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Re: [PATCH v4 2/3] x86: use __builtin_popcountl() for arch_hweightl() when available
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 15 Oct 2026 14:22:46 +0200
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- Cc: Teddy Astie <teddy.astie@xxxxxxxxxx>, Roger Pau Monné <roger@xxxxxxxxxxxxxx>, Jason Andryuk <jason.andryuk@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 15 Oct 2026 12:22:57 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 15.10.2026 12:37, Andrew Cooper wrote:
> On 15/10/2026 10:10 am, Jan Beulich wrote:
>> Use the respective compiler builtin when available; no need to play any
>> alternatives games in such configurations.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> v4: Re-base (re-do).
>>
>> --- a/xen/arch/x86/include/asm/bitops.h
>> +++ b/xen/arch/x86/include/asm/bitops.h
>> @@ -500,6 +500,9 @@ static always_inline unsigned int arch_f
>>
>> unsigned int arch_generic_hweightl(unsigned long x);
>>
>> +#ifdef __POPCNT__
>> +#define arch_hweightl __builtin_popcountl
>> +#else /* !__POPCNT__ */
>> static always_inline unsigned int arch_hweightl(unsigned long x)
>> {
>> unsigned int r;
>> @@ -525,5 +528,6 @@ static always_inline unsigned int arch_h
>> return r;
>> }
>> #define arch_hweightl arch_hweightl
>> +#endif /* __POPCNT__ */
>>
>> #endif /* _X86_BITOPS_H */
>>
>
> This is needlessly complex.
I don't mind doing it this way. (It's less complex in source then, but more
complex for the compiler.)
Jan
> --- a/xen/arch/x86/include/asm/bitops.h
> +++ b/xen/arch/x86/include/asm/bitops.h
> @@ -504,6 +504,10 @@ static always_inline unsigned int
> arch_hweightl(unsigned long x)
> {
> unsigned int r;
>
> + /* If compiled with a suitable baseline ISA. */
> + if ( IS_ENABLED(__POPCNT__) )
> + return __builtin_popcountl(x);
> +
> /*
> * arch_generic_hweightl() is written in ASM in order to preserve all
> * registers, as the compiler can't see the call.
>
>
> ~Andrew
|
