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

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Date:	Thu, 10 Oct 2013 17:04:16 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jakub Jelinek <jakub@...hat.com>, 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


* Peter Zijlstra <peterz@...radead.org> wrote:

> > 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.

Basically not a monolithic 1-dimensional feature value but a more 
finegrained multi-dimensional feature vector would be nice.

It could be used not just for bugs but for all regular features as well, 
as they are added.

Something like:

	__builtin_feature(asm_goto)

would return 1 for a basic asm goto implementation and 0 for unknown 
feature strings. Any individual bug, if applications are affected by it, 
would then would be discoverable:

	__builtin_feature(asm_goto_bug_58670_fixed)

(or a fully symbolic name would suffice as well.) On old GCC this would 
return 1, on anything 4.8.2 and higher it would return 1.

Thanks,

	Ingo
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