Re: [Xen-devel] [PATCH RFC 3/4] Arm64: further speed-up to hweight{32, 64}()

[Robin Murphy <robin.murphy@xxxxxxx>]

On 04/06/2019 17:11, Julien Grall wrote:
> 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.cooper3@xxxxxxxxxx>
> > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> > ---
> > 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.
IIRC it did look like Linux's hweight() routines could possibly be 
further tuned for the A64 ISA to shave off one or two more instructions, 
but it's yet to be proven that performance is anywhere near critical 
enough to justify maintaining arch-specific versions. It costs us 
basically nothing to switch between the two generic implementations 
though, so since the smaller-and-no-slower code can be considered a net 
win (however minor), there seemed no reason *not* to apply the existing 
option appropriately.
Robin.

> 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_MULTIPLY
> >   config ARM
> >       def_bool y
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