linux-kernel - Re: [RFC PATCH 00/15] Provide atomics and bitops implemented with ISO C++11 atomics

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Message-ID: <20160518172258.GB3206@twins.programming.kicks-ass.net>
Date:	Wed, 18 May 2016 19:22:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Howells <dhowells@...hat.com>
Cc:	linux-arch@...r.kernel.org, x86@...nel.org, will.deacon@....com,
	linux-kernel@...r.kernel.org, ramana.radhakrishnan@....com,
	paulmck@...ux.vnet.ibm.com, dwmw2@...radead.org
Subject: Re: [RFC PATCH 00/15] Provide atomics and bitops implemented with
 ISO C++11 atomics

On Wed, May 18, 2016 at 04:10:37PM +0100, David Howells wrote:
> There are some advantages to using ISO C++11 atomics:
> 
>  (1) The compiler can make use of extra information, such as condition
>      flags, that are tricky to get out of inline assembly in an efficient
>      manner.  This should reduce the number of instructions required in
>      some cases - such as in x86 where we use SETcc to store the condition
>      inside the inline asm and then CMP outside to put it back again.
> 
>      Whilst this can be alleviated by the use of asm-goto constructs, this
>      adds mandatory conditional jumps where the use of CMOVcc and SETcc
>      might be better.

Supposedly gcc-6.1 has cc-output which makes this go away.