From: Roland Dreier <rdreier@cisco.com> To: David Miller <davem@davemloft.net> Cc: swise@opengridcomputing.com, randy.dunlap@oracle.com, linux-next@vger.kernel.org, linux-kernel@vger.kernel.org, general@lists.openfabrics.org Subject: Re: [ofa-general] [PATCH 2.6.30] RDMA/cxgb3: Remove modulo math. Date: Tue, 10 Feb 2009 17:18:49 -0800 [thread overview] Message-ID: <adaeiy5ahza.fsf@cisco.com> (raw) In-Reply-To: <20090210.170740.208470781.davem@davemloft.net> (David Miller's message of "Tue, 10 Feb 2009 17:07:40 -0800 (PST)") > > 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.
WARNING: multiple messages have this Message-ID (diff)
From: Roland Dreier <rdreier@cisco.com> To: David Miller <davem@davemloft.net> 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: Tue, 10 Feb 2009 17:18:49 -0800 [thread overview] Message-ID: <adaeiy5ahza.fsf@cisco.com> (raw) In-Reply-To: <20090210.170740.208470781.davem@davemloft.net> (David Miller's message of "Tue, 10 Feb 2009 17:07:40 -0800 (PST)") > > 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.
next prev parent reply other threads:[~2009-02-11 1:19 UTC|newest] Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top 2009-02-10 18:44 [PATCH 2.6.30] RDMA/cxgb3: Remove modulo math Steve Wise 2009-02-10 18:44 ` [ofa-general] " Steve Wise 2009-02-10 19:04 ` Randy Dunlap 2009-02-10 19:04 ` [ofa-general] " Randy Dunlap 2009-02-10 19:10 ` Steve Wise 2009-02-10 19:10 ` [ofa-general] " 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 2009-02-11 1:07 ` David Miller 2009-02-11 1:18 ` Roland Dreier [this message] 2009-02-11 1:18 ` Roland Dreier 2009-02-11 1:23 ` David Miller 2009-02-11 1:23 ` David Miller 2009-02-11 7:20 ` Roland Dreier 2009-02-11 7:20 ` Roland Dreier 2009-02-11 8:00 ` David Miller 2009-02-11 1:03 ` Steve Wise 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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