Re: [tip:x86/urgent] x86: Add memory modify constraints to xchg()and cmpxchg()

From: H. Peter Anvin
Date: Wed Jul 28 2010 - 12:01:16 EST

On 07/28/2010 08:45 AM, Linus Torvalds wrote:
> On Tue, Jul 27, 2010 at 10:33 PM, tip-bot for H. Peter Anvin
> <hpa@xxxxxxxxx> wrote:
>> x86: Add memory modify constraints to xchg() and cmpxchg()
> Ack. I assume this doesn't really change the code generated? At least
> not with a gcc that honors the whole memory clobber thing properly?

I didn't do a binary A:B comparison, on the assumption it would preturb
the code too much to be equal, but I guess it would be a good idea to at
least try.

> I also suspect that we can/should get rid of the __xg() thing - it was
> there just to make sure gcc didn't see the memory read as a single
> word and tried to optimize it. With the "+m" it probably doesn't
> matter any more (don't know if it ever did)

"+m" would still see as a single word, but the __xg() thing should be
obsoleted by asm volatile + "memory", neither of which were in the
1.3.11 version.


H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.

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