Author: Eugene Nalimov
Date: 17:09:27 11/18/02
Go up one level in this thread
Indirect call costs *at least* the same as conditional branch, sometimes much
more.
Thanks,
Eugene
On November 18, 2002 at 20:04:23, Russell Reagan wrote:
>Branchless code has the potential to run faster than the equivalent code that
>uses conditionals, or so I've heard. I was wondering about how people go about
>getting rid of conditional code, since it would seem to make a significant
>difference in some cases (such as in move generation or evaluation).
>
>One idea that came to mind is to use an array of function pointers instead of a
>conditional. For example, instead of something like:
>
>if (side_to_move == WHITE) {
> // do white stuff here
>}
>else {
> // do black stuff here
>}
>
>You could do something like:
>
>// Forgive me if my syntax for using function pointers is wrong
>Function do_stuff[] = { DoWhiteStuff, DoBlackStuff };
>do_stuff[side_to_move]();
>
>That would eliminate the potential branch misprediction penalty, and only cost a
>call and ret instruction. So as long as the call+ret instructions don't cost
>more than 10-20 cycles that you would suffer for a branch misprediction, you win
>here.
>
>In thinking about this, it sounded similar to the OOP refactoring technique
>known as "replace conditional with polymorphism". Is that essentially what is
>happening when I replace a conditional with a lookup table of function pointers?
>
>Anyway, I was wondering if someone more knowledgable than myself about these
>things could shed some light on which method would be more efficient, and about
>the real worth of branchless code. Is it worth a great amount of extra work to
>achieve branchless code? Or is it only going to provide minimal speedups?
>
>Thanks,
>Russell
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