* Peter Zijlstra <pet...@infradead.org> wrote:
> On Fri, May 01, 2015 at 06:33:29PM +0200, Jakub Jelinek wrote:
> > On Fri, May 01, 2015 at 09:03:32AM -0700, Linus Torvalds wrote:
> > > > PPS. Jakub, I see gcc5.1 still hasn't got output operands for asm goto;
> > > > is this something we can get 'fixed' ?
> >
> > CCing Richard as author of asm goto and Vlad as register allocator
> > maintainer. There are a few enhancement requests to support this,
> > like http://gcc.gnu.org/PR59615 and http://gcc.gnu.org/PR52381 ,
> > but indeed the reason why no outputs are allowed is the register
> > allocation issue. Don't know if LRA would be better suited to
> > handle that case, but it would indeed be pretty hard.
>
> So it would b awesome if we could use these freshly modeled flags as
> output for regular asm stmts; that would obviate much of the asm
> goto hackery we now do/have and allow gcc to pick the right branch
> for likely/unlikely.
If I may hijack the discussion a bit: it would also be awesome if
there was a GCC flag that would allow us to use __builtin_expect()
hints even when automatic branch heuristics are disabled:
I.e. very similar to -fno-guess-branch-probability, just that explicit
__builtin_expect() hints would not be ignored (like
-fno-guess-branch-probability does it today).
We could use this to compress the kernel instruction cache footprint
by about 5% on x86-64, while still having all the hand-made
optimizations that __builtin_expect() allows us.
It would be a perfect solution if -fno-guess-branch-probability just
stopped ignoring __builtin_expect().
Thanks,
Ingo
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