linux-kernel - Re: [tip:x86/asm] x86: Prefer TZCNT over BFS

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Message-ID: <CA+55aFxs3FUETs830p-ow29RDibu9=d77gxqaueX-wZyr8-40A@mail.gmail.com>
Date:	Fri, 14 Sep 2012 14:14:57 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	mingo@...nel.org, hpa@...or.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, JBeulich@...e.com,
	tglx@...utronix.de
Cc:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/asm] x86: Prefer TZCNT over BFS

On Thu, Sep 13, 2012 at 11:23 PM, tip-bot for Jan Beulich
<JBeulich@...e.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.

                Linus
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