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

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Message-ID: <4B7C29C1.10906@suse.cz>
Date:	Wed, 17 Feb 2010 18:39:13 +0100
From:	Michal Marek <mmarek@...e.cz>
To:	Borislav Petkov <bp@...64.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	linux-kbuild <linux-kbuild@...r.kernel.org>,
	Borislav Petkov <petkovbb@...glemail.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	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>, Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH 2/5] bitops: compile time optimization for hweight_long(CONSTANT)

On 17.2.2010 18:31, Michal Marek wrote:
> On 17.2.2010 18:20, Borislav Petkov wrote:
>> On Wed, Feb 17, 2010 at 02:57:42PM +0100, Michal Marek wrote:
>>> On 12.2.2010 20:05, H. Peter Anvin wrote:
>>>> On 02/12/2010 09:47 AM, Borislav Petkov wrote:
>>>>>
>>>>> However, this is generic code and for the above to work we have to
>>>>> enforce x86-specific CFLAGS for it. What is the preferred way to do
>>>>> that?
>>>>>
>>>>
>>>> That's a question for Michal and the kbuild list.  Michal?
>>>
>>> (I was offline last week).
>>>
>>> The _preferred_ way probably is not to do it :), but otherwise you can
>>> set CFLAGS_hweight.o depending on CONFIG_X86(_32|_64), just like you do
>>> in arch/x86/lib/Makefile already.
>>
>> Wouldn't it be better if we had something like ARCH_CFLAGS_hweight.o
>> which gets set in the arch Makefile instead?
> 
> We could, but is it worth it if there is only one potential user so far?
> IMO just put the condition to lib/Makefile now and if there turn out to
> be more cases like this, we can add support for ARCH_CFLAGS_foo.o then.

It wouldn't work actually, because such variable would then apply to all
hweight.o targets in the tree. But another way would be:

arch/x86/Kconfig
config ARCH_HWEIGHT_CFLAGS
        string
        default "..." if X86_32
        default "..." if X86_64

lib/Makefile
CFLAGS_hweight.o = $(CONFIG_ARCH_HWEIGHT_CFLAGS)

Michal
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