> > why not move code blocks in macros ( like that in mm module ) or better in
> > inline functions.
>
> That would not have the intended effect - to keep jumps out of the
> fast path.
>
> Normally gcc codes a normal if like that (pseudo, m68k like assembly)
>
> if (x < 0) {
> /* error */
> }
> /* rest of fast path */
>
> tst x
> bgr L1 /* jump taken in most cases */
> /* error */
> L1:
> /* rest of fast path */
>
> this means there is a jump in the fast path (on modern CPUs that can be
> quite costly because of the pipeline stall when the jump is mispredicted).
> It is usually faster to have code like this:
What about heuristic which says
if (...) {
...
return;
}
should be moved out of line?
Pavel
> I would prefer a cleaner way too, e.g.
>
> if (__builtin_predict(err < 0, 0)) {
> /* unlikely error code */
> }
maybe
#define unlikely __builtin_one_unlikely()
if (is_error() && unlikely) {
}
:-). Where unlikely is evaluated as 1, but gcc knows that unlikely is
not evaluated in common case. It would permit other nice tricks, like
(dreaming)
if (unlikely) {
...
}
which means code in brackets hardly ever executes, so optimize it for
size, instead ;-).
Pavel
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