Re: [PATCH 0/5] powerpc: Get rid of redundant arch specific swab functions

'David Gibson'
2015-02-04T13:42:48Z

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On Wed, Feb 04, 2015 at 11:54:39AM +0000, David Laight wrote:
> From: David Gibson
> > arch/powerpc/include/asm/swab.h includes some powerpc specific
> > byteswapping functions, which are implemented in terms of powerpc's
> > built in byte reversed load/store instructions. There are two problems with this:
> >
> > 1) They're not necessary - gcc is perfectly capable of generating the
> > byte-reversed load and store instructions when using the normal,
> > generic byteswapping functions (tested with gcc (GCC) 4.8.3
> > 20140911 (Red Hat 4.8.3-9))
>
> Should you be worrying about older versions of gcc?
> IIRC the internal byteswap 'stuff' is relatively recent (like
> the last couple of years) so people building current kernels
> on older distributions might have issues.

Well.. even then, surely the worst that will happen is that there will
be a few extra instructions to do the byteswap in registers. Given
that these are mostly used for IO, I find it hard to imagine that
would make a measurable performance difference.

--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson

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