From: David Miller <davem@davemloft•net>
To: rdreier@cisco•com
Cc: randy.dunlap@oracle•com, linux-next@vger•kernel.org,
general@lists•openfabrics.org, linux-kernel@vger•kernel.org
Subject: Re: [ofa-general] [PATCH 2.6.30] RDMA/cxgb3: Remove modulo math.
Date: Wed, 11 Feb 2009 00:00:49 -0800 (PST) [thread overview]
Message-ID: <20090211.000049.193727089.davem@davemloft.net> (raw)
In-Reply-To: <ada4oz1a188.fsf@cisco.com>
From: Roland Dreier <rdreier@cisco•com>
Date: Tue, 10 Feb 2009 23:20:39 -0800
> > unsigned long page_size[4];
> >
> > int main(int argc)
> > {
> > unsigned long long x = argc;
> >
> > return x % (1UL << (12 + page_size[argc]));
> > }
> >
> > I get a call to __umoddi3:
>
> You're not testing the same thing. The original code was:
>
> wqe->recv.sgl[i].to = cpu_to_be64(((u32) wr->sg_list[i].addr) %
> (1UL << (12 + page_size[i])));
>
> and it's not that easy to see with all the parentheses, but the
> expression being done is (u32) % (unsigned long). So rather than
> unsigned long long in your program, you should have just done unsigned
> (u32 is unsigned int on all Linux architectures). In that case gcc does
> not generate a call to any library function in all the versions I have
> handy, although gcc 4.1 does do a div instead of an and. (And I don't
> think any 32-bit architectures require a library function for (unsigned)
> % (unsigned), so the code should be OK)
>
> Your example shows that gcc is missing a strength reduction opportunity
> in not handling (u64) % (unsigned long) on 32 bit architectures, but I
> guess it is a more difficult optimization to do, since gcc has to know
> that it can simply zero the top 32 bits.
Indeed, I get the divide if I use "unsigned int" for "x".
I still think you should make this change, as many systems out
there are getting the expensive divide.
main:
sethi %hi(page_size), %g1
or %g1, %lo(page_size), %g1
mov %o0, %g3
sll %o0, 2, %g4
ld [%g1+%g4], %g2
mov 1, %g1
add %g2, 12, %g2
sll %g1, %g2, %g1
wr %g0, %g0, %y
nop
nop
nop
udiv %o0, %g1, %o0
smul %o0, %g1, %o0
jmp %o7+8
sub %g3, %o0, %o0
next prev parent reply other threads:[~2009-02-11 8:00 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-02-10 18:44 [ofa-general] [PATCH 2.6.30] RDMA/cxgb3: Remove modulo math Steve Wise
2009-02-10 19:04 ` [ofa-general] " Randy Dunlap
2009-02-10 19:10 ` Steve Wise
2009-02-10 19:12 ` Randy Dunlap
2009-02-11 0:38 ` [ofa-general] " Roland Dreier
2009-02-11 1:03 ` Steve Wise
[not found] ` <499223F8.1010204@opengridcomputing.com>
2009-02-11 1:07 ` David Miller
2009-02-11 1:18 ` Roland Dreier
2009-02-11 1:23 ` David Miller
2009-02-11 7:20 ` Roland Dreier
2009-02-11 8:00 ` David Miller [this message]
2009-02-11 15:44 ` Steve Wise
2009-02-11 18:12 ` Roland Dreier
2009-02-11 18:32 ` Steve Wise
2009-02-11 18:36 ` Roland Dreier
2009-02-11 18:44 ` Steve Wise
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