Hi Jan, On 5/31/19 10:53 AM, Jan Beulich wrote:
According to Linux commit e75bef2a4f ("arm64: Select ARCH_HAS_FAST_MULTIPLIER") this is a further improvement over the variant using only bitwise operations on at least some hardware, and no worse on other.Suggested-by: Andrew Cooper <andrew.coop...@citrix.com> Signed-off-by: Jan Beulich <jbeul...@suse.com> --- RFC: To be honest I'm not fully convinced this is a win in particular in the hweight32() case, as there's no actual shift insn which gets replaced by the multiplication. Even for hweight64() the compiler could emit better code and avoid the explicit shift by 32 (which it emits at least for me).
I can see multiplication instruction used in both hweight32() and hweight64() with the compiler I am using.
I would expect the compiler could easily replace a multiply by a series of shift but it would be more difficult to do the invert.
Also, this has been in Linux for a year now, so I am assuming Linux folks are happy with changes (CCing Robin just in case I missed anything). Therefore I am happy to give it a go on Xen as well.
Cheers,
--- a/xen/arch/arm/Kconfig +++ b/xen/arch/arm/Kconfig @@ -12,6 +12,7 @@ config ARM_32 config ARM_64 def_bool y depends on 64BIT + select HAS_FAST_MULTIPLYconfig ARMdef_bool y
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