Re: [PATCH v4 2/3] x86: use __builtin_popcountl() for arch_hweightl() when available

[Andrew Cooper <andrew.cooper3@citrix.com>]

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@suse.com>
> ---
> 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.

diff --git a/xen/arch/x86/include/asm/bitops.h
b/xen/arch/x86/include/asm/bitops.h
index 557cee03baeb..c38680e8888c 100644
--- 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