Question

以下C或C ++代码应具有输出“11,11,11” 但是使用Visual Studio Professional 2013（版本12.0.40629.00 Update 5），输出为“11,0,0”！这仅发生在发布版本中，并在关闭优化时消失。这是编译器错误吗？

#include <stdio.h>

int main(void)
{
    int A[100] = { 0 };
    int row = 0;   // BUG disappears if we make this const or short or char...
    int ncols = 3; // BUG disappears if we make this const or short or char...

    for (int y = row; y <= row; ++y)
    {
        for (int x = 0; x < ncols; ++x)
        {
            const int index = y * ncols + x;

            //A[index] = 11;           // (no bug !)
            *(A + index) = 11;       // BUG!!!
            //*(A + y*ncols+x) = 11;   // (no bug !)
            //*(A + (y*ncols+x)) = 11; // BUG!!!
        }
    }

    for (int x = 0; x < ncols; ++x)
    {
        printf("%d,", A[x]);
    }

    return 0;
}

Answer 1

是的，它似乎是一个编译器错误。在Win32版本的代码中，编译器使用寄存器esi来表示y并注册edx来表示x。正如@Ajay Brahmakshatriya在评论中正确指出的那样，似乎编译器尝试交换周期（将外部与内部交换），但结果却是错误的代码。最后一个条件跳转指令，应该代表[交换]内部循环，由于某种原因将控制转移到检查esi的位置。该检查过早地结束了迭代。

0018206B  xor         esi,esi             ; This is `y`
0018206D  xor         edx,edx             ; This is `x`
...
00182070  test        esi,esi  
00182072  jg          main+5Ch (018209Ch) ; Exit from the outer cycle?

00182074  lea         eax,[edx+esi*2]     ; Recalculate the starting storage location
00182077  add         eax,esi             ; for the next cycle:
00182079  lea         ecx,[A]             ; eax = esi * 3 + edx
0018207F  lea         eax,[ecx+eax*4]     ; eax = &A[eax]
...
00182082  mov         ecx,1               ; It is not exactly clear to me what this is 
00182087  sub         ecx,esi             ; supposed to do, but when `esi` is `0`, it 
00182089  add         esi,ecx             ; leaves `ecx` as 1, which is correct 
                                          ; number of iterations for outer cycle
...
00182090  mov         dword ptr [eax],0Bh ; Storing the value
00182096  lea         eax,[eax+0Ch]       ; Updating the pointer for the next storage location 
00182099  dec         ecx  
0018209A  jne         main+50h (0182090h) ; Outer cycle [exchanged]

0018209C  inc         edx  
0018209D  cmp         edx,3  
001820A0  jl          main+30h (0182070h) ; Inner cycle [exchanged]: for some reason it
                                          ; jumps to `test esi,esi`, which is what
                                          ; suddenly terminates the iterations

Visual Studio 2013中可能的C / C ++编译器错误

1 个答案: