c++ - Replacing IF statement (random condition) with boolean logic- execution time identical? -

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(setup: win 7 64, msvc, 3rd generation core i7, 64-bit compliation, -o2 enabled)

the below code has 3 functions- 1 has if statement executes different code depending on whether condition has been met. replaced if statement boolean logic. timings identical.... expecting lack of branch prediction yield faster code:

#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; }

update asm:

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    ,         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    ,         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);

the generated code doesn't contain internal branches either function, why there no mis-prediction penalty.

in first 1 converts boolean either 0 or -1 (around sbb eax,eax) , adds 5. pretty standard optimisation when working booleans.

in second 1 multiplies 9 (rcx+rcx*8), because have 5 * condition not 5 * !condition.


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