linux-kernel - Re: [PATCH 13/15] x86: add cmpxchg

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Message-ID: <4E53FDDA.4020304@zytor.com>
Date:	Tue, 23 Aug 2011 12:22:02 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Christoph Lameter <cl@...ux.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH 13/15] x86: add cmpxchg_flag() variant

On 08/23/2011 12:01 PM, Christoph Lameter wrote:
> On Mon, 22 Aug 2011, Jeremy Fitzhardinge wrote:
> 
>> Most callers of cmpxchg() direcly compare RETURN with OLD to see if it was
>> successful.  This results in unnecessary conditional comparisons
>> and conditionals since the cmpxchg instruction directly sets the flags
>> to indicate success/failure.
> 
>> Add cmpxchg_flag() variants which return a boolean flag directly indicating
>> success.  Unfortunately an asm() statement can't directly export status
>> status flags, but sete isn't too bad.
> 
> And so what happens through this patch is that a cmp with a value that is
> likely in a register is replaced by a sete. Is there really a benefit?
> 
> What I wish we would have is the actual use of the processor flag.
> 
> if (cmpxchg_flags(....)) {
> }
> 
> where the cmpxchg is followed immediately by a jump. I tried in the past
> to pass a goto label to cmpxchg but that did not work.
> 

Yes; asm goto can't have output values :(

I keep meaning to write up a proposal for the gcc people to output
arithmetic flag status.

	-hpa

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