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

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Message-ID: <CAFbHwiTCN5sFadXX7z7Smwd17tEZ7WmfKhG_6b3psu0bx228ew@mail.gmail.com>
Date:	Thu, 7 Feb 2013 10:19:05 +0000
From:	Will Newton <will.newton@...il.com>
To:	Kim Phillips <kim.phillips@...escale.com>
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 Thu, Feb 7, 2013 at 1:19 AM, Kim Phillips <kim.phillips@...escale.com> wrote:
> On Wed, 6 Feb 2013 09:02:04 +0000
> "Woodhouse, David" <david.woodhouse@...el.com> wrote:
>
>> On Tue, 2013-02-05 at 21:04 -0600, Kim Phillips wrote:
>> > gcc -Os emits calls to __bswapsi2 on those platforms to save space
>> > because they don't have the single rev byte swap instruction.
>>
>> Is that the right thing for GCC to do in that situation?
>
> if it saves space, why wouldn't it be?
>
> "Many of these functions are only optimized in certain cases; if they
> are not optimized in a particular case, a call to the library
> function is emitted." [1]
>
> I see "(arm_arch6 || !optimize_size)" in gcc's define_expand
> "bswapsi2" source, so GCC considers size optimization as a
> legitimate one of those cases.
>
>> If so, perhaps we should be *providing* __bswap[sd]i2 functions for it
>> to use?
>
> either that, or link with libgcc - why does arch/arm64 do this and
> arch/arm not?  It's not obvious from git log.

One reason I have found, I don't know if it is the canonical one, is
that linking with libgcc allows people to use all intrinsics e.g. soft
float routines in the kernel without noticing it. If you limit the
intrinsics to the ones linked into the kernel explicitly then this
cannot happen.

I have also seen cases where the libgcc intrinsics are improved over
time, having the code in the kernel allows these improvements to be
rolled into the kernel even if the user has an older toolchain.

A number of ports link against libgcc and a roughly equal number do
not, so it isn't clear that there's any consensus on the issue.
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