From 8507272844932342665 X-Google-Thread: f78e5,174aa7b34b06a51 X-Google-Attributes: gidf78e5,public X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news1.google.com!news4.google.com!news.glorb.com!news.wiretrip.org!border2.nntp.ams.giganews.com!nntp.giganews.com!newshub2.home.nl!newshub1.home.nl!home.nl!skynet.be!peer-uk.news.demon.net!kibo.news.demon.net!news.demon.co.uk!demon!stump.algebra.com!devnull From: SeeWebsiteForEmail@moderncppdesign.com ("Andrei Alexandrescu (See Website For Email)") Newsgroups: comp.std.c++ Subject: Re: Is this really unspecified behavior? Date: Sat, 10 Dec 2005 04:58:00 GMT Organization: Computer Science & Engineering, U of Washington, Seattle Lines: 42 Sender: mail2news@demon.net Approved: fjh@cs.mu.oz.au (Fergus Henderson , moderator of comp.std.c++) Message-ID: References: <1132759176.368850.264850@g43g2000cwa.googlegroups.com> <1132933832.802917.153350@g14g2000cwa.googlegroups.com> <1133551647.532929.325730@z14g2000cwz.googlegroups.com> <1133583412.951239.208510@g49g2000cwa.googlegroups.com> <1133910546.716987.80030@g14g2000cwa.googlegroups.com> <0l5ep1ppfhdj5p1pcu7e4na4ub0vjajlq3@4ax.com> <67pgp1p61j2vvlr8iphl28clsplb9lf6jk@4ax.c NNTP-Posting-Host: news.news.demon.net Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Trace: news.demon.co.uk 1134190686 8290 158.152.254.254 (10 Dec 2005 04:58:06 GMT) X-Complaints-To: abuse@demon.net NNTP-Posting-Date: Sat, 10 Dec 2005 04:58:06 +0000 (UTC) X-Nntp-Posting-Host: parakeet.ee.washington.edu X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.10) Gecko/20050716 Thunderbird/1.0.6 Mnenhy/0.7.2.0 X-Accept-Language: en-us, en X-Virus-Scanned: amavisd-new at cs.mu.OZ.AU X-Path: comp-std-cpp-robomod!not-for-mail X-Received: (from fjh@localhost) by mulga.cs.mu.OZ.AU (8.12.10+Sun/8.12.9/Submit) id jBA4w0mY007820; Sat, 10 Dec 2005 15:58:00 +1100 (EST) X-Authentication-Warning: mulga.cs.mu.OZ.AU: fjh set sender to devnull@stump.algebra.com using -f X-Newsgroups: comp.std.c++ Xref: g2news1.google.com comp.std.c++:2727 Herb Sutter wrote: > After sending this, I thought I should clarify something that on rereading > wasn't clear the way I first wrote it: > > hsutter@gotw.ca (Herb Sutter) wrote: > >> a) how much optimization loss there actually is on popular >>implementations and platforms (note this is likely to vary greatly by >>application) > > [...] > >>The >>most concrete information I know of, but which still needs measuring, is >>that a) is likely to cost several _times_ the throughput on some standard >>Spec microbenchmarks where we can get an order-of-magnitude perf gain by >>doing things like choosing to stride the other way across arrays (i.e., >>the program's loop strides rows then columns, and we know we'll get better >>cache behavior by reordering all or just chunks of the loop by striding >>columns then rows). But we don't know the cost on typical app code (e.g., >>some smart people I know expect <10%, maybe <5%, but we need to measure). > > > Specifically, the stride issue was specifically about the performance gain > from of a memory model latitude for reordering reads/writes, not latitude > for reordering expression evaluation. Ah, now that makes sense. (I was a bit confused.) So the jury is still out on finding cases (that are not source-level optimizable in an obvious way) in which a specified order of argument evaluation forces the compiler to generate pessimized code. Andrei --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ]