From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00,FREEMAIL_FROM autolearn=ham autolearn_force=no version=3.4.4 X-Google-Thread: 103376,103803355c3db607 X-Google-NewGroupId: yes X-Google-Attributes: gida07f3367d7,domainid0,public,usenet X-Google-Language: ENGLISH,ASCII-7-bit Received: by 10.68.228.227 with SMTP id sl3mr6581294pbc.5.1342340859908; Sun, 15 Jul 2012 01:27:39 -0700 (PDT) Path: l9ni11846pbj.0!nntp.google.com!news1.google.com!postnews.google.com!glegroupsg2000goo.googlegroups.com!not-for-mail From: Keean Schupke Newsgroups: comp.lang.ada Subject: Re: GNAT (GCC) Profile Guided Compilation Date: Sun, 15 Jul 2012 01:27:39 -0700 (PDT) Organization: http://groups.google.com Message-ID: <505460ad-495d-41b4-a88f-e95eb99a6be3@googlegroups.com> References: <38b9c365-a2b2-4b8b-8d2a-1ea39d08ce86@googlegroups.com> <982d531a-3972-4971-b802-c7e7778b8649@googlegroups.com> <520bdc39-6004-4142-a227-facf14ebb0e8@googlegroups.com> <4ff08cb2$0$6575$9b4e6d93@newsspool3.arcor-online.net> <4ff1d731$0$6582$9b4e6d93@newsspool3.arcor-online.net> <4ff41d38$0$6577$9b4e6d93@newsspool3.arcor-online.net> <26b778c4-5abc-4fbf-94b0-888c2ce71831@googlegroups.com> <4ff43956$0$6576$9b4e6d93@newsspool3.arcor-online.net> <2dba1140-4f28-4fb8-ace4-2c10f3a02313@googlegroups.com> NNTP-Posting-Host: 82.44.19.199 Mime-Version: 1.0 X-Trace: posting.google.com 1342340859 6417 127.0.0.1 (15 Jul 2012 08:27:39 GMT) X-Complaints-To: groups-abuse@google.com NNTP-Posting-Date: Sun, 15 Jul 2012 08:27:39 +0000 (UTC) In-Reply-To: Complaints-To: groups-abuse@google.com Injection-Info: glegroupsg2000goo.googlegroups.com; posting-host=82.44.19.199; posting-account=T5Z2vAoAAAB8ExE3yV3f56dVATtEMNcM User-Agent: G2/1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Date: 2012-07-15T01:27:39-07:00 List-Id: On Sunday, 15 July 2012 08:15:35 UTC+1, Niklas Holsti wrote: > On 12-07-15 02:40 , Keean Schupke wrote: > > After a bit of checking line-by-line to make sure I am using 'an= d > > then', 'or else' and 'constant' everywhere I can= in the code, Ada is > > outperforming C++ when using profile-guided compilation for the firs= t > > time. both C++ and Ada are getting about 40k simulations per second > > with normal compilation, C++ is achieving 56k simulations per second > > profile-guided, and Ada 57k per second. >=20 > Good news. >=20 > An Ada copmiler is of course free to implement the ordinary=20 > "long-circuit" operators "and"/"or" using s= hort-circuit code, if the=20 > evaluation of the operands has no side effects. What are the operands=20 > typically like in your program? Are they function calls, or simple=20 > variables? >=20 > It seems to me that the general belief, regarding the expected relative= =20 > speeds of the short-circuit code versus the long-circuit code for=20 > Boolean expressions with simple operands, is that the branch penalties=20 > on modern processors are so large that the short-circuit form is not=20 > obviously faster. This may explain why the Ada compiler is not using the= =20 > short-circuit code automatically. >=20 > Clearly, if the expression is "(simple operand likely to be True) an= d=20 > (longer and longer expression)", at some point the short-circuit cod= e=20 > (or changing to "and then") will become faster than the long-ci= rcuit=20 > code, whatever the branch penalty. This point will come sooner if=20 > profile-guidance is used to reduce the branch penalty. >=20 > --=20 > Niklas Holsti > Tidorum Ltd > niklas holsti tidorum fi > . @ . The code is side effect free. I wonder if using "pragma Pure_Function(X)" w= ould help here? Actually I think the assumption that branch misprediction has a high cost i= s wrong on modern speculative processors. In my tests on different core2 pr= ocessors simple branching beats all the branchless methods. This is probabl= y because they can decode up to multiple instructions per cycle, and execut= e both branch paths in parallel speculatively, so a branch misprediction ju= st throws away the micro-ops from the not-taken path, and the cost is fairl= y low. I realise this somewhat contradicts what I said about profile guided= compilation, but there is still a higher cost to misprediction, and for my= code that was worth about 6k simulations using profile-guided compilation.= However the short-circuit execution plus profile guided compilation was wo= rth another 10k simulations. It would appear the biggest benefit is in re-a= rranging the order of the clauses of boolean logic so the most often taken = short-circuit path is first, although the re-arranging of general branches = still has a significant effect. The most critical Boolean expression has the a format like this: function F(N : Node, V : Value) return Boolean is begin ( return (N.Enum =3D Const) or else ((N.Enum =3D V) =3D (N.Number =3D 0))= ; ) B : constant Boolean =3D F(N1, V)=20 and then F(N2, V) and then F(N3, V) and then F(N4, V); Cheers, Keean.