Comparing to zero

Once I was told I should prefer comparing with zero whenever possible, inside for loops, instead of comparing to other values, because it’s faster. But I never knew why. And I decided to understand what actually happens.

These are two programs with the same result and their assembly code. The first one is a normal for loop, the other one is a reversed for (going from n down to 0).

int main() {
    int n = 10;
    int s = 0;
    for (int i = 1; i <= n; ++i)  {
        s += i;
    }
}
main:
        push    rbp
        mov     rbp, rsp
        mov     DWORD PTR [rbp-12], 10
        mov     DWORD PTR [rbp-4], 0
        mov     DWORD PTR [rbp-8], 1
.L3:
        mov     eax, DWORD PTR [rbp-8]
        cmp     eax, DWORD PTR [rbp-12]
        jg      .L2
        mov     eax, DWORD PTR [rbp-8]
        add     DWORD PTR [rbp-4], eax
        add     DWORD PTR [rbp-8], 1
        jmp     .L3
.L2:
        mov     eax, 0
        pop     rbp
        ret

 

int main() {
    int n = 10;
    int s = 0;
    for (int i = n; i > 0; --i) {
        s += i;
    }
}
main:
        push    rbp
        mov     rbp, rsp
        mov     DWORD PTR [rbp-12], 10
        mov     DWORD PTR [rbp-4], 0
        mov     eax, DWORD PTR [rbp-12]
        mov     DWORD PTR [rbp-8], eax
.L3:
        cmp     DWORD PTR [rbp-8], 0
        jle     .L2
        mov     eax, DWORD PTR [rbp-8]
        add     DWORD PTR [rbp-4], eax
        sub     DWORD PTR [rbp-8], 1
        jmp     .L3
.L2:
        mov     eax, 0
        pop     rbp
        ret

 

The for loops are represented by the L3 labels.

For the normal for loop, the i variable (rbp-8) is loaded into the eax registry, then the registry is compared to n (rbp-12).

mov     eax, DWORD PTR [rbp-8]
cmp     eax, DWORD PTR [rbp-12]

As for the reversed for loop, i is always compared to 0 and this can be done directly, without first copying it into the eax registry.

cmp     DWORD PTR [rbp-8], 0

So the difference is of one instruction, the first for does an extra copy.

With O3 optimization level, comparing to 0 does not need the cmp instruction.

Does this matter? I know too little assembly to have a good opinion on this, but it could matter When a Microsecond Is an Eternity. Otherwise, it would be early optimization and maybe confusing for others.

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