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Re: Any way to do branch hints?
Andrew Pinski <pinskia@physics.uc.edu> writes:
> > __builtin_expect.
On Tue, May 25, 2004 at 06:56:15PM +0200, Andi Kleen wrote:
> Intel P4 has a new branch hint prefix for conditional jumps, but afaik
> gcc doesn't generate it right now even with -march=pentium4 and
> __builtin_expect. I think that is what Kean asked about.
>
> Of course you get some part of this from the block reordering
> and the "if it jumps backwards it must be taken" heuristics,
> but telling the CPU explicitely may be a stronger hint.
Ah, but the conditional branch hint field just changes the default
branch hint, right? (This is typically to assume that forward branches
aren't taken and backward branches are). So if gcc arranges the code
such that the __builtin_expect direction is the fall-through direction,
then it would be no faster to use the branch hint prefix. Right?
Of course, the ability to use the prefix might add more flexibility.