没有GCC为什么产生MOV EAX％，％EAX什么意思呢？ [英] Why did GCC generate mov %eax,%eax and what does it mean?

问题描述

GCC 4.4.3生成以下x86_64的组装。这混淆我的部分是 MOV EAX％，％EAX 。移动寄存器本身呢？为什么呢？

GCC 4.4.3 generated the following x86_64 assembly. The part that confuses me is the mov %eax,%eax. Move the register to itself? Why?

   23b6c:       31 c9                   xor    %ecx,%ecx        ; the 0 value for shift
   23b6e:       80 7f 60 00             cmpb   $0x0,0x60(%rdi)  ; is it shifted?
   23b72:       74 03                   je     23b77
   23b74:       8b 4f 64                mov    0x64(%rdi),%ecx  ; is shifted so load shift value to ecx
   23b77:       48 8b 57 38             mov    0x38(%rdi),%rdx  ; map base
   23b7b:       48 03 57 58             add    0x58(%rdi),%rdx  ; plus offset to value
   23b7f:       8b 02                   mov    (%rdx),%eax      ; load map_used value to eax
   23b81:       89 c0                   mov    %eax,%eax        ; then what the heck is this? promotion from uint32 to 64-bit size_t?
   23b83:       48 d3 e0                shl    %cl,%rax         ; shift rax/eax by cl/ecx
   23b86:       c3                      retq   

C ++的code此功能是：

The C++ code for this function is:

    uint32_t shift = used_is_shifted ? shift_ : 0;
    le_uint32_t le_map_used = *used_p();
    size_t map_used = le_map_used;
    return map_used << shift;

这是 le_uint32_t 是包裹在大端机字节交换操作的类。在x86它什么都不做。在 used_p（）函数计算从地图基础指针+偏移量并返回正确类型的指针。

An le_uint32_t is a class which wraps byte-swap operations on big-endian machines. On x86 it does nothing. The used_p() function computes a pointer from the map base + offset and returns a pointer of the correct type.

推荐答案

在X86-64 32位指令隐含零扩展：位32-63被清除。所以有时候这就是为什么你会看到古怪的指令。

In x86-64, 32-bit instructions implicitly zero-extend: bits 32-63 are cleared. So sometimes thats why you'll see odd-looking instructions.

不过，previous MOV 也是32位的，所以高半％RAX 的被清除。在 MOV EAX％，％EAX 看起来是一个NOP。

However, the previous mov is also 32-bit, so the high half of %rax is already cleared. The mov %eax,%eax appears to be a NOP.

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