在这种从VC++进行的反汇编中,正在执行一个函数调用。编译器在将本地指针压入寄存器之前先将其移动到寄存器:
memcpy( nodeNewLocation, pNode, sizeCurrentNode );
0041A5DA 8B 45 F8 mov eax,dword ptr [ebp-8]
0041A5DD 50 push eax
0041A5DE 8B 4D 0C mov ecx,dword ptr [ebp+0Ch]
0041A5E1 51 push ecx
0041A5E2 8B 55 D4 mov edx,dword ptr [ebp-2Ch]
0041A5E5 52 push edx
0041A5E6 E8 67 92 FF FF call 00413852
0041A5EB 83 C4 0C add esp,0Ch
为什么不直接插入它们呢? IE
push dword ptr [ebp-8]
另外,如果您要进行单独的推送,为什么不手动进行。换句话说,不要在上面做“推eax”,而是
mov [esp], eax
这样做的好处是,在执行了3次移动之后,您可以执行一次减法来设置新的堆栈指针,而不是通过插入隐式地减去3次。
更新-发布版本
这是为发布而编译的相同代码:
; 741 : memcpy( nodeNewLocation, pNode, sizeCurrentNode );
00087 8b 45 f8 mov eax, DWORD PTR _sizeCurrentNode$[ebp]
0008a 8b 7b 04 mov edi, DWORD PTR [ebx+4]
0008d 50 push eax
0008e 56 push esi
0008f 57 push edi
00090 e8 00 00 00 00 call _memcpy
00095 83 c4 0c add esp, 12 ; 0000000cH
绝对比调试版本有效,但它仍在执行MOV/PUSH组合。
最佳答案
这是一个优化。在英特尔处理器手册第4卷第12.3.3.6节中明确提到:
In Intel Atom microarchitecture, using PUSH/POP instructions to manage stack space and address adjustment between function calls/returns will be more optimal than using ENTER/LEAVE alternatives. This is because PUSH/POP will not need MSROM flows and stack pointer address update is done at AGU. When a callee function need to return to the caller, the callee could issue POP instruction to restore data and restore the stack pointer from the EBP.
Assembly/Compiler Coding Rule 19. (MH impact, M generality) For Intel Atom processors, favor register form of PUSH/POP and avoid using LEAVE; Use LEA to adjust ESP instead of ADD/SUB.
手册的其余部分不清楚原因,但确实提到了隐式ESP调整可能造成的3个周期的AGU停顿。
关于visual-c++ - 为什么VC++编译器MOV + PUSH args不仅仅是PUSH它们? x86,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/13124501/