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

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Message-ID: <CAFULd4ZT5mjjEXW3SnGQVMo18fO8CapFS-ikLQvYFw5EKtEfmA@mail.gmail.com>
Date: Sun, 30 Mar 2025 17:15:16 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Borislav Petkov <bp@...en8.de>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>, 
	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

On Tue, Mar 25, 2025 at 6:11 PM Borislav Petkov <bp@...en8.de> wrote:
>
> On Tue, Mar 25, 2025 at 05:48:38PM +0100, Uros Bizjak 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
>
> A whopping 599 bytes which makes the asm more ugly.
>
> Not worth the effort IMO.

You missed this part:

--q--
... where there is no need for an entry in the .altinstr_replacement
section, shrinking all text sections by 9476 bytes:

            text           data     bss      dec            hex filename
        27267068        4643047  814852 32724967        1f357e7 vmlinux-old.o
        27257592        4643047  814852 32715491        1f332e3 vmlinux-new.o
--/q--

Thanks,
Uros.