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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.06.19 at 11:24, <julien.grall@xxxxxxx> wrote:
> On 05/06/2019 08:42, Jan Beulich wrote:
>>>>> On 04.06.19 at 18:11, <julien.grall@xxxxxxx> wrote:
>>> 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.
>> 
>> That is for which exact implementation?
> 
> A simple call hweight64().

That's as ambiguous as it was before: Are you talking about
un-patched code (before this series), or patches up to at
least this one applied. What I'm trying to understand is if your
compiler is smart enough to use MUL without us telling it to.

Jan



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel