linux-kernel - Re: [PATCH RFC 0/7] x86: convert ticketlocks to C and remove duplicate code

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Date:	Wed, 22 Jun 2011 13:19:39 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	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>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH RFC 0/7] x86: convert ticketlocks to C and remove duplicate
 code

On 06/22/2011 12:21 PM, Jeremy Fitzhardinge wrote:
> 
> A friend just pointed out that gcc has a "__sync_fetch_and_add()"
> intrinsic, which compiles to xadd when used in this context.  What's the
> general feeling about using these kinds of gcc features?
> 

In general they are good, IF:

a) they cover all versions of gcc we care about (or we have a fallback),
and
b) they have the right semantics.

Using gcc intrinsics can generate better code than we can in inline
assembly.

However, (b) is a killer since gcc doesn't have a way to generate our
lock prefix annotations, and so it isn't really useful here.

	-hpa
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