| Message ID | 877b532d8d240c1d9e9db923c84b924443a218ed.1524583390.git.robin.murphy@arm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Tue, Apr 24, 2018 at 04:25:47PM +0100, Robin Murphy wrote: > It is probably safe to assume that all Armv8-A implementations have a > multiplier whose efficiency is comparable or better than a sequence of > three or so register-dependent arithmetic instructions. Select > ARCH_HAS_FAST_MULTIPLIER to get ever-so-slightly nicer codegen in the > few dusty old corners which care. > > In a contrived benchmark calling hweight64() in a loop, this does indeed > turn out to be a small win overall, with no measurable impact on > Cortex-A57 but about 5% performance improvement on Cortex-A53. > > Signed-off-by: Robin Murphy <robin.murphy@arm.com> > --- Acked-by: Will Deacon <will.deacon@arm.com> Will
On Tue, Apr 24, 2018 at 04:25:47PM +0100, Robin Murphy wrote: > It is probably safe to assume that all Armv8-A implementations have a > multiplier whose efficiency is comparable or better than a sequence of > three or so register-dependent arithmetic instructions. Select > ARCH_HAS_FAST_MULTIPLIER to get ever-so-slightly nicer codegen in the > few dusty old corners which care. > > In a contrived benchmark calling hweight64() in a loop, this does indeed > turn out to be a small win overall, with no measurable impact on > Cortex-A57 but about 5% performance improvement on Cortex-A53. > > Signed-off-by: Robin Murphy <robin.murphy@arm.com> Queued for 4.18. Thanks.
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig index eb2cf4938f6d..9c850f3b398f 100644 --- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig @@ -12,6 +12,7 @@ config ARM64 select ARCH_HAS_DEVMEM_IS_ALLOWED select ARCH_HAS_ACPI_TABLE_UPGRADE if ACPI select ARCH_HAS_ELF_RANDOMIZE + select ARCH_HAS_FAST_MULTIPLIER select ARCH_HAS_FORTIFY_SOURCE select ARCH_HAS_GCOV_PROFILE_ALL select ARCH_HAS_GIGANTIC_PAGE if (MEMORY_ISOLATION && COMPACTION) || CMA
It is probably safe to assume that all Armv8-A implementations have a multiplier whose efficiency is comparable or better than a sequence of three or so register-dependent arithmetic instructions. Select ARCH_HAS_FAST_MULTIPLIER to get ever-so-slightly nicer codegen in the few dusty old corners which care. In a contrived benchmark calling hweight64() in a loop, this does indeed turn out to be a small win overall, with no measurable impact on Cortex-A57 but about 5% performance improvement on Cortex-A53. Signed-off-by: Robin Murphy <robin.murphy@arm.com> --- Apropos of stumbling across this option whilst digging down into some bitmap-juggling code... arch/arm64/Kconfig | 1 + 1 file changed, 1 insertion(+)