linux-kernel - Re: [PATCH] x86/hweight: Fix and improve __arch

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Message-ID: <CAFULd4byd1v26FVJFEWedPCvpCSsSKUj0PKz8gOzdfMqdcOoAQ@mail.gmail.com>
Date: Mon, 10 Mar 2025 22:25:15 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, 
	Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/hweight: Fix and improve __arch_hweight{32,64}() assembly

On Mon, Mar 10, 2025 at 9:16 PM Ingo Molnar <mingo@...nel.org> wrote:
>
>
> * Uros Bizjak <ubizjak@...il.com> wrote:
>
> > a) Use ASM_CALL_CONSTRAINT to prevent inline asm that includes call
> > instruction from being scheduled before the frame pointer gets set
> > up by the containing function, causing objtool to print a "call
> > without frame pointer save/setup" warning.
> >
> > b) Use asm_inline to instruct the compiler that the size of asm()
> > is the minimum size of one instruction, ignoring how many instructions
> > the compiler thinks it is. ALTERNATIVE macro that expands to several
> > pseudo directives causes instruction length estimate to count
> > more than 20 instructions.
> >
> > c) Use named operands in inline asm.
> >
> > More inlining causes slight increase in the code size:
> >
> >    text    data     bss     dec     hex filename
> > 27261832        4640296  814660 32716788        1f337f4 vmlinux-new.o
> > 27261222        4640320  814660 32716202        1f335aa vmlinux-old.o
>
> What is the per call/inlining-instance change in code size, measured in
> fast-path instruction bytes? Also, exception code or cold branches near
> the epilogue of the function after the main RET don't fully count as a
> size increase.
>
> This kind of normalization and filtering of changes to relevant
> generated instructions is a better metric than some rather meaningless
> '+610 bytes of code' figure.
>
> Also, please always specify the kind of config you used for building
> the vmlinux.

Sorry, this just slipped my mind. x86_64 defconfig - I'll note this in
the revised commit entry.

BTW: The difference between old and new number of inlined __sw_hweight
calls is: 367 -> 396. I'll try to analyze this some more.

Thanks,
Uros.