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

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Message-ID: <1359470601.3529.120.camel@shinybook.infradead.org>
Date:	Tue, 29 Jan 2013 14:43:23 +0000
From:	"Woodhouse, David" <david.woodhouse@...el.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Kim Phillips <kim.phillips@...escale.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Santos <daniel.santos@...ox.com>,
	Borislav Petkov <bp@...en8.de>,
	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>
Subject: Re: [RFC] arm: use built-in byte swap function

On Tue, 2013-01-29 at 14:13 +0000, Russell King - ARM Linux wrote:
> 
> So, this compiler (4.5.4) has support for 32-bit and 64-bit bswaps
> across all our architectures, but not the 16-bit ones.

That observation is consistent with my dig through GCC history. I had
come to the conclusion that the 32-bit and 64-bit versions were added
*generically* in 4.4, and that the 16-bit version was added in 4.6 to
that PowerPC back end, and made generic in 4.8. So I *had* put that
arch-specific check into compiler-gcc4.h, for PowerPC. It's just outside
the context of Kim's patch. If it really does end up being different for
every arch, I may reconsider that.

As for the __bswapsi2() calls... if it's ever emitting an out-of-line
call for something like that, that seems like a really dubious decision;
surely it's better being inlined? So rather than adding it to your
bits-of-libgcc.a in the kernel, I'd suggest just *not* using
ARCH_USE_BUILTIN_BSWAP for the offending compilers, and filing a bug to
get them fixed.

But really, this is why I created ARCH_USE_BUILTIN_BSWAP and left it to
architecture maintainers to enable it at their leisure.... :)

-- 
                   Sent with MeeGo's ActiveSync support.

David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation

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