From -7403499999666511977 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,fea2c3a38bda88aa X-Google-Attributes: gidf78e5,public From: "Paul D. DeRocco" Subject: Re: memcmp source Date: 1997/12/17 Message-ID: <34986660.F999A5E8@ix.netcom.com>#1/1 X-Deja-AN: 299188827 References: <01bd073a$216f93e0$245b9b26@GMorris.dallasmfg.com> <34921798.9351A27@ix.netcom.com> X-Original-Date: Wed, 17 Dec 1997 18:55:12 -0500 Originator: austern@isolde.mti.sgi.com Organization: The Booboisie X-NETCOM-Date: Wed Dec 17 3:57:00 PM PST 1997 X-Auth: PGPMoose V1.1 PGP comp.std.c++ iQBVAwUBNJhwUUy4NqrwXLNJAQHivQH8CLjZp6P2fMrZ8qNphTS7+jCs/aXTFUTL zCACEb5uoS1C2amEcq1HFrmmCJmznAT9nO7O3vWP16wOrhk08k84sA== =t5EQ Newsgroups: comp.std.c++ J. Kanze wrote: > > If given constants, or values about which the compiler has some > information, the compiler can generate optimal inline code without the > tests. On an Intel architecture, for example, it would be a very poor > compiler that would still use movsb when the length were an even > constant, for example. From actual measurements, done a long time ago > on an 8086, over actual code, using different implementations of memcpy > (as a function, thus, always with run-time tests), shifting the count > right and copying words was a definite win, even with relatively short > strings, despite the overhead of the extra tests. On the other hand, > trying to align wasn't: statistically, enough of the > sources/destinations were already aligned so that the rare improvement > didn't offset the tests of the other calls. What you're saying is undoubtedly correct for memcpy, but the original question was about memcmp. While it is certainly common to do big memcpy's (e.g., disk buffers), I think memcmp's are typically much shorter, so it may not be worth even the small optimization you suggest. For instance, the Borland compiler's library functions (which are used when intrinsics are disabled) perform this optimization for memcpy but not memcmp. I did a version of memcpy once that did a simple test at the beginning: if the count was greater than a certain value (I think I used 16), I did the full-blown optimization, including alignment. Otherwise, it just moved bytes. -- Ciao, Paul --- [ comp.std.c++ is moderated. To submit articles: Try just posting with your newsreader. If that fails, use mailto:std-c++@ncar.ucar.edu comp.std.c++ FAQ: http://reality.sgi.com/austern/std-c++/faq.html Moderation policy: http://reality.sgi.com/austern/std-c++/policy.html Comments? mailto:std-c++-request@ncar.ucar.edu ]