optimization - GCC 不使用 inc

Question

GCC 编译器

$ gcc --version
gcc (GCC) 4.8.2
...

不会生成 inc汇编指令，它实际上可能很有用，就像在这个 C 程序中一样:

int main(int argc, char **argv)
{
    int sum = 0;
    int i;
    for(i = 0; i < 1000000000L; i++)                     <---- that "i++"
        sum += i;
    return sum;
}

相反，它生成一个 add操作说明:

0000000000000000 <main>:
   0:   31 d2                   xor    %edx,%edx
   2:   31 c0                   xor    %eax,%eax
   4:   0f 1f 40 00             nopl   0x0(%rax)
   8:   01 d0                   add    %edx,%eax
   a:   83 c2 01                add    $0x1,%edx         <---- HERE
   d:   81 fa 00 ca 9a 3b       cmp    $0x3b9aca00,%edx
  13:   75 f3                   jne    8 <main+0x8>
  15:   f3 c3                   repz retq 

它为什么这样做？

编辑 : 我用过 gcc -O2编译这个。 gcc -Os确实会生成 inc操作说明。不使用 inc与其说是空间优化，不如说是速度优化？

Answer 1

用 -march=<your machine> 试试.结果可能会有所不同。

但是，请注意 add $1, %reg不一定是一个糟糕的选择。虽然 inc和 dec具有较小的编码，这很有吸引力，但它们仅部分更新标志，导致错误的依赖问题。英特尔优化手册包含此评论(我的重点):

The INC and DEC instructions modify only a subset of the bits in the flag register. This creates a dependence on all previous writes of the flag register. This is especially problematic when these instructions are on the critical path because they are used to change an address for a load on which many other instructions depend. Assembly/Compiler Coding Rule 33. (M impact, H generality) INC and DEC instructions should be replaced with ADD or SUB instructions, because ADD and SUB overwrite all flags, whereas INC and DEC do not, therefore creating false dependencies on earlier instructions that set the flags.