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This single patch is effectively a no-op for now. It enables architectures

Browse source 
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.
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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
This commit is contained in:
Linus Torvalds 2012-12-19 07:52:48 -08:00
commit 7f2de8171d
4 changed files with 45 additions and 3 deletions
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19
arch/Kconfig
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@ -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