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

On Wed, Feb 04, 2015 at 11:54:39AM +0000, David Laight wrote:

From: David Gibson

quoted

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