linux-kernel - Re: [PATCH v2] x86/hweight: Fix and improve __arch

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Message-ID: <Z9F5h7cEiV55dWMB@gmail.com>
Date: Wed, 12 Mar 2025 13:09:43 +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 v2] 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. This unconstrained scheduling might
> cause objtool to print a "call without frame pointer save/setup"
> warning. Current versions of compilers don't seem to trigger this
> condition, but without this constraint there's nothing to prevent
> the compiler from scheduling the insn in front of frame creation.
> 
> 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.

I think this should be a 3-patch series. While it all modifies the same 
code, each change could introduce separate issues, so it's better to be 
individually bisectable IMO ...

Thanks,

	Ingo