c++ - 用 bool 逻辑替换 IF 语句(随机条件) - 执行时间是否相同？

Question

(设置:Win 7 64、MSVC、第三代酷睿 i7、64 位编译、启用 -O2)

下面的代码有三个功能——一个有一个 IF 语句，它根据是否满足条件执行不同的代码。我用一些 bool 逻辑替换了这个 IF 语句。然而，时间是相同的......我期待缺乏分支预测来产生更快的代码:

#include <iostream>

unsigned long long iterations = 1000000000;

void test1(){
    volatile int c = 0;

    for(int i=0; i<iterations; i++){
        bool condition = __rdtsc() % 2 == 0;
        if(condition){
            c = 4;
        }
        else{
            c = 5;
        }
    }
}

void test2(){
    volatile int c = 0;

    for(int i=0; i<iterations; i++){
        bool condition = __rdtsc() % 2 == 0;
        c = (4 * condition) + (5 * !condition);
    }
}

int main(){
    unsigned long long s = 0;
    unsigned long long f = 0;
    unsigned long long s2 = 0;
    unsigned long long f2 = 0;
    unsigned int x = 0;
    unsigned int y = 0;

    start = __rdtscp(&x);
    test1();
    finish = __rdtscp(&y);

    start2 = __rdtscp(&x);
    test2();
    finish2 = __rdtscp(&y);

    std::cout << "1: " << f - s<< std::endl;
    std::cout << "2: " << f2- s2<< std::endl;
}

更新汇编:

int main(){
 push        rbp  
 push        rsi  
 push        rdi  
 push        r14  
 sub         rsp,20h  
    unsigned long long start = 0;
    unsigned long long finish = 0;
    unsigned long long start2 = 0;
    unsigned long long finish2 = 0;
    unsigned long long start3 = 0;
    unsigned long long finish3 = 0;
    unsigned int x = 0;
 xor         r8d,r8d  
 mov         dword ptr [x],r8d  
    unsigned int y = 0;
 mov         dword ptr [y],r8d  

    start = __rdtscp(&x);
 rdtscp  
 lea         r9,[x]  
 shl         rdx,20h  
 mov         dword ptr [r9],ecx  
 or          rax,rdx  
    test1();
 mov         dword ptr [rsp+60h],r8d  
 mov         ecx,r8d  

    start = __rdtscp(&x);
 mov         r10,rax  
 nop         word ptr [rax+rax]  
    test1();
 rdtsc  
 shl         rdx,20h  
 or          rax,rdx  
 xor         al,0FFh  
 and         al,1  
 neg         al  
 sbb         eax,eax  
 inc         ecx  
 add         eax,5  
 mov         dword ptr [rsp+60h],eax  
 movsxd      rax,ecx  
 cmp         rax,3E8h  
    test1();
 jb          main+40h (013FFE1280h)  
    finish = __rdtscp(&y);
 rdtscp  
 lea         r9,[y]  
 shl         rdx,20h  
 or          rax,rdx  
 mov         dword ptr [r9],ecx  
 mov         rbp,rax  

    start2 = __rdtscp(&x);
 rdtscp  
 lea         r9,[x]  
 shl         rdx,20h  
 mov         dword ptr [r9],ecx  
 or          rax,rdx  
    test2();
 mov         dword ptr [rsp+60h],r8d  
 mov         r9d,r8d  

    start2 = __rdtscp(&x);
 mov         r14,rax  
 nop         word ptr [rax+rax]  
    test2();
 rdtsc  
 shl         rdx,20h  
 inc         r9d  
 or          rax,rdx  
 xor         al,0FFh  
 and         al,1  
    test2();
 movzx       ecx,al  
 lea         eax,[rcx+rcx*8]  
 mov         dword ptr [rsp+60h],eax  
 movsxd      rax,r9d  
 cmp         rax,3E8h  
 jb          main+0A0h (013FFE12E0h)  
    finish2 = __rdtscp(&y);

Answer 1

生成的代码不包含任何函数的任何内部分支，这就是没有错误预测惩罚的原因。

在第一个中，它将 bool 值转换为零或 -1(围绕 sbb eax,eax)并将其加到 5。这是使用 bool 值时非常标准的优化。

在第二个中它乘以九 (rcx+rcx*8)，因为你有 5 * condition 而不是 5 * !condition.