linux-kernel - Re: [PATCH v2] x86: page: get

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Date:	Tue, 29 Mar 2011 09:27:01 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Maksym Planeta <mcsim.planeta@...il.com>, mingo@...hat.com,
	kernel-janitors@...r.kernel.org, namhyung@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86: page: get_order() optimization


* H. Peter Anvin <hpa@...or.com> wrote:

> On 03/27/2011 01:45 AM, Maksym Planeta wrote:
> > For x86 architecture get_order function can be optimized due to
> > assembler instruction bsr.
> > 
> > This is second version of patch where for constants gcc precompute the
> > result.
> > 
> > Signed-off-by: Maksym Planeta <mcsim.planeta@...il.com>
> 
> gcc 4.x has an intrinsic, __builtin_clz(), which does the opposite of
> the bsr instruction; specifically:
> 
> 	__builtin_clz(x) ^ 31
> 
> ... generates a bsrl instruction if x is variable.  This tends to
> generate much better code than any assembly hacks.

Indeed, that should work better and should be tried - and it can probably 
propagate the flags result sensibly (which GCC's asm() cannot, unfortunately).

Thanks,

	Ingo
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