c++ - 为什么这会立即运行？

Question

我在 Windows 和 Linux 的不同编译器上对数组、 vector 、boost::array 进行基准测试。我遇到了以下奇怪的事情。

我在 Linux 3.7.0.7 上安装了 gcc 4.7.2，带有标志:

g++ -O3 -g -Wall -c -fmessage-length=0 -std=c++11 -MMD -MP -MF"main.d"-MT"main.d"-o "main .o""../main.cpp"

还有这段代码:

    const int arrLength = 5;
    int a[arrLength];
    for (int i = 0; i < arrLength; i++) {
        a[i] = i * 5;
    }

    srand(time(0)); // randomise at run time so it cannot be precomputed by the compiler
    int numbers[10];
    for (auto &i : numbers)
        i = rand();

    clock_t c;

    c = clock();
    for (int i = 0; i < 100000000; i++) {
        for (int j = 0; j < arrLength; j++)
            a[j] += numbers[j%10];
    }

    // write it out so the compiler doesn't omit the whole operation if the values in the array are not being used
    for (int x : a)
        cout << x;
    cout << endl;
    cout << (float) (clock() - c) << endl;

它实际上在 0 秒内运行...这怎么会发生？

Answer 1

我的编译器只计算最终结果。这是我带注释的源代码:

asm volatile("DEBUG_IN");

for (int i = 0; i < 100000000; i++)
{
    for (int j = 0; j < arrLength; j++)
    {
        a[j] += numbers[j % 10];
    }
}

asm volatile("DEBUG_OUT");

调用是 g++ -std=c++11 -O3 -S -masm=intel。

结果:

#APP
# 21 "/tmp/x.cpp" 1
        DEBUG_IN
# 0 "" 2
#NO_APP
        mov     ecx, DWORD PTR [esp+60]
        imul    edi, DWORD PTR [esp+56], 100000000
        mov     eax, DWORD PTR [esp+64]
        mov     edx, DWORD PTR [esp+68]
        mov     DWORD PTR [esp+36], edi
        imul    edi, ecx, 99999999
        lea     ecx, [ecx+5+edi]
        mov     DWORD PTR [esp+40], ecx
        imul    ecx, eax, 99999999
        lea     eax, [eax+10+ecx]
        mov     DWORD PTR [esp+44], eax
        imul    eax, edx, 99999999
        lea     eax, [edx+15+eax]
        mov     edx, DWORD PTR [esp+72]
        mov     DWORD PTR [esp+48], eax
        imul    eax, DWORD PTR [esp+72], 99999999
        lea     eax, [edx+20+eax]
        mov     DWORD PTR [esp+52], eax
#APP
# 31 "/tmp/x.cpp" 1
        DEBUG_OUT
# 0 "" 2
#NO_APP

如您所见，只有五个简单的作业。请注意，[esp+36] 到 [esp+52] 指的是 numbers 的各个元素。