linux-kernel - Re: [PATCH 2/5] bitops: compile time optimization for hweight

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Date:	Wed, 03 Feb 2010 09:10:19 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Jamie Lokier <jamie@...reable.org>,
	Roland Dreier <rdreier@...co.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 2/5] bitops: compile time optimization for	hweight_long(CONSTANT)

On 02/03/2010 05:39 AM, Wu Fengguang wrote:
> 
>> (I wonder if it could have used __builtin_popcount)
>>
>> (I wonder which gcc versions support __builtin_popcount)
> 

I think 4.0 or later, but it often generates calls to libgcc, which
would mean adding libgcc functions to the kernel (which some arches
already do.)

> This is how Jamie Lokier first recommended it to me:
> 
>         Checked GCC 3.4.3 / 4.1 / 4.4: It expands as a compile-time
>         constant if the argument is a compile-time constant, so can be
>         used in BUILD_BUG_ON() and even for array sizes etc.
> 
> Is there an official lowest GCC version that Linux supports?

3.4, I believe.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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