linux-kernel - Re: [ofa-general] [PATCH 2.6.30] RDMA/cxgb3: Remove modulo math.

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Date:	Tue, 10 Feb 2009 17:07:40 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	swise@...ngridcomputing.com
Cc:	rdreier@...co.com, randy.dunlap@...cle.com,
	linux-next@...r.kernel.org, linux-kernel@...r.kernel.org,
	general@...ts.openfabrics.org
Subject: Re: [ofa-general] [PATCH 2.6.30] RDMA/cxgb3: Remove modulo math.

From: Steve Wise <swise@...ngridcomputing.com>
Date: Tue, 10 Feb 2009 19:03:52 -0600

> Roland Dreier wrote:
> > I'll roll this into the offending patch (that is in -next).
> >
> > But:
> >
> >  > -		wqe->recv.sgl[i].to = cpu_to_be64(((u32) wr->sg_list[i].addr) %
> >  > -				(1UL << (12 + page_size[i])));
> >  > +		wqe->recv.sgl[i].to = cpu_to_be64(((u64) wr->sg_list[i].addr) &
> >  > +				((1UL << (12 + page_size[i]))-1));
> >
> > Is this required?  Strength reduction optimization should do this
> > automatically (and the code has been there for quite a while, so
> > obviously it isn't causing problems)
> >
> >  - R.
> >   
> Ok.

GCC won't optimize that modulus the way you expect, try for yourself
and look at the assembler if you don't believe me. :-)

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