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From: "Craig S. Kaplan" <csk@cgl.uwaterloo.ca> To: nobody@gcc.gnu.org Cc: gcc-prs@gcc.gnu.org, Subject: Re: other/10417: Inconsistent results in floating point comparison. Date: Wed, 16 Apr 2003 14:46:00 -0000 [thread overview] Message-ID: <20030416144600.14364.qmail@sources.redhat.com> (raw) The following reply was made to PR other/10417; it has been noted by GNATS. From: "Craig S. Kaplan" <csk@cgl.uwaterloo.ca> To: tprince@computer.org Cc: gcc-gnats@gcc.gnu.org Subject: Re: other/10417: Inconsistent results in floating point comparison. Date: Wed, 16 Apr 2003 10:40:02 -0400 (EDT) Tim, Thanks for the insights. If you've got the time, I have some additional comments below. > > --------------------- > > #include <stdio.h> > > #include <stdlib.h> > > > > int main( int argc, char ** argv ) > > { > > double x = atof( argv[1] ); > > double d = x*x; > > > > printf( "%d\n", (x*x<d) ); > > return 0; > > } > > ----------------------- > > > > Ideally, this program should always print 0, since a number shouldn't be > > strictly less than itself. When compiled without optimization, the program > > will print 0 or 1 depending on the input (I get 0.3 --> 0, 0.4 --> 1, for > > instance). > > > > Now I understand that floating point numbers are far from ideal, and that > > this behaviour might not be a bug. Still, I would love to be able to > > characterize for which numbers the program will print 0 or 1. Any > > thoughts? > If you ask the compiler to generate x87 code (the probable default for your > configuration), it could interpret the expression as > (long double)x*x < d; > so the expression would be 1 every time d has been rounded up. > If you use the command > gcc -march=pentium4 -mfpmath=sse *.c > as you might normally do for a P4, I doubt you could get the results you > mention. Surprisingly, I _do_ still get the strange behaviour with those switches. But I am beginning to understand where the behaviour may be coming from, based on your comments and the comments of another maintainer who emailed me (ebotcazou). When d is stored in a variable, some extra bits of precision that existed in the FPU are discarded. Now, you seem to suggest that the way the FPU does this is to _round_ d (rather than just truncating). My simple program seems to generate random output for different floats, but actually I'm just seeing the FPU's rounding semantics -- its decision whether to round up or down. Does that sound believable to you? Of course, if this is the case, we shouldn't expect -mfpmath=sse to solve the 'problem' -- we're still rounding x*x to store the answer in d. The other maintainer suggested that I needed '-ffloat-store' to guarantee strict IEEE semantics. I tried that switch, and it seemed to make the behaviour a little more consistent. It does remove the randomness from the large program I'm working on, but amazingly it doesn't fix my toy example. Using gcc 3.2.1, the only way I can make the short program above output 0 consistently is to compile it -O2. The -ffloat-store, -march=pentium4, and -mfpmath=sse switches don't seem to do anything. Any thoughts on why I'm not seeing a difference here? Thanks again. -- Craig S. Kaplan ``Evolution drives a steamroller, School of Computer Science disguised as a stationary bike.'' University of Waterloo http://www.cgl.uwaterloo.ca/~csk/
next reply other threads:[~2003-04-16 14:46 UTC|newest] Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top 2003-04-16 14:46 Craig S. Kaplan [this message] -- strict thread matches above, loose matches on Subject: below -- 2003-04-16 7:30 ebotcazou 2003-04-16 4:56 Tim Prince 2003-04-15 19:06 csk
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