Starting a function with a branch

Question

From optimization and branch predictor point of view, is there any difference between those two codes?

First:

void think_and_do(){
    if(expression){
        //Set_A of instructions
    }
    else{
         //Set_B of instructions
    }
}

int main(){
    think_and_do();
}

Second:

void do_A(){
    //Set_A of instructions
}

void do_B(){
    //Set_B of instructions
}

int main(){
    if(expression){
        do_A();
    }
    else{
        do_B();
    }
}

Answer 1

I've made an test on godbolt.org think_and_do and in main

First observation, if your examples are trivial they mostly get optimized away. Without the cin both examples should have compiled to:

    xor     eax, eax
    add     rsp, 8 #may or may not be present.
    ret 

Second observation is that the code is exactly the same in main: and none of the functions are called, everything is inlined.

Third observation is that both examples makes the following code

    mov     edx, DWORD PTR a[rip]
    mov     eax, DWORD PTR b[rip]
    cmp     edx, eax
    je      .L8

That is they fill one cycle of 4 instruction to make the most of issuing (and ignore the possibility of macro-fusion of the cmp and jump).

If they had started with an

    cmp     edx, eax
    je      .L8

Half of the issue bandwidth would potentially have been wasted.