/* 优化生成的汇编代码很容易就能够看出max对应的值以及相关循环控制
 * 而未优化生成的汇编代码没有对应的max值?```cmpq -8(%rbp), %rax```
 * 看了半天都感觉这个是无限循环啊，%rax(这个也没溢出啊)总是大于-8(%rbp)(为0)的，jl .L3就
 * 一直循环。*/

#include <stdio.h>
#include <math.h>

int main(int argc, const char *argv[])
{
  long i, max;
  long sum = 0;
  max = (long)pow(2,32);

  for (i = 0; i < max; i++) {
    sum += i; 
  }
  printf("%ld\n", sum);
  return 0;
}

用 gcc -S main.c 生成的汇编代码为：

 .file   "main.c"
    .section    .rodata
.LC0:
    .string "%ld\n"
    .text
    .globl  main
    .type   main, @function
main:
.LFB0:
    .cfi_startproc
    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6, -16
    movq    %rsp, %rbp
    .cfi_def_cfa_register 6
    subq    $48, %rsp
    movl    %edi, -36(%rbp)
    movq    %rsi, -48(%rbp)
    movq    $0, -16(%rbp)
    movl    $0, -8(%rbp)
    movl    $1, -4(%rbp)
    movq    $0, -24(%rbp)
    jmp .L2
.L3:
    movq    -24(%rbp), %rax
    addq    %rax, -16(%rbp)
    addq    $1, -24(%rbp)
.L2:
    movq    -24(%rbp), %rax
    cmpq    -8(%rbp), %rax
    jl  .L3
    movl    $.LC0, %eax
    movq    -16(%rbp), %rdx
    movq    %rdx, %rsi
    movq    %rax, %rdi
    movl    $0, %eax
    call    printf
    movl    $0, %eax
    leave
    .cfi_def_cfa 7, 8
    ret
    .cfi_endproc
.LFE0:
    .size   main, .-main
    .ident  "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
    .section    .note.GNU-stack,"",@progbits

而优化 gcc -S -O1 main.c 后的汇编代码为：

 .file   "main.c"
    .section    .rodata.str1.1,"aMS",@progbits,1
.LC0:
    .string "%ld\n"
    .text
    .globl  main
    .type   main, @function
main:
.LFB29:
    .cfi_startproc
    subq    $8, %rsp
    .cfi_def_cfa_offset 16
    movl    $0, %eax
    movabsq $4294967296, %rdx
.L2:
    addq    $1, %rax
    cmpq    %rdx, %rax
    jne .L2
    movabsq $9223372034707292160, %rdx
    movl    $.LC0, %esi
    movl    $1, %edi
    movl    $0, %eax
    call    __printf_chk
    movl    $0, %eax
    addq    $8, %rsp
    .cfi_def_cfa_offset 8
    ret
    .cfi_endproc
.LFE29:
    .size   main, .-main
    .ident  "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
    .section    .note.GNU-stack,"",@progbits