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

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Message-ID: <CA+55aFzh7OBG66-bgyY5=bC3RF59c1mxY_ej-7k2SFpjv9veFg@mail.gmail.com>
Date:	Fri, 1 May 2015 09:45:35 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jakub Jelinek <jakub@...hat.com>
Cc:	Richard Henderson <rth@...hat.com>,
	Vladimir Makarov <vmakarov@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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 1, 2015 at 9:33 AM, Jakub Jelinek <jakub@...hat.com> wrote:
>
> 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.

So would it help if it was documented that output registers would only
be valid for the "fallthrough" case of an asm goto, and would be
considered undefined for any jump targets?

It wouldn't be the perfect situation, but it would be better than not
having any outputs at all, and as mentioned, it would probably be
sufficient for a fair number of cases.

                              Linus
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