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Message-ID: <20150501171728.GD1657@gmail.com>
Date: Fri, 1 May 2015 19:17:28 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Jakub Jelinek <jakub@...hat.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Richard Henderson <rth@...hat.com>,
Vladimir Makarov <vmakarov@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] x86: Optimize variable_test_bit()
* Peter Zijlstra <peterz@...radead.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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