linux-kernel - Re: [PATCH 2/2] x86/bitops: Fix false output register dependency of TZCNT insn

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Message-ID: <Z-MjgKSzJKIVooFI@gmail.com>
Date: Tue, 25 Mar 2025 22:43:28 +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 2/2] x86/bitops: Fix false output register dependency of
 TZCNT insn


* Uros Bizjak <ubizjak@...il.com> wrote:

> On Haswell and later Intel processors, the TZCNT instruction appears
> to have a false dependency on the destination register. Even though
> the instruction only writes to it, the instruction will wait until
> destination is ready before executing. This false dependency
> was fixed for Skylake (and later) processors.
> 
> Fix false dependency by clearing the destination register first.
> 
> The x86_64 defconfig object size increases by 4215 bytes:
> 
> 	    text           data     bss      dec            hex filename
> 	27342396        4642999  814852 32800247        1f47df7 vmlinux-old.o
> 	27346611        4643015  814852 32804478        1f48e7e vmlinux-new.o

Yeah, so Skylake was released in 2015, about a decade ago.

So we'd be making the kernel larger for an unquantified 
micro-optimization for CPUs that almost nobody uses anymore.
That's a bad trade-off.

Thanks,

	Ingo