linux-kernel - Re: [PATCH -tip 2/2] x86/hweight: Use POPCNT when available with X86_NATIVE

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Message-ID: <Z-mZfCvep7iMrqoU@gmail.com>
Date: Sun, 30 Mar 2025 21:20:28 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Borislav Petkov <bp@...en8.de>
Cc: Uros Bizjak <ubizjak@...il.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH -tip 2/2] x86/hweight: Use POPCNT when available with
 X86_NATIVE_CPU option


* Borislav Petkov <bp@...en8.de> wrote:

> On Sun, Mar 30, 2025 at 08:47:31PM +0200, Ingo Molnar wrote:
> >  +#ifdef __POPCNT__
> >  +     asm_inline (ASM_FORCE_CLR "popcntl %[val], %[cnt]"
> >  +                 : [cnt] "=&r" (res)
> >  +                 : [val] ASM_INPUT_RM (w));
> >  +#else
> >        asm_inline (ALTERNATIVE(ANNOTATE_IGNORE_ALTERNATIVE
> >                                "call __sw_hweight32",
> >                                ASM_CLR "popcntl %[val], %[cnt]",
> >                                X86_FEATURE_POPCNT)
> >                         : [cnt] "=a" (res), ASM_CALL_CONSTRAINT
> >                         : [val] REG_IN (w));
> >  -
> >  +#endif
> 
> That ifdeffery.
> 
> The alternative only is fine as this is the usual way we do those 
> insns.
> 
> The ifdeffery around it is ugly and is pushing it and it would be 
> fine if it would bring anything but it doesn't. It is making the code 
> ugly for no good reason whatsoever.

Tangible code size reduction, if it can be realized, is definitely 
'something', so your claim is simply false.

> > Which is 3 straightforward lines of assembly code and a 
> > straightforward #ifdef.
> 
> And they bring what exactly?
> 
> I haven't seen anything besides some super minor, completely 
> pointless, hm, "savings". So much so that the uglification of the 
> function is not worth it in the *least*.

Even 0.5K of .text reduction is a tangible benefit.

The kernel's 35 years long history comprises of literally over a 
million patches, which were small and inconsequential 99% of the time.

> > My main objection is different: if __POPCNT__ isn't defined during 
> > the kernel build of major Linux distros, then this optimization 
> > almost doesn't exist to our users. And I don't think it's defined.
> 
> Yah, that too.
> 
> This whole effort is a total waste of time and energy.

We don't know yet for sure, but I don't think an absolutist "can't do" 
approach is very productive.

Thanks,

	Ingo