Question

我在C++中写了这一微薄的方案,以便检查CPU的负荷情况。

#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <windows.h>
#include <time.h>
int main()
{

    double x = 1;
    int t1 = GetTickCount();
    srand(10000);

    for (unsigned long i = 0; i < 10000000; i++)
    {
        int r = rand();
        double l = sqrt((double)r);
        x *= log(l/3) * pow(x, r);
    }

    int t2 = GetTickCount();
    printf("Time: %d
", t2-t1);
    getchar();
}

I compiled it both for x86 and for x64 on win7 x64.
For some reason when I ran the x64 version it finished running in about 3 seconds
but when I tried it with the x86 version it took 48 (!!!) seconds.
I tried it many times and always got similar results.
What could cause this difference?

Answer 1

考察组群的输出与/Ox(最大优化),发现x86和x64之间的速度差异:

; cl /Ox /Fa tick.cpp
; x86 Line 17: x *= log(l/3) * pow(x, r)
fld     QWORD PTR _x$[esp+32]
mov     eax, esi
test    esi, esi
; ...

我们看到,这项计算使用了x87指示。相比之下,X64号建筑:

; cl /Ox /Fa tick.cpp
; x64 Line 17: x *= log(l/3) * pow(x, r)
movapd  xmm1, xmm8
mov     ecx, ebx
movapd  xmm5, xmm0
test    ebx, ebx
; ...

我们现在看到,使用SSE指令。

页: 1 “2010年视觉演播室”制作类似指示,但“64位轨道汇编者”仅生产“~~、更好的~~faster组件,供你手。

Finally, if you relax the floating point model the x86 and x64 perform nearly identically.

时间选择,最不敏感的是:

x86, /Ox: 22704 ticks
x64, /Ox: 822 ticks
x86, /Ox /arch:SSE2: 3432 ticks
x64, /Ox /favor:INTEL64: 1014 ticks
x86, /Ox /arch:SSE2 /fp:fast: 834 ticks

Answer 2

其原因确实与特别安全局有关。在VS中,64个轨道释放是在违约时生成SSE2指示的,但必须使用<代码>/arch:SSE2的开关,使SSE2。当你这样做时,你有32个和64个轨道的可比运行时间。

Answer 3

Many of the possibilities here have little or nothing to do with x86 vs. x64. One obvious possibility is that most (all?) compilers use SSE for floating point, where most normally use 8087-style instructions in x86 mode. Since your code is heavy on floating point, this could make a significant difference.

Another possibility is that in the process or rewriting for x64, they noticed/fixed some problems in their code generator that let it produce substantially better code, at least under certain circumstances.

虽然它不像此处那样看待,但有些法典也大大得益于64个轨道上登记的面积和/或数量的增加。

Answer 4

Part of it is definitely the SSE, but there s a huge reason why x64 uses SSE mode: all AMD64 CPUs are required to have SSE2. Another part could also be the increased register count

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