Comment by stassats
The text mentions that it can also do multiplication but doesn't expand on that.
E.g. for x * 5 gcc issues lea eax, [rdi+rdi*4].
The text mentions that it can also do multiplication but doesn't expand on that.
E.g. for x * 5 gcc issues lea eax, [rdi+rdi*4].
The tricks to avoid multiplication (and division) are probably worth a whole post.
x * 6:
lea eax, [rdi+rdi*2]
add eax, eax
x * 7:
lea eax, [0+rdi*8]
sub eax, edi
x * 11:
lea eax, [rdi+rdi*4]
lea eax, [rdi+rax*2]
And on modern CPUs multiplication might not be actually all that slow (but there may be fewer multiply units).
Hey now; let's not get ahead too far :) I'm trying to keep each one bite-sized...I don't think you'll be (too) disappointed at the next few episodes :)
It also says the multiplier must be one of 2, 4 or 8.
So I guess this trick then only works for multiplication by 2, 3, 4, 5, 8 or 9?