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

[Kim Phillips]

On Wed, 30 Jan 2013 10:22:15 +0000
"Woodhouse, David" <david.woodhouse@xxxxxxxxx> wrote:

> On Tue, 2013-01-29 at 19:10 +0100, Borislav Petkov wrote:
> > So, IMHO it sounds to me like we want to explicitly state for each arch
> > separately that it is ok to use the __builtin_bswapXX things. This also
> > takes care of the case where the compiler is doing something suboptimal
> > by excluding the affected versions.
> 
> Well, if it really does end up being different for every architecture,
> then that means I probably made the wrong decision when I chose to make
> it "generic", and override the __arch_swabXX() macros. I could have just
> pushed all the architectures to use the builtins in their __arch_swabXX
> macros instead, as appropriate.
> 
> Let's see how many special cases we actually end up with, and perhaps
> we'll end up switching that round. For now, let's just make ARM set
> __HAVE_BUILTIN_BSWAPxx__ for the appropriate sizes in <asm/swab.h>,
> according to whatever criteria it needs.

thanks - I've attempted to do this - see v2 patch below.

> It's not entirely clear how much of a win it is on ARM anyway; we don't
> have load-and-swap or store-and-swap instructions so there are only a
> few added opportunities for optimisation that we get by letting the
> compiler see what's going on.

I've added some text size figures to the patch description.  
They are indeed very small, but it should help drivers for
big-endian devices.