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

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Message-ID: <Z-cib74Y1NjB4huZ@gmail.com>
Date: Fri, 28 Mar 2025 23:27:59 +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 Tue, Mar 25, 2025 at 10:43 PM Ingo Molnar <mingo@...nel.org> wrote:
> >
> >
> > * 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.
> 
> Yes, 4.2k seems a bit excessive. OTOH, I'd not say that the issue is 
> a micro-optimization, it is bordering on the hardware bug.

Has this been quantified, and do we really care about the 
micro-performance of ~10-year old CPUs, especially at the
expense of modern CPUs?

Thanks,

	Ingo