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

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Message-ID: <Z89ImJlHFLJUDhFM@gmail.com>
Date: Mon, 10 Mar 2025 21:16:24 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Uros Bizjak <ubizjak@...il.com>
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


* 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.

Thanks,

	Ingo