public inbox for gcc-prs@sourceware.org help / color / mirror / Atom feed
From: sayle@gcc.gnu.org To: gcc-bugs@gcc.gnu.org, gcc-prs@gcc.gnu.org, murph@winface.com, nobody@gcc.gnu.org Subject: Re: other/5574: "http://ego.uwaterloo.ca/SpamGate?ip=216.94.64Off by two error produced by gcc -o test sqr.c -lm Date: Fri, 02 Aug 2002 20:38:00 -0000 [thread overview] Message-ID: <20020803033852.14638.qmail@sources.redhat.com> (raw) Synopsis: "http://ego.uwaterloo.ca/SpamGate?ip=216.94.64Off by two error produced by gcc -o test sqr.c -lm State-Changed-From-To: open->closed State-Changed-By: sayle State-Changed-When: Fri Aug 2 20:38:52 2002 State-Changed-Why: This is a well known issue with floating point calculations. See the section of "non-bugs" section of GCC's bugs WWW page http://gcc.gnu.org/bugs.html#nonbugs, particularly the paragraph entitled "Problems with floating point computations". The issue is "real" values cannot be represented precisely in the "floating point" representations used by current computer hardware. In your test case, when compiling without optimizations the program calls the system sqrt routine, and rounds both the sqrt result and the result of each addition to 64 bit precision by writing the results to memory on each iteration. With optimizations, the program uses the Pentium's fsqrt instruction and accumulates the result in floating point registers, both to 80 bits of precision. For more information, see the Goldberg paper linked to from the above URL, and the documentation of the "-ffloat-store" command line option in the GCC manual: http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#Optimize%20Options I hope this explains the odd behaviour. Roger -- http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=5574
next reply other threads:[~2002-08-03 3:38 UTC|newest] Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top 2002-08-02 20:38 sayle [this message] -- strict thread matches above, loose matches on Subject: below -- 2002-02-01 11:16 murph
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=20020803033852.14638.qmail@sources.redhat.com \ --to=sayle@gcc.gnu.org \ --cc=gcc-bugs@gcc.gnu.org \ --cc=gcc-gnats@gcc.gnu.org \ --cc=gcc-prs@gcc.gnu.org \ --cc=murph@winface.com \ --cc=nobody@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).