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| author | David Woodhouse <David.Woodhouse@intel.com> | 2012-12-03 16:25:40 +0000 |
|---|---|---|
| committer | David Woodhouse <David.Woodhouse@intel.com> | 2012-12-06 01:22:31 +0000 |
| commit | cf66bb93e0f75e0a4ba1ec070692618fa028e994 (patch) | |
| tree | 0ae48658adb29f50bdd85a94cbb84670a234f441 /arch/Kconfig | |
| parent | 27d7c2a006a81c04fab00b8cd81b99af3b32738d (diff) | |
| download | op-kernel-dev-cf66bb93e0f75e0a4ba1ec070692618fa028e994.zip op-kernel-dev-cf66bb93e0f75e0a4ba1ec070692618fa028e994.tar.gz | |
byteorder: allow arch to opt to use GCC intrinsics for byteswapping
Since GCC 4.4, there have been __builtin_bswap32() and __builtin_bswap16()
intrinsics. A __builtin_bswap16() came a little later (4.6 for PowerPC,
48 for other platforms).
By using these instead of the inline assembler that most architectures
have in their __arch_swabXX() macros, we let the compiler see what's
actually happening. The resulting code should be at least as good, and
much *better* in the cases where it can be combined with a nearby load
or store, using a load-and-byteswap or store-and-byteswap instruction
(e.g. lwbrx/stwbrx on PowerPC, movbe on Atom).
When GCC is sufficiently recent *and* the architecture opts in to using
the intrinsics by setting CONFIG_ARCH_USE_BUILTIN_BSWAP, they will be
used in preference to the __arch_swabXX() macros. An architecture which
does not set ARCH_USE_BUILTIN_BSWAP will continue to use its own
hand-crafted macros.
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Acked-by: H. Peter Anvin <hpa@linux.intel.com>
Diffstat (limited to 'arch/Kconfig')
| -rw-r--r-- | arch/Kconfig | 19 |
1 files changed, 19 insertions, 0 deletions
diff --git a/arch/Kconfig b/arch/Kconfig index 366ec06..c31416b 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -112,6 +112,25 @@ 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 |
