Merge tag 'byteswap-for-linus-20121219' of git://git.infradead.org/users/dwmw2/byteswap

Pull preparatory gcc intrisics bswap patch from David Woodhouse:
 "This single patch is effectively a no-op for now.  It enables
  architectures to opt in to using GCC's __builtin_bswapXX() intrinsics
  for byteswapping, and if we merge this now then the architecture
  maintainers can enable it for their arch during the next cycle without
  dependency issues.

  It's worth making it a par-arch opt-in, because although in *theory*
  the compiler should never do worse than hand-coded assembler (and of
  course it also ought to do a lot better on platforms like Atom and
  PowerPC which have load-and-swap or store-and-swap instructions), that
  isn't always the case.  See

     http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46453

  for example."

* tag 'byteswap-for-linus-20121219' of git://git.infradead.org/users/dwmw2/byteswap:
  byteorder: allow arch to opt to use GCC intrinsics for byteswapping

1 files changed, 19 insertions, 0 deletions

diff --git a/arch/Kconfig b/arch/Kconfig
index 54ffd0f9df21..8e9e3246b2b4 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -113,6 +113,25 @@ config HAVE_EFFICIENT_UNALIGNED_ACCESS
           See Documentation/unaligned-memory-access.txt for more
           information on the topic of unaligned memory accesses.
 
+config ARCH_USE_BUILTIN_BSWAP
+       bool
+       help
+         Modern versions of GCC (since 4.4) have builtin functions
+         for handling byte-swapping. Using these, instead of the old
+         inline assembler that the architecture code provides in the
+         __arch_bswapXX() macros, allows the compiler to see what's
+         happening and offers more opportunity for optimisation. In
+         particular, the compiler will be able to combine the byteswap
+         with a nearby load or store and use load-and-swap or
+         store-and-swap instructions if the architecture has them. It
+         should almost *never* result in code which is worse than the
+         hand-coded assembler in <asm/swab.h>.  But just in case it
+         does, the use of the builtins is optional.
+
+         Any architecture with load-and-swap or store-and-swap
+         instructions should set this. And it shouldn't hurt to set it
+         on architectures that don't have such instructions.
+
 config HAVE_SYSCALL_WRAPPERS
         bool