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

Ingo Molnar
2025-03-28T22:28:03Z

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

> On Tue, Mar 25, 2025 at 10:43 PM Ingo Molnar <mingo@kernel.org> wrote:
> >
> >
> > * Uros Bizjak <ubizjak@gmail.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