linux-kernel - Re: [RFC] arm: use built-in byte swap function

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Message-ID: <20130220214943.9b28a5b208da9f081387c55e@freescale.com>
Date:	Wed, 20 Feb 2013 21:49:43 -0600
From:	Kim Phillips <kim.phillips@...escale.com>
To:	Nicolas Pitre <nico@...xnic.net>
CC:	"Woodhouse, David" <david.woodhouse@...el.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Santos <daniel.santos@...ox.com>,
	David Rientjes <rientjes@...gle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <robherring2@...il.com>
Subject: Re: [RFC] arm: use built-in byte swap function

On Wed, 20 Feb 2013 10:43:18 -0500
Nicolas Pitre <nico@...xnic.net> wrote:

> On Wed, 20 Feb 2013, Woodhouse, David wrote:
> > On Wed, 2013-02-20 at 09:06 -0500, Nicolas Pitre wrote:
> > > ... in which case there is no harm shipping a .c file and trivially 
> > > enforcing -O2, the rest being equal.
> > 
> > For today's compilers, unless the wind changes.
> 
> We'll adapt if necessary.  Going with -O2 should remain pretty safe anyway.

Alas, not so for gcc 4.4 - I had forgotten I had tested
Ubuntu/Linaro 4.4.7-1ubuntu2 here:

https://patchwork.kernel.org/patch/2101491/

add -O2 to that test script and gcc 4.4 *always* emits calls to
__bswap[sd]i2, even with -march=armv6k+.

I'll try working on an assembly version given it probably
makes more sense, future-gcc-immunity-wise.

Otherwise we're back to the old 'if GCC_VERSION >= 40500' in
arch/arm/include/asm/swab.h...

Kim

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