linux-kernel - Re: [PATCH] __div64_32: implement division by multiplication for 32-bit arches

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Date:	Fri, 30 Oct 2015 15:54:02 +0000
From:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>
CC:	"shemminger@...ux-foundation.org" <shemminger@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Vineet.Gupta1@...opsys.com" <Vineet.Gupta1@...opsys.com>,
	"linux-snps-arc@...ts.infradead.org" 
	<linux-snps-arc@...ts.infradead.org>,
	"mans@...sr.com" <mans@...sr.com>,
	"rmk+kernel@....linux.org.uk" <rmk+kernel@....linux.org.uk>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"mingo@...e.hu" <mingo@...e.hu>
Subject: Re: [PATCH] __div64_32: implement division by multiplication for
 32-bit arches

Hi Nicolas,

On Fri, 2015-10-30 at 11:17 -0400, Nicolas Pitre wrote:
> On Fri, 30 Oct 2015, Måns Rullgård wrote:
> 
> > Nicolas Pitre <nicolas.pitre@...aro.org> writes:
> > 
> > > OK... I was intrigued, so I adapted my ARM code to the generic case, 
> > > including the overflow avoidance optimizations.  Please have look and 
> > > tell me how this works for you.
> > > 
> > > If this patch is accepted upstream, then it could be possible to 
> > > abstract only the actual multiplication part with some architecture 
> > > specific assembly.
> > 
> > Good idea.
> 
> Could you please provide a reviewed-by or acked-by tag?

Sure!

Acked-by: Alexey Brodkin <abrodkin@...opsys.com>

BTW I thought about that optimization a bit more and now I think
we may even skip addition of arch-specific assembly insertions.

That's because that kind of division as discussed many times
should be used as limited as possible, in other words there should be
just a very few usages of it especially in very frequently used code paths.
And in that case there might be not much of benefit having do_div()
even faster and smaller than the one we're about to get with your change.

-Alexey