Re: [tip:x86/asm] x86: Prefer TZCNT over BFS
From: Linus Torvalds
Date: Fri Sep 14 2012 - 17:15:11 EST
On Thu, Sep 13, 2012 at 11:23 PM, tip-bot for Jan Beulich
> 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
Stop this kind of idiocy. The code looks bad, and the logic is pure shit too.
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