public inbox for gcc-bugs@sourceware.org help / color / mirror / Atom feed
From: "fb.programming at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug other/51041] New: g++ strange optimisation behaviour Date: Tue, 08 Nov 2011 22:21:00 -0000 [thread overview] Message-ID: <bug-51041-4@http.gcc.gnu.org/bugzilla/> (raw) http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51041 Bug #: 51041 Summary: g++ strange optimisation behaviour Classification: Unclassified Product: gcc Version: 4.6.2 Status: UNCONFIRMED Severity: normal Priority: P3 Component: other AssignedTo: unassigned@gcc.gnu.org ReportedBy: fb.programming@gmail.com The attached code repeatedly executes a vector * vector product to test the performance of the system. Compiled with g++ -Wall -O2 file.cpp it results in a performance of about 1.7 Gflops on an Intel i5-750, ie the output is adding: 0.059 s, 1.695 GFlops, sum=0.000000 However, when adding another printf (remove the comment in front of the last printf) the performance deteriorates strongly (same compiler options): adding: 0.195 s, 0.512 GFlops, sum=0.000000 sum=0.000000 It seems the last printf confuses the compiler optimisation completely, although it shouldn't make a difference at all, as the same variable is already printed a few lines above. This is worrying as it seems the compiler fails to fully optimise the code under odd circumstances. I've used compiler version 4.6.2 as well as 4.4.1 which is the default compiler on the system. $ gcc-4.6.2 --version gcc-4.6.2 (GCC) 4.6.2 $ gcc --version gcc (SUSE Linux) 4.4.1 [gcc-4_4-branch revision 150839] $ uname -a Linux localhost 2.6.31.14-0.8-desktop #1 SMP PREEMPT 2011-04-06 18:09:24 +0200 x86_64 x86_64 x86_64 GNU/Linux
next reply other threads:[~2011-11-08 22:19 UTC|newest] Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top 2011-11-08 22:21 fb.programming at gmail dot com [this message] 2011-11-08 22:23 ` [Bug other/51041] " fb.programming at gmail dot com 2011-11-08 22:31 ` [Bug rtl-optimization/51041] " pinskia at gcc dot gnu.org 2023-07-07 15:13 ` [Bug rtl-optimization/51041] register allocation of SSE register in loop with across eh edges vmakarov at gcc dot gnu.org
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=bug-51041-4@http.gcc.gnu.org/bugzilla/ \ --to=gcc-bugzilla@gcc.gnu.org \ --cc=gcc-bugs@gcc.gnu.org \ /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 a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).