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 23:20:39 -0800 [thread overview] Message-ID: <ada4oz1a188.fsf@cisco.com> (raw) In-Reply-To: <20090210.172347.189515015.davem@davemloft.net> (David Miller's message of "Tue, 10 Feb 2009 17:23:47 -0800 (PST)") > Must be compiler and platform specific because with gcc-4.1.3 on > sparc with -O2, for the test program: > > 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. - 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, 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 23:20:39 -0800 [thread overview] Message-ID: <ada4oz1a188.fsf@cisco.com> (raw) In-Reply-To: <20090210.172347.189515015.davem@davemloft.net> (David Miller's message of "Tue, 10 Feb 2009 17:23:47 -0800 (PST)") > Must be compiler and platform specific because with gcc-4.1.3 on > sparc with -O2, for the test program: > > 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. - R.
next prev parent reply other threads:[~2009-02-11 7:20 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 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 [this message] 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
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=ada4oz1a188.fsf@cisco.com \ --to=rdreier@cisco.com \ --cc=davem@davemloft.net \ --cc=general@lists.openfabrics.org \ --cc=linux-kernel@vger.kernel.org \ --cc=linux-next@vger.kernel.org \ --cc=randy.dunlap@oracle.com \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is an external index of several public inboxes, see mirroring instructions on how to clone and mirror all data and code used by this external index.