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

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Message-ID: <20100208092845.GB12618@a1.tnic>
Date:	Mon, 8 Feb 2010 10:28:45 +0100
From:	Borislav Petkov <petkovbb@...glemail.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Borislav Petkov <bp@...64.org>,
	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 Sat, Feb 06, 2010 at 05:55:47PM -0800, H. Peter Anvin wrote:
> > Well, the example Brian pointed me to - __mutex_fastpath_lock - lists
> > the full set of clobbered registers. Please elaborate on the assembly
> > wrapper for the function, wouldn't I need to list all the clobbered
> > registers there too or am I missing something?
> > 
> 
> The notion there would be that you do push/pop in the assembly wrapper.

Oh yes, something similar to SAVE/RESTORE_ALL in
<arch/x86/kernel/entry_32.S> could work. Good idea!

> >> d) On the other hand, you do *not* need a "memory" clobber.
> > 
> > Right, in this case we have all non-barrier like inlines so no memory
> > clobber, according to the comment above alternative() macro.
> 
> OK, I'm missing something here.
> 
> A few more notions:
> 
> a. This is exactly the kind of code where you don't want to put
> "volatile" on your asm statement, because it's a pure compute.
> 
> b. It is really rather pointless to go through the whole alternatives
> work if you are then going to put it inside a function which isn't an
> inline ...

Well, in the second version I did replace a 'call _hweightXX' with
the actual popcnt opcode so the alternatives is only needed to do the
replacement during boot. We might just as well do

if (X86_FEATURE_POPCNT)
	__hw_popcnt()
else
	__software_hweight()

The only advantage of the alternatives is that it would save us the
if-else test above each time we do cpumask_weight. However, the if-else
approach is much more readable and obviates the need for all that macro
magic and taking special care of calling c function from within asm. And
since we do not call cpumask_weight all that often I'll honestly opt for
alternative-less solution...

Hmm...

Thanks,
Boris.
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