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

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Mon, Jan 28, 2013 at 07:30:33PM -0600, Kim Phillips wrote:
> AFAICT, arm gcc got __builtin_bswap{32,64} support in 4.6,
> and for the 16-bit version in 4.8.

Hmm.

$ /usr/local/aeabi/bin/arm-linux-gcc --version
arm-linux-gcc (GCC) 4.5.4
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ for a in armv3 armv4 armv4t armv5t armv5te armv6k armv6 armv7-a; do \
   echo $a:; \
   for f in 16 32 64; do \
      echo 'unsigned foo(unsigned val) { return __builtin_bswap'$f'(val); }' | \
      /usr/local/aeabi/bin/arm-linux-gcc -x c -S -o - - -march=$a | grep 'bl'; \
   done; \
done

produces:

armv3:
        bl __builtin_bswap16
armv4:
        bl __builtin_bswap16
armv4t:
        bl __builtin_bswap16
armv5t:
        bl __builtin_bswap16
armv5te:
        bl __builtin_bswap16
armv6k:
        bl __builtin_bswap16
armv6:
        bl __builtin_bswap16
armv7-a:
        bl __builtin_bswap16

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.