linux-kernel - Re: [PATCH -tip 1/2] x86/hweight: Fix false output register dependency of POPCNT insn

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Message-ID: <20250325174430.GEZ-LrfugbyA0OxOnH@fat_crate.local>
Date: Tue, 25 Mar 2025 18:44:30 +0100
From: Borislav Petkov <bp@...en8.de>
To: Uros Bizjak <ubizjak@...il.com>
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 1/2] x86/hweight: Fix false output register
 dependency of POPCNT insn

On Tue, Mar 25, 2025 at 06:17:01PM +0100, Uros Bizjak wrote:
> Please see [1], errata 026.

Then next time put at least a reference to such official source. Because right
now it reads like a wild guess.

> There is a lot of performance analysis at [2] and [3].

You mean the commit message should not mention it - people should use a search
engine to find that?!

Lemme quote our documentation for you:

"Quantify optimizations and trade-offs. If you claim improvements in
performance, memory consumption, stack footprint, or binary size, include
numbers that back them up. But also describe non-obvious costs. Optimizations
usually aren’t free but trade-offs between CPU, memory, and readability; or,
when it comes to heuristics, between different workloads. Describe the
expected downsides of your optimization so that the reviewer can weigh costs
against benefits."

From: https://kernel.org/doc/html/latest/process/submitting-patches.html

Please peruse that document when you get a chance.

So my question still is: does that change has *any* *impact* on *kernel*
workloads?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette