From mboxrd@z Thu Jan  1 00:00:00 1970
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Mon, 28 Mar 2011 19:47:57 +0000
Subject: Re: [PATCH v2] x86: page: get_order() optimization
Message-Id: <4D90E5ED.3080604@zytor.com>
List-Id: <kernel-janitors.vger.kernel.org>
References: <1301215556-8898-1-git-send-email-mcsim.planeta@gmail.com>
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To: Maksym Planeta <mcsim.planeta@gmail.com>
Cc: mingo@redhat.com, kernel-janitors@vger.kernel.org, namhyung@gmail.com, linux-kernel@vger.kernel.org

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@gmail.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.

	-hpa