我在Windows和Linux上的不同编译器上对数组,向量,boost :: array进行了基准测试。我遇到了以下奇怪的事情。
我在Linux 3.7.0.7上有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"
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......这怎么可能发生?
答案 0 :(得分:3)
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]
会引用numbers
的相应元素。