Diffstat (limited to 'arch/Kconfig')
1 files changed, 19 insertions, 0 deletions
diff --git a/arch/Kconfig b/arch/Kconfig
index 366ec06a518..c31416b1058 100644
@@ -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.
+ 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.