When is assembly faster than C?

Question

One of the stated reasons for knowing assembler is that, on occasion, it can be employed to write code that will be more performant than writing that code in a higher-level

Answer 1

I'm surprised no one said this. The strlen() function is much faster if written in assembly! In C, the best thing you can do is

int c;
for(c = 0; str[c] != '\0'; c++) {}

while in assembly you can speed it up considerably:

mov esi, offset string
mov edi, esi
xor ecx, ecx

lp:
mov ax, byte ptr [esi]
cmp al, cl
je  end_1
cmp ah, cl
je end_2
mov bx, byte ptr [esi + 2]
cmp bl, cl
je end_3
cmp bh, cl
je end_4
add esi, 4
jmp lp

end_4:
inc esi

end_3:
inc esi

end_2:
inc esi

end_1:
inc esi

mov ecx, esi
sub ecx, edi

the length is in ecx. This compares 4 characters at time, so it's 4 times faster. And think using the high order word of eax and ebx, it will become 8 times faster that the previous C routine!