gcc源代码分析之varasm.c

rtx
expand_expr (exp, target, tmode, modifier)
     register tree exp;
     rtx target;
     enum machine_mode tmode;
     enum expand_modifier modifier;
{

.....

    case STRING_CST:

      if (! TREE_CST_RTL (exp))
    output_constant_def (exp);

      /* TREE_CST_RTL probably contains a constant address.
     On RISC machines where a constant address isn't valid,
     make some insns to get that address into a register.  */
      if (GET_CODE (TREE_CST_RTL (exp)) == MEM
      && modifier != EXPAND_CONST_ADDRESS
      && !memory_address_p (mode, XEXP (TREE_CST_RTL (exp), 0)))
    return change_address (TREE_CST_RTL (exp), VOIDmode,
                   copy_rtx (XEXP (TREE_CST_RTL (exp), 0)));

      return TREE_CST_RTL (exp);

/* Return an rtx representing a reference to constant data in memory
   for the constant expression EXP.
   If assembler code for such a constant has already been output,
   return an rtx to refer to it.
   Otherwise, output such a constant in memory and generate
   an rtx for it.  The TREE_CST_RTL of EXP is set up to point to that rtx.  */

rtx
output_constant_def (exp)
     tree exp;
{
  register rtx def;
  int temp_p = allocation_temporary_p ();

  if (TREE_CODE (exp) == INTEGER_CST)
    abort ();            /* No TREE_CST_RTL slot in these.  */

  if (TREE_CST_RTL (exp))
    return TREE_CST_RTL (exp);

  if (TREE_PERMANENT (exp))
    end_temporary_allocation ();

  def = gen_rtx (SYMBOL_REF, Pmode, get_or_assign_label (exp));

  TREE_CST_RTL (exp)
    = gen_rtx (MEM, TYPE_MODE (TREE_TYPE (exp)), def);
  RTX_UNCHANGING_P (TREE_CST_RTL (exp)) = 1;

  if (temp_p && TREE_PERMANENT (exp))
    resume_temporary_allocation ();

  return TREE_CST_RTL (exp);
}

expand_expr code = 26
 <string_cst 84014
    type <array_type 94ef4
        type <integer_type 825bc char permanent QI
            size <integer_cst 82638 literal permanent 1
            align 8 size_unit 8 sep_unit 8 symtab 0
            sep <integer_cst 82608 literal permanent -128 precision 8 min <integer_cst 82608 -128>
            max <integer_cst 82620 literal permanent 127
            pointer_to_this <pointer_type 88a44> chain <integer_type 826a8 long int>
        BLK
        size <integer_cst 94f40 literal 15
        align 8 size_unit 8 sep_unit 8 symtab 0 sep <integer_cst 82638 1>
        domain <integer_type 94ea8 SI
            size <integer_cst 8254c literal permanent 4
            align 32 size_unit 8 sep_unit 32 symtab 0
            sep <integer_cst 84048 literal 0 precision 32 min <integer_cst 84048 0>
            max <integer_cst 84078 literal 14
        pointer_to_this <pointer_type 94f58> chain <pointer_type 94f58>
    static literal "Hello, world!
"

(symbol_ref:SI ("*LC0"))

(mem:BLK (symbol_ref:SI ("*LC0")))

上面的代码可以解释上面两个rtx的生成过程。

    .file    "hello.c"
gcc_compiled.:
.text
LC0:
    .ascii "Hello, world!\12\0"
    .align 2
.globl _main
_main:
    pushl %ebp
    movl %esp,%ebp
    pushl $LC0
    call _printf
    xorl %eax,%eax
    jmp L1
    .align 2
L1:
    leave
    ret

Hello.world!\12\0 是个字符串常量，编译过程给了一个符号LC0。