linux-kernel - Re: [PATCH] x86: fix inline assembly constraints

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Message-ID: <4909C9D9.7040105@zytor.com>
Date:	Thu, 30 Oct 2008 07:51:05 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jike Song <albcamus@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>, tglx@...utronix.de, mingo@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: fix inline assembly constraints

Jike Song wrote:
> 
> Yes, sometimes gcc did have bugs with its obscure inline asm
> conventions. But I think the change of x86-64 atomic operations should
> be OK. Anyway, the "+" constraint is more clear than a "=m" output and
>  a "m" input.
> 
> The 32-bit atomic ops were already changed to "+m".(commit
> b862f3b099f3ea672c7438c0b282ce8201d39dfc)
> 

You *THINK*.  It's very easy to *THINK* that gcc won't do something
utterly moronic, and you'd be wrong.

Just changing it for the sake of churn is pointless... if there is a
bug, then we have to take the risk anyway, but if it is already correct,
then there is no point in provoking a bug.  Not *your* bug, because your
code is correct, but gcc's bug.

FWIW, the reason that code doesn't use "+m" is because a version of gcc
which we no longer support didn't handle it.  That by itself isn't a
reason to keep it, but there is also no reason to just "tidy" it, IMNSHO.

	-hpa
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