From: Borislav Petkov <bp@alien8.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: mingo@kernel.org, hpa@zytor.com, linux-kernel@vger.kernel.org,
JBeulich@suse.com, tglx@linutronix.de,
linux-tip-commits@vger.kernel.org
Subject: Re: [tip:x86/asm] x86: Prefer TZCNT over BFS
Date: Fri, 14 Sep 2012 23:32:39 +0200 [thread overview]
Message-ID: <20120914213239.GB29307@liondog.tnic> (raw)
In-Reply-To: <CA+55aFxs3FUETs830p-ow29RDibu9=d77gxqaueX-wZyr8-40A@mail.gmail.com>
On Fri, Sep 14, 2012 at 02:14:57PM -0700, Linus Torvalds wrote:
> On Thu, Sep 13, 2012 at 11:23 PM, tip-bot for Jan Beulich
> <JBeulich@suse.com> wrote:
> >
> > x86: Prefer TZCNT over BFS
>
> This patch is insane.
>
> > For the moment, only do this when the respective generic-CPU
> > option is selected (as there are no specific-CPU options
> > covering the CPUs supporting TZCNT), and don't do that when size
> > optimization was requested.
>
> This is pure garbage.
>
> Anybody who thinks this:
>
> > +#if (defined(CONFIG_X86_GENERIC) || defined(CONFIG_GENERIC_CPU)) \
> > + && !defined(CONFIG_CC_OPTIMIZE_FOR_SIZE)
>
> is a good idea should be shot. Don't do it.
>
> Introduce a new CONFIG variable with a sane name, for chrissake, the
> same way we have CONFIG_X86_XADD etc. It would be logical to call it
> X86_TZCNT, wouldn't it?
>
> And then add sane rules for that in the x86 config file. And no, the
> above is *NOT* a sane rule at all. If I read that right, it will
> enable TZCNT even for old 32-bit CPU's, for example. That's just
> f*cking insane.
>
> Stop this kind of idiocy. The code looks bad, and the logic is pure shit too.
And the other important question is, is this even worth the complexity?
I mean "may execute that faster than 'bsf ...'" doesn't mean a lot so
can anyone remind me again why we're doing this?
Any hot paths I've missed?
Thanks.
--
Regards/Gruss,
Boris.
next prev parent reply other threads:[~2012-09-14 21:32 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-09-10 11:24 [PATCH] x86: prefer tzcnt over bsf Jan Beulich
2012-09-14 6:23 ` [tip:x86/asm] x86: Prefer TZCNT over BFS tip-bot for Jan Beulich
2012-09-14 21:14 ` Linus Torvalds
2012-09-14 21:32 ` Borislav Petkov [this message]
2012-09-17 7:41 ` Jan Beulich
2012-09-17 10:00 ` Ingo Molnar
2012-09-17 10:58 ` Jan Beulich
2012-09-17 17:00 ` Linus Torvalds
2012-09-17 17:05 ` H. Peter Anvin
2012-09-17 17:13 ` Linus Torvalds
2012-09-18 6:40 ` Jan Beulich
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