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

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Message-ID: <4B6AE4BE.70600@zytor.com>
Date:	Thu, 04 Feb 2010 07:16:14 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...64.org>
CC:	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>
Subject: Re: [PATCH 2/5] bitops: compile time optimization for hweight_long(CONSTANT)

On 02/04/2010 07:10 AM, Borislav Petkov wrote:
>>
>> Arguably the "best" option is to have the alternative being a jump to an
>> out-of-line stub which does the necessary parameter marshalling before
>> calling a stub.  This technique is already used in a few other places.
> 
> Ok, here's a first alpha prototype and completely untested. The asm
> output looks ok though. I've added separate 32-bit and 64-bit helpers in
> order to dispense with the if-else tests. The hw-popcnt versions are the
> opcodes for "popcnt %eax, %eax" and "popcnt %rax, %rax", respectively,
> so %rAX has to be preloaded with the bitmask and the computed value
> has to be retrieved from there afterwards. And yes, it looks not that
> elegant so I'm open for suggestions.
> 
> The good thing is, this should work on any toolchain since we don't rely
> on the compiler to know about popcnt and we're protected by CPUID flag
> so that the hw-popcnt version is used only on processors which support
> it.
> 
> Please take a good look and let me know what do you guys think.
> 

I think you misunderstood me.  The idea was to use the alternatives
mechanism to put the popcnt instruction inline instead of the call.

	-hpa

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

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