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

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Date:	Tue, 10 Feb 2009 17:18:49 -0800
From:	Roland Dreier <rdreier@...co.com>
To:	David Miller <davem@...emloft.net>
Cc:	swise@...ngridcomputing.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.

> > 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)

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

Are you thinking of the case when there are signed integers involved and
so "% modulus" might produce a different result than "& (modulus - 1)"
(because the compiler can't know that things are never negative)?
Because in this case the compiler seems to do what I thought it would;
the relevant part of the i386 assembly for

		wqe->recv.sgl[i].to = cpu_to_be64(((u32) wr->sg_list[i].addr) %
				(1UL << (12 + page_size[i])));

is

        movl    %eax, 28(%edi,%ebx)     # <variable>.length,
        <variable>.len
        movzbl  28(%esp,%esi), %ecx     # page_size, tmp89
        movl    $1, %eax        #, tmp92
        addl    $12, %ecx       #, tmp90
        sall    %cl, %eax       # tmp90, tmp92
        movl    (%esp), %ecx    # wr,
        decl    %eax    # tmp93
        movl    12(%ecx), %edx  # <variable>.sg_list, <variable>.sg_list
        andl    (%edx,%ebx), %eax       # <variable>.addr, tmp93

ie the compiler computes the modulus, then does decl to compute
modulus-1 and then &s with it.

Or am I misunderstanding your point?

 - R.
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