darwin.h

gcc-you can use this code to learn something about gcc, and inquire further into linux,

/* Target definitions for x86 running Darwin.
   Copyright (C) 2001, 2002 Free Software Foundation, Inc.
   Contributed by Apple Computer Inc.

This file is part of GNU CC.

GNU CC is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2, or (at your option)
any later version.

GNU CC is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with GNU CC; see the file COPYING.  If not, write to
the Free Software Foundation, 59 Temple Place - Suite 330,
Boston, MA 02111-1307, USA.  */

/* Enable Mach-O bits in generic x86 code.  */
#undef TARGET_MACHO
#define TARGET_MACHO 1

#define TARGET_VERSION fprintf (stderr, " (i386 Darwin)");

#define TARGET_OS_CPP_BUILTINS()                \
  do                                            \
    {                                           \
      builtin_define ("__i386__");              \
      builtin_define ("__LITTLE_ENDIAN__");     \
      builtin_define ("__MACH__");              \
      builtin_define ("__APPLE__");             \
    }                                           \
  while (0)

/* We want -fPIC by default, unless we're using -static to compile for
   the kernel or some such.  */

#undef CC1_SPEC
#define CC1_SPEC "%{!static:-fPIC}"

/* The Darwin assembler mostly follows AT&T syntax.  */
#undef ASSEMBLER_DIALECT
#define ASSEMBLER_DIALECT ASM_ATT

/* Define macro used to output shift-double opcodes when the shift
   count is in %cl.  Some assemblers require %cl as an argument;
   some don't.  This macro controls what to do: by default, don't
   print %cl.  */

#define SHIFT_DOUBLE_OMITS_COUNT 0

/* Define the syntax of pseudo-ops, labels and comments.  */

/* String containing the assembler's comment-starter.  */

#define ASM_COMMENT_START "#"

/* By default, target has a 80387, uses IEEE compatible arithmetic,
   and returns float values in the 387.  */

#define TARGET_SUBTARGET_DEFAULT (MASK_80387 | MASK_IEEE_FP | MASK_FLOAT_RETURNS)

/* TARGET_DEEP_BRANCH_PREDICTION is incompatible with Mach-O PIC.  */

#undef TARGET_DEEP_BRANCH_PREDICTION
#define TARGET_DEEP_BRANCH_PREDICTION   0

/* Define the syntax of pseudo-ops, labels and comments.  */

#define LPREFIX "L"

/* Assembler pseudos to introduce constants of various size.  */

#define ASM_BYTE_OP "\t.byte\t"
#define ASM_SHORT "\t.word\t"
#define ASM_LONG "\t.long\t"
/* Darwin as doesn't do ".quad".  */

#undef ASM_OUTPUT_ALIGN
#define ASM_OUTPUT_ALIGN(FILE,LOG)	\
 do { if ((LOG) != 0)			\
        {				\
          if (in_text_section ())	\
            fprintf (FILE, "\t%s %d,0x90\n", ALIGN_ASM_OP, (LOG)); \
          else				\
            fprintf (FILE, "\t%s %d\n", ALIGN_ASM_OP, (LOG)); \
        }				\
    } while (0)

/* This says how to output an assembler line
   to define a global common symbol.  */

#define ASM_OUTPUT_COMMON(FILE, NAME, SIZE, ROUNDED)  \
( fputs (".comm ", (FILE)),			\
  assemble_name ((FILE), (NAME)),		\
  fprintf ((FILE), ",%u\n", (ROUNDED)))

/* This says how to output an assembler line
   to define a local common symbol.  */

#define ASM_OUTPUT_LOCAL(FILE, NAME, SIZE, ROUNDED)  \
( fputs (".lcomm ", (FILE)),			\
  assemble_name ((FILE), (NAME)),		\
  fprintf ((FILE), ",%u\n", (ROUNDED)))

/* Darwin profiling -- call mcount.  */
#undef FUNCTION_PROFILER
#define FUNCTION_PROFILER(FILE, LABELNO)				\
    do {								\
      if (MACHOPIC_INDIRECT)						\
	{								\
	  const char *name = machopic_stub_name ("*mcount");		\
	  fprintf (FILE, "\tcall %s\n", name+1);  /*  skip '&'  */	\
	  machopic_validate_stub_or_non_lazy_ptr (name, /*stub:*/1);	\
	}								\
      else fprintf (FILE, "\tcall mcount\n");				\
    } while (0)