @samp{tcc -c asmfile.S}
Preprocess with C preprocess and assemble @file{asmfile.S} and generate
object file @file{asmfile.o}.

@item @samp{tcc -c asmfile.s}
Assemble (but not preprocess) @file{asmfile.s} and generate object file
@file{asmfile.o}.

@item @samp{tcc -r -o ab.o a.c b.c}
Compile @file{a.c} and @file{b.c}, link them together and generate the object file @file{ab.o}.

@end table

Scripting:

TCC can be invoked from @emph{scripts}, just as shell scripts. You just
need to add @code{#!/usr/local/bin/tcc -run} at the start of your C source:

@example
#!/usr/local/bin/tcc -run
#include <stdio.h>

int main() 
@{
    printf("Hello World\n");
    return 0;
@}
@end example

TCC can read C source code from @emph{standard input} when @option{-} is used in 
place of @option{infile}. Example:

@example
echo 'main()@{puts("hello");@}' | tcc -run -
@end example
@c man end

@section Option summary

General Options:

@c man begin OPTIONS
@table @option
@item -v
Display current TCC version, increase verbosity.

@item -c
Generate an object file (@option{-o} option must also be given).

@item -o outfile
Put object file, executable, or dll into output file @file{outfile}.

@item -Bdir
Set the path where the tcc internal libraries can be found (default is
@file{PREFIX/lib/tcc}).

@item -bench
Output compilation statistics.

@item -run source [args...]
Compile file @var{source} and run it with the command line arguments
@var{args}. In order to be able to give more than one argument to a
script, several TCC options can be given @emph{after} the
@option{-run} option, separated by spaces. Example:

@example
tcc "-run -L/usr/X11R6/lib -lX11" ex4.c
@end example

In a script, it gives the following header:

@example
#!/usr/local/bin/tcc -run -L/usr/X11R6/lib -lX11
#include <stdlib.h>
int main(int argc, char **argv)
@{
    ...
@}
@end example

@end table

Preprocessor options:

@table @option
@item -Idir
Specify an additional include path. Include paths are searched in the
order they are specified.

System include paths are always searched after. The default system
include paths are: @file{/usr/local/include}, @file{/usr/include}
and @file{PREFIX/lib/tcc/include}. (@file{PREFIX} is usually
@file{/usr} or @file{/usr/local}).

@item -Dsym[=val]
Define preprocessor symbol @samp{sym} to
val. If val is not present, its value is @samp{1}. Function-like macros can
also be defined: @option{-DF(a)=a+1}

@item -Usym
Undefine preprocessor symbol @samp{sym}.
@end table

Compilation flags:

Note: each of the following warning options has a negative form beginning with
@option{-fno-}.

@table @option
@item -funsigned-char
Let the @code{char} type be unsigned.

@item -fsigned-char
Let the @code{char} type be signed.

@item -fno-common
Do not generate common symbols for uninitialized data.

@item -fleading-underscore
Add a leading underscore at the beginning of each C symbol.

@end table

Warning options:

@table @option
@item -w
Disable all warnings.

@end table

Note: each of the following warning options has a negative form beginning with
@option{-Wno-}.

@table @option
@item -Wimplicit-function-declaration
Warn about implicit function declaration.

@item -Wunsupported
Warn about unsupported GCC features that are ignored by TCC.

@item -Wwrite-strings
Make string constants be of type @code{const char *} instead of @code{char
*}.

@item -Werror
Abort compilation if warnings are issued.

@item -Wall 
Activate all warnings, except @option{-Werror}, @option{-Wunusupported} and
@option{-Wwrite-strings}.

@end table

Linker options:

@table @option
@item -Ldir
Specify an additional static library path for the @option{-l} option. The
default library paths are @file{/usr/local/lib}, @file{/usr/lib} and @file{/lib}.

@item -lxxx
Link your program with dynamic library libxxx.so or static library
libxxx.a. The library is searched in the paths specified by the
@option{-L} option.

@item -shared
Generate a shared library instead of an executable (@option{-o} option
must also be given).

@item -static
Generate a statically linked executable (default is a shared linked
executable) (@option{-o} option must also be given).

@item -rdynamic
Export global symbols to the dynamic linker. It is useful when a library
opened with @code{dlopen()} needs to access executable symbols.

@item -r
Generate an object file combining all input files (@option{-o} option must
also be given).

@item -Wl,-Ttext,address
Set the start of the .text section to @var{address}.

@item -Wl,--oformat,fmt
Use @var{fmt} as output format. The supported output formats are:
@table @code
@item elf32-i386
ELF output format (default)
@item binary
Binary image (only for executable output)
@item coff
COFF output format (only for executable output for TMS320C67xx target)
@end table

@end table

Debugger options:

@table @option
@item -g
Generate run time debug information so that you get clear run time
error messages: @code{ test.c:68: in function 'test5()': dereferencing
invalid pointer} instead of the laconic @code{Segmentation
fault}.

@item -b
Generate additional support code to check
memory allocations and array/pointer bounds. @option{-g} is implied. Note
that the generated code is slower and bigger in this case.

@item -bt N
Display N callers in stack traces. This is useful with @option{-g} or
@option{-b}.

@end table

Note: GCC options @option{-Ox}, @option{-fx} and @option{-mx} are
ignored.
@c man end

@ignore

@setfilename tcc
@settitle Tiny C Compiler

@c man begin SEEALSO
gcc(1)
@c man end

@c man begin AUTHOR
Fabrice Bellard
@c man end

@end ignore

@node Clang
@chapter C language support

@section ANSI C

TCC implements all the ANSI C standard, including structure bit fields
and floating point numbers (@code{long double}, @code{double}, and
@code{float} fully supported).

@section ISOC99 extensions

TCC implements many features of the new C standard: ISO C99. Currently
missing items are: complex and imaginary numbers and variable length
arrays.

Currently implemented ISOC99 features:

@itemize

@item 64 bit @code{long long} types are fully supported.

@item The boolean type @code{_Bool} is supported.

@item @code{__func__} is a string variable containing the current
function name.

@item Variadic macros: @code{__VA_ARGS__} can be used for
   function-like macros:
@example
    #define dprintf(level, __VA_ARGS__) printf(__VA_ARGS__)
@end example

@noindent
@code{dprintf} can then be used with a variable number of parameters.

@item Declarations can appear anywhere in a block (as in C++).

@item Array and struct/union elements can be initialized in any order by
  using designators:
@example
    struct @{ int x, y; @} st[10] = @{ [0].x = 1, [0].y = 2 @};

    int tab[10] = @{ 1, 2, [5] = 5, [9] = 9@};
@end example
    
@item Compound initializers are supported:
@example
    int *p = (int [])@{ 1, 2, 3 @};
@end example
to initialize a pointer pointing to an initialized array. The same
works for structures and strings.

@item Hexadecimal floating point constants are supported:
@example
          double d = 0x1234p10;
@end example

@noindent
is the same as writing 
@example
          double d = 4771840.0;
@end example

@item @code{inline} keyword is ignored.

@item @code{restrict} keyword is ignored.
@end itemize

@section GNU C extensions

TCC implements some GNU C extensions:

@itemize

@item array designators can be used without '=': 
@example
    int a[10] = @{ [0] 1, [5] 2, 3, 4 @};
@end example

@item Structure field designators can be a label: 
@example
    struct @{ int x, y; @} st = @{ x: 1, y: 1@};
@end example
instead of
@example
    struct @{ int x, y; @} st = @{ .x = 1, .y = 1@};
@end example

@item @code{\e} is ASCII character 27.

@item case ranges : ranges can be used in @code{case}s:
@example
    switch(a) @{
    case 1 @dots{} 9:
          printf("range 1 to 9\n");
          break;
    default:
          printf("unexpected\n");
          break;
    @}
@end example

@cindex aligned attribute
@cindex packed attribute
@cindex section attribute
@cindex unused attribute
@cindex cdecl attribute
@cindex stdcall attribute
@cindex regparm attribute
@cindex dllexport attribute

@item The keyword @code{__attribute__} is handled to specify variable or
function attributes. The following attributes are supported:
  @itemize

  @item @code{aligned(n)}: align a variable or a structure field to n bytes
(must be a power of two).

  @item @code{packed}: force alignment of a variable or a structure field to
  1.

  @item @code{section(name)}: generate function or data in assembly section
name (name is a string containing the section name) instead of the default
section.

  @item @code{unused}: specify that the variable or the function is unused.

  @item @code{cdecl}: use standard C calling convention (default).

  @item @code{stdcall}: use Pascal-like calling convention.

  @item @code{regparm(n)}: use fast i386 calling convention. @var{n} must be
between 1 and 3. The first @var{n} function parameters are respectively put in
registers @code{%eax}, @code{%edx} and @code{%ecx}.

  @item @code{dllexport}: export function from dll/executable (win32 only)

  @end itemize

Here are some examples:
@example
    int a __attribute__ ((aligned(8), section(".mysection")));
@end example

@noindent
align variable @code{a} to 8 bytes and put it in section @code{.mysection}.

@example
    int my_add(int a, int b) __attribute__ ((section(".mycodesection"))) 
    @{
        return a + b;
    @}
@end example

@noindent
generate function @code{my_add} in section @code{.mycodesection}.

@item GNU style variadic macros:
@example
    #define dprintf(fmt, args@dots{}) printf(fmt, ## args)

    dprintf("no arg\n");
    dprintf("one arg %d\n", 1);
@end example

@item @code{__FUNCTION__} is interpreted as C99 @code{__func__} 
(so it has not exactly the same semantics as string literal GNUC
where it is a string literal).

@item The @code{__alignof__} keyword can be used as @code{sizeof} 
to get the alignment of a type or an expression.

@item The @code{typeof(x)} returns the type of @code{x}. 
@code{x} is an expression or a type.

@item Computed gotos: @code{&&label} returns a pointer of type 
@code{void *} on the goto label @code{label}. @code{goto *expr} can be
used to jump on the pointer resulting from @code{expr}.

@item Inline assembly with asm instruction:
@cindex inline assembly
@cindex assembly, inline
@cindex __asm__
@example
static inline void * my_memcpy(void * to, const void * from, size_t n)
@{
int d0, d1, d2;
__asm__ __volatile__(
        "rep ; movsl\n\t"
        "testb $2,%b4\n\t"
        "je 1f\n\t"
        "movsw\n"
        "1:\ttestb $1,%b4\n\t"
        "je 2f\n\t"
        "movsb\n"
        "2:"
        : "=&c" (d0), "=&D" (d1), "=&S" (d2)
        :"0" (n/4), "q" (n),"1" ((long) to),"2" ((long) from)
        : "memory");
return (to);
@}
@end example

@noindent
@cindex gas
TCC includes its own x86 inline assembler with a @code{gas}-like (GNU
assembler) syntax. No intermediate files are generated. GCC 3.x named
operands are supported.

@item @code{__builtin_types_compatible_p()} and @code{__builtin_constant_p()} 
are supported.

@item @code{#pragma pack} is supported for win32 compatibility.

@end itemize

@section TinyCC extensions

@itemize

@item @code{__TINYC__} is a predefined macro to @code{1} to
indicate that you use TCC.

@item @code{#!} at the start of a line is ignored to allow scripting.

@item Binary digits can be entered (@code{0b101} instead of
@code{5}).

@item @code{__BOUNDS_CHECKING_ON} is defined if bound checking is activated.

@end itemize

@node asm
@chapter TinyCC Assembler

Since version 0.9.16, TinyCC integrates its own assembler. TinyCC
assembler supports a gas-like syntax (GNU assembler). You can
desactivate assembler support if you want a smaller TinyCC executable
(the C compiler does not rely on the assembler).

TinyCC Assembler is used to handle files with @file{.S} (C
preprocessed assembler) and @file{.s} extensions. It is also used to
handle the GNU inline assembler with the @code{asm} keyword.

@section Syntax

TinyCC Assembler supports most of the gas syntax. The tokens are the
same as C.

@itemize

@item C and C++ comments are supported.

@item Identifiers are the same as C, so you cannot use '.' or '$'.

@item Only 32 bit integer numbers are supported.

@end itemize

@section Expressions

@itemize

@item Integers in decimal, octal and hexa are supported.

@item Unary operators: +, -, ~.

@item Binary operators in decreasing priority order:

@enumerate
@item *, /, %
@item &, |, ^
@item +, -
@end enumerate

@item A value is either an absolute number or a label plus an offset. 
All operators accept absolute values except '+' and '-'. '+' or '-' can be
used to add an offset to a label. '-' supports two labels only if they
are the same or if they are both defined and in the same section.

@end itemize

@section Labels

@itemize

@item All labels are considered as local, except undefined ones.

@item Numeric labels can be used as local @code{gas}-like labels. 
They can be defined several times in the same source. Use 'b'
(backward) or 'f' (forward) as suffix to reference them:

@example
 1:
      jmp 1b /* jump to '1' label before */
      jmp 1f /* jump to '1' label after */
 1:
@end example

@end itemize

@section Directives
@cindex assembler directives
@cindex directives, assembler
@cindex align directive
@cindex skip directive
@cindex space directive
@cindex byte directive
@cindex word directive
@cindex short directive
@cindex int directive
@cindex long directive
@cindex quad directive
@cindex globl directive
@cindex global directive
@cindex section directive
@cindex text directive
@cindex data directive
@cindex bss directive
@cindex fill directive
@cindex org directive
@cindex previous directive
@cindex string directive
@cindex asciz directive
@cindex ascii directive

All directives are preceeded by a '.'. The following directives are
supported:

@itemize
@item .align n[,value]
@item .skip n[,value]
@item .space n[,value]
@item .byte value1[,...]
@item .word value1[,...]
@item .short value1[,...]
@item .int value1[,...]
@item .long value1[,...]
@item .quad immediate_value1[,...]
@item .globl symbol
@item .global symbol
@item .section section
@item .text
@item .data
@item .bss
@item .fill repeat[,size[,value]]
@item .org n
@item .previous
@item .string string[,...]
@item .asciz string[,...]
@item .ascii string[,...]
@end itemize

@section X86 Assembler
@cindex assembler

All X86 opcodes are supported. Only ATT syntax is supported (source
then destination operand order). If no size suffix is given, TinyCC
tries to guess it from the operand sizes.

Currently, MMX opcodes are supported but not SSE ones.

@node linker
@chapter TinyCC Linker
@cindex linker

@section ELF file generation
@cindex ELF

TCC can directly output relocatable ELF files (object files),
executable ELF files and dynamic ELF libraries without relying on an
external linker.

Dynamic ELF libraries can be output but the C compiler does not generate
position independent code (PIC). It means that the dynamic library
code generated by TCC cannot be factorized among processes yet.

TCC linker eliminates unreferenced object code in libraries. A single pass is
done on the object and library list, so the order in which object files and
libraries are specified is important (same constraint as GNU ld). No grouping
options (@option{--start-group} and @option{--end-group}) are supported.

@section ELF file loader

TCC can load ELF object files, archives (.a files) and dynamic
libraries (.so).

@section PE-i386 file generation
@cindex PE-i386

TCC for Windows supports the native Win32 executable file format (PE-i386).  It
generates EXE files (console and gui) and DLL files.

For usage on Windows, see also tcc-win32.txt.

@section GNU Linker Scripts
@cindex scripts, linker
@cindex linker scripts
@cindex GROUP, linker command
@cindex FILE, linker command
@cindex OUTPUT_FORMAT, linker command
@cindex TARGET, linker command

Because on many Linux systems some dynamic libraries (such as
@file{/usr/lib/libc.so}) are in fact GNU ld link scripts (horrible!),
the TCC linker also supports a subset of GNU ld scripts.

The @code{GROUP} and @code{FILE} commands are supported. @code{OUTPUT_FORMAT}
and @code{TARGET} are ignored.

Example from @file{/usr/lib/libc.so}:
@example
/* GNU ld script
   Use the shared library, but some functions are only in
   the static library, so try that secondarily.  */
GROUP ( /lib/libc.so.6 /usr/lib/libc_nonshared.a )
@end example

@node Bounds
@chapter TinyCC Memory and Bound checks
@cindex bound checks
@cindex memory checks

This feature is activated with the @option{-b} (@pxref{Invoke}).

Note that pointer size is @emph{unchanged} and that code generated
with bound checks is @emph{fully compatible} with unchecked
code. When a pointer comes from unchecked code, it is assumed to be
valid. Even very obscure C code with casts should work correctly.

For more information about the ideas behind this method, see
@url{http://www.doc.ic.ac.uk/~phjk/BoundsChecking.html}.

Here are some examples of caught errors:

@table @asis

@item Invalid range with standard string function:
@example
@{
    char tab[10];
    memset(tab, 0, 11);
@}
@end example

@item Out of bounds-error in global or local arrays:
@example
@{
    int tab[10];
    for(i=0;i<11;i++) @{
        sum += tab[i];
    @}
@}
@end example

@item Out of bounds-error in malloc'ed data:
@example
@{
    int *tab;
    tab = malloc(20 * sizeof(int));
    for(i=0;i<21;i++) @{
        sum += tab4[i];
    @}
    free(tab);
@}
@end example

@item Access of freed memory:
@example
@{
    int *tab;
    tab = malloc(20 * sizeof(int));
    free(tab);
    for(i=0;i<20;i++) @{
        sum += tab4[i];
    @}
@}
@end example

@item Double free:
@example
@{
    int *tab;
    tab = malloc(20 * sizeof(int));
    free(tab);
    free(tab);
@}
@end example

@end table

@node Libtcc
@chapter The @code{libtcc} library

The @code{libtcc} library enables you to use TCC as a backend for
dynamic code generation. 

Read the @file{libtcc.h} to have an overview of the API. Read
@file{libtcc_test.c} to have a very simple example.

The idea consists in giving a C string containing the program you want
to compile directly to @code{libtcc}. Then you can access to any global
symbol (function or variable) defined.

@node devel
@chapter Developer's guide

This chapter gives some hints to understand how TCC works. You can skip
it if you do not intend to modify the TCC code.

@section File reading

The @code{BufferedFile} structure contains the context needed to read a
file, including the current line number. @code{tcc_open()} opens a new
file and @code{tcc_close()} closes it. @code{inp()} returns the next
character.

@section Lexer

@code{next()} reads the next token in the current
file. @code{next_nomacro()} reads the next token without macro
expansion.

@code{tok} contains the current token (see @code{TOK_xxx})
constants. Identifiers and keywords are also keywords. @code{tokc}
contains additional infos about the token (for example a constant value
if number or string token).

@section Parser

The parser is hardcoded (yacc is not necessary). It does only one pass,
except:

@itemize

@item For initialized arrays with unknown size, a first pass 
is done to count the number of elements.

@item For architectures where arguments are evaluated in 
reverse order, a first pass is done to reverse the argument order.

@end itemize

@section Types

The types are stored in a single 'int' variable. It was choosen in the
first stages of development when tcc was much simpler. Now, it may not
be the best solution.

@example
#define VT_INT        0  /* integer type */
#define VT_BYTE       1  /* signed byte type */
#define VT_SHORT      2  /* short type */
#define VT_VOID       3  /* void type */
#define VT_PTR        4  /* pointer */
#define VT_ENUM       5  /* enum definition */
#define VT_FUNC       6  /* function type */
#define VT_STRUCT     7  /* struct/union definition */
#define VT_FLOAT      8  /* IEEE float */
#define VT_DOUBLE     9  /* IEEE double */
#define VT_LDOUBLE   10  /* IEEE long double */
#define VT_BOOL      11  /* ISOC99 boolean type */
#define VT_LLONG     12  /* 64 bit integer */
#define VT_LONG      13  /* long integer (NEVER USED as type, only
                            during parsing) */
#define VT_BTYPE      0x000f /* mask for basic type */
#define VT_UNSIGNED   0x0010  /* unsigned type */
#define VT_ARRAY      0x0020  /* array type (also has VT_PTR) */
#define VT_BITFIELD   0x0040  /* bitfield modifier */

#define VT_STRUCT_SHIFT 16   /* structure/enum name shift (16 bits left) */
@end example

When a reference to another type is needed (for pointers, functions and
structures), the @code{32 - VT_STRUCT_SHIFT} high order bits are used to
store an identifier reference.

The @code{VT_UNSIGNED} flag can be set for chars, shorts, ints and long
longs.

Arrays are considered as pointers @code{VT_PTR} with the flag
@code{VT_ARRAY} set.

The @code{VT_BITFIELD} flag can be set for chars, shorts, ints and long
longs. If it is set, then the bitfield position is stored from bits
VT_STRUCT_SHIFT to VT_STRUCT_SHIFT + 5 and the bit field size is stored
from bits VT_STRUCT_SHIFT + 6 to VT_STRUCT_SHIFT + 11.

@code{VT_LONG} is never used except during parsing.

During parsing, the storage of an object is also stored in the type
integer:

@example
#define VT_EXTERN  0x00000080  /* extern definition */
#define VT_STATIC  0x00000100  /* static variable */
#define VT_TYPEDEF 0x00000200  /* typedef definition */
@end example

@section Symbols

All symbols are stored in hashed symbol stacks. Each symbol stack
contains @code{Sym} structures.

@code{Sym.v} contains the symbol name (remember
an idenfier is also a token, so a string is never necessary to store
it). @code{Sym.t} gives the type of the symbol. @code{Sym.r} is usually
the register in which the corresponding variable is stored. @code{Sym.c} is
usually a constant associated to the symbol.

Four main symbol stacks are defined:

@table @code

@item define_stack
for the macros (@code{#define}s).

@item global_stack
for the global variables, functions and types.

@item local_stack
for the local variables, functions and types.

@item global_label_stack
for the local labels (for @code{goto}).

@item label_stack
for GCC block local labels (see the @code{__label__} keyword).

@end table

@code{sym_push()} is used to add a new symbol in the local symbol
stack. If no local symbol stack is active, it is added in the global
symbol stack.

@code{sym_pop(st,b)} pops symbols from the symbol stack @var{st} until
the symbol @var{b} is on the top of stack. If @var{b} is NULL, the stack
is emptied.

@code{sym_find(v)} return the symbol associated to the identifier
@var{v}. The local stack is searched first from top to bottom, then the
global stack.

@section Sections

The generated code and datas are written in sections. The structure
@code{Section} contains all the necessary information for a given
section. @code{new_section()} creates a new section. ELF file semantics
is assumed for each section.

The following sections are predefined:

@table @code

@item text_section
is the section containing the generated code. @var{ind} contains the
current position in the code section.

@item data_section
contains initialized data

@item bss_section
contains uninitialized data

@item bounds_section
@itemx lbounds_section
are used when bound checking is activated

@item stab_section
@itemx stabstr_section
are used when debugging is actived to store debug information

@item symtab_section
@itemx strtab_section
contain the exported symbols (currently only used for debugging).

@end table

@section Code generation
@cindex code generation

@subsection Introduction

The TCC code generator directly generates linked binary code in one
pass. It is rather unusual these days (see gcc for example which
generates text assembly), but it can be very fast and surprisingly
little complicated.

The TCC code generator is register based. Optimization is only done at
the expression level. No intermediate representation of expression is
kept except the current values stored in the @emph{value stack}.

On x86, three temporary registers are used. When more registers are
needed, one register is spilled into a new temporary variable on the stack.

@subsection The value stack
@cindex value stack, introduction

When an expression is parsed, its value is pushed on the value stack
(@var{vstack}). The top of the value stack is @var{vtop}. Each value
stack entry is the structure @code{SValue}.

@code{SValue.t} is the type. @code{SValue.r} indicates how the value is
currently stored in the generated code. It is usually a CPU register
index (@code{REG_xxx} constants), but additional values and flags are
defined:

@example
#define VT_CONST     0x00f0
#define VT_LLOCAL    0x00f1
#define VT_LOCAL     0x00f2
#define VT_CMP       0x00f3
#define VT_JMP       0x00f4
#define VT_JMPI      0x00f5
#define VT_LVAL      0x0100
#define VT_SYM       0x0200
#define VT_MUSTCAST  0x0400
#define VT_MUSTBOUND 0x0800
#define VT_BOUNDED   0x8000
#define VT_LVAL_BYTE     0x1000
#define VT_LVAL_SHORT    0x2000
#define VT_LVAL_UNSIGNED 0x4000
#define VT_LVAL_TYPE     (VT_LVAL_BYTE | VT_LVAL_SHORT | VT_LVAL_UNSIGNED)
@end example

@table @code

@item VT_CONST
indicates that the value is a constant. It is stored in the union
@code{SValue.c}, depending on its type.

@item VT_LOCAL
indicates a local variable pointer at offset @code{SValue.c.i} in the
stack.

@item VT_CMP
indicates that the value is actually stored in the CPU flags (i.e. the
value is the consequence of a test). The value is either 0 or 1. The
actual CPU flags used is indicated in @code{SValue.c.i}. 

If any code is generated which destroys the CPU flags, this value MUST be
put in a normal register.

@item VT_JMP
@itemx VT_JMPI
indicates that the value is the consequence of a conditional jump. For VT_JMP,
it is 1 if the jump is taken, 0 otherwise. For VT_JMPI it is inverted.

These values are used to compile the @code{||} and @code{&&} logical
operators.

If any code is generated, this value MUST be put in a normal
register. Otherwise, the generated code won't be executed if the jump is
taken.

@item VT_LVAL
is a flag indicating that the value is actually an lvalue (left value of
an assignment). It means that the value stored is actually a pointer to
the wanted value. 

Understanding the use @code{VT_LVAL} is very important if you want to
understand how TCC works.

@item VT_LVAL_BYTE
@itemx VT_LVAL_SHORT
@itemx VT_LVAL_UNSIGNED
if the lvalue has an integer type, then these flags give its real
type. The type alone is not enough in case of cast optimisations.

@item VT_LLOCAL
is a saved lvalue on the stack. @code{VT_LLOCAL} should be eliminated
ASAP because its semantics are rather complicated.

@item VT_MUSTCAST
indicates that a cast to the value type must be performed if the value
is used (lazy casting).

@item VT_SYM
indicates that the symbol @code{SValue.sym} must be added to the constant.

@item VT_MUSTBOUND
@itemx VT_BOUNDED
are only used for optional bound checking.

@end table

@subsection Manipulating the value stack
@cindex value stack

@code{vsetc()} and @code{vset()} pushes a new value on the value
stack. If the previous @var{vtop} was stored in a very unsafe place(for
example in the CPU flags), then some code is generated to put the
previous @var{vtop} in a safe storage.

@code{vpop()} pops @var{vtop}. In some cases, it also generates cleanup
code (for example if stacked floating point registers are used as on
x86).

The @code{gv(rc)} function generates code to evaluate @var{vtop} (the
top value of the stack) into registers. @var{rc} selects in which
register class the value should be put. @code{gv()} is the @emph{most
important function} of the code generator.

@code{gv2()} is the same as @code{gv()} but for the top two stack
entries.

@subsection CPU dependent code generation
@cindex CPU dependent
See the @file{i386-gen.c} file to have an example.

@table @code

@item load()
must generate the code needed to load a stack value into a register.

@item store()
must generate the code needed to store a register into a stack value
lvalue.

@item gfunc_start()
@itemx gfunc_param()
@itemx gfunc_call()
should generate a function call

@item gfunc_prolog()
@itemx gfunc_epilog()
should generate a function prolog/epilog.

@item gen_opi(op)
must generate the binary integer operation @var{op} on the two top
entries of the stack which are guaranted to contain integer types.

The result value should be put on the stack.

@item gen_opf(op)
same as @code{gen_opi()} for floating point operations. The two top
entries of the stack are guaranted to contain floating point values of
same types.

@item gen_cvt_itof()
integer to floating point conversion.

@item gen_cvt_ftoi()
floating point to integer conversion.

@item gen_cvt_ftof()
floating point to floating point of different size conversion.

@item gen_bounded_ptr_add()
@item gen_bounded_ptr_deref()
are only used for bounds checking.

@end table

@section Optimizations done
@cindex optimizations
@cindex constant propagation
@cindex strength reduction
@cindex comparison operators
@cindex caching processor flags
@cindex flags, caching
@cindex jump optimization
Constant propagation is done for all operations. Multiplications and
divisions are optimized to shifts when appropriate. Comparison
operators are optimized by maintaining a special cache for the
processor flags. &&, || and ! are optimized by maintaining a special
'jump target' value. No other jump optimization is currently performed
because it would require to store the code in a more abstract fashion.

@unnumbered Concept Index
@printindex cp

@bye

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