linux-kernel - Re: [PATCH] x86: Optimize variable_test

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Message-ID: <20150501164639.GA19158@dyad.arnhem.chello.nl>
Date:	Fri, 1 May 2015 18:46:40 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jakub Jelinek <jakub@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Richard Henderson <rth@...hat.com>,
	Vladimir Makarov <vmakarov@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	"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()

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.
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