linux-kernel - Re: [PATCH, -v2] compiler/gcc4: Add quirk for 'asm goto' miscompilation bug

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Date:	Thu, 10 Oct 2013 15:10:34 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jakub Jelinek <jakub@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Richard Henderson <rth@...ddle.net>
Subject: Re: [PATCH, -v2] compiler/gcc4: Add quirk for 'asm goto'
 miscompilation bug

On Thu, Oct 10, 2013 at 02:32:36PM +0200, Jakub Jelinek wrote:
> On Thu, Oct 10, 2013 at 01:56:17PM +0200, Peter Zijlstra wrote:
> > We didn't do version checks for CC_HAVE_ASM_GOTO because of vendor
> > backports; can't we detect this in the same way?
> 
> The problem is that it will be harder to check for this as compile time only
> check, and for runtime check you'd need to have the assembly string for
> every architecture and you couldn't do it for cross-compiling anyway.

> So for >= 4.8.3 just assume no
> workaround is needed, otherwise scan assembly.

Right, tedious and error prone it is.. :-)

Would it make sense to create something whereby GCC can tell us about
these things? Maybe something like:

  __builtin_bug_fixed(58670)

Which would default return 0, and only return 1 when its a known number.

But yes, I also see why you'd not want to do that. I suppose all I'm
saying is it would be nice to be able to detect some arbitrary issue
being fixed.

> See the follow-up mails, I think placing it immediately after asm goto might
> be better.

Duh indeed, must've missed that when I scanned through the thread :/
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