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

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Date:	Thu, 30 Oct 2008 14:40:37 +0800
From:	"Jike Song" <albcamus@...il.com>
To:	"H. Peter Anvin" <hpa@...or.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

On Thu, Oct 30, 2008 at 11:54 AM, H. Peter Anvin <hpa@...or.com> wrote:
> Jike Song wrote:
>>
>> Besides, by looking at the inline assembly in kernel,  I found lots of
>> codes like this:
>>
>>         static inline void atomic_add(int i, atomic_t *v)
>>         {
>>               asm volatile(LOCK_PREFIX "addl %1,%0"
>>                            : "=m" (v->counter)
>>                            : "ir" (i), "m" (v->counter));
>>         }
>>
>>
>> Yes, it works. But a little ugly..  Should this be cleaned-up with the
>> following?
>>
>>         : "+m" (v->counter)
>>         : "ir" (i)
>>
>> If you agrees, I'll send out the patch; otherwise I won't wasting your time ;-)
>>
>
> Please don't change them just to change them, if there is no actual
> error.  You never know when you're going to trigger a new bug in some
> weird version of gcc.
>
>        -hpa

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)

-- 
Thanks,
Jike
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