digitalmars.D.bugs - [Issue 3171] New: % not implemented correctly for floats
- d-bugmail puremagic.com (20/20) Jul 13 2009 http://d.puremagic.com/issues/show_bug.cgi?id=3171
- d-bugmail puremagic.com (13/13) Jul 13 2009 http://d.puremagic.com/issues/show_bug.cgi?id=3171
- d-bugmail puremagic.com (16/21) Jul 14 2009 http://d.puremagic.com/issues/show_bug.cgi?id=3171
- d-bugmail puremagic.com (7/7) Jul 14 2009 http://d.puremagic.com/issues/show_bug.cgi?id=3171
- d-bugmail puremagic.com (20/22) Jul 14 2009 http://d.puremagic.com/issues/show_bug.cgi?id=3171
- d-bugmail puremagic.com (7/7) Jul 14 2009 http://d.puremagic.com/issues/show_bug.cgi?id=3171
- d-bugmail puremagic.com (11/11) Dec 06 2009 http://d.puremagic.com/issues/show_bug.cgi?id=3171
http://d.puremagic.com/issues/show_bug.cgi?id=3171 Summary: % not implemented correctly for floats Product: D Version: unspecified Platform: Other OS/Version: Linux Status: NEW Severity: normal Priority: P2 Component: DMD AssignedTo: nobody puremagic.com ReportedBy: andrei metalanguage.com Pasting a posting from Walter in digitalmars.d: Hmm, I just noticed that the code generator should use FPREM1 instead to get IEEE conformance. Darn. http://www.sesp.cse.clrc.ac.uk/html/SoftwareTools/vtune/users_guide/mergedProjects/analyzer_ec/mergedProjects/reference_olh/mergedProjects/instructions/instruct32_hh/vc108.htm http://www.sesp.cse.clrc.ac.uk/html/SoftwareTools/vtune/users_guide/mergedProjects/analyzer_ec/mergedProjects/reference_olh/mergedProjects/instructions/instruct32_hh/vc109.htm -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Jul 13 2009
http://d.puremagic.com/issues/show_bug.cgi?id=3171 Walter Bright <bugzilla digitalmars.com> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |bugzilla digitalmars.com 13:08:41 PDT --- As to why the code generator doesn't use FPREM1 instead of FPREM, there's the following comment: "We don't use fprem1 because for some inexplicable reason we get -5 when we do _modulo(15, 10)" This could be a bug in older CPUs. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Jul 13 2009
http://d.puremagic.com/issues/show_bug.cgi?id=3171 Don <clugdbug yahoo.com.au> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |clugdbug yahoo.com.auAs to why the code generator doesn't use FPREM1 instead of FPREM, there's the following comment: "We don't use fprem1 because for some inexplicable reason we get -5 when we do _modulo(15, 10)" This could be a bug in older CPUs.It isn't a bug. That's what the IEEE remainder specifies. Note that C's fmod is NOT the same as IEEE remainder. 15/10 = 1.5, so there's a choice of n == 1 or n==2. The standard specifies even n in such cases, so r == a - b*n == 15 - 2*10 == -5. That's kind of... weird, highly non-intuitive, and not terribly useful. I'm pretty sure that that behaviour would be unpopular. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Jul 14 2009
http://d.puremagic.com/issues/show_bug.cgi?id=3171 02:27:46 PDT --- Thanks for the explanation. At least I know why that happens, now. What do you suggest, then? Staying with FPREM or going with FPREM1 ? -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Jul 14 2009
http://d.puremagic.com/issues/show_bug.cgi?id=3171Thanks for the explanation. At least I know why that happens, now. What do you suggest, then? Staying with FPREM or going with FPREM1 ?It's hard to justify including a primitive built-in operator that differs from IEEE. But it may be justifiable when it's the only way to avoid a major break from C and intuition. int x = 15 % 10; int y = cast(int)((cast(float)15) % 10); // Are we really comfortable with these being completely different? You know, all this time I was thinking that the behaviour of % for negative integers was because it needed to be consistent with floating-point modulus... Now it just seems to be wrong. But I think I have the answer. In IEEE, the preferred conversion from float to int uses round-to-nearest. IEEE remainder makes sense in that context. Since in cast(int), D has inherited 'chop' rounding from C, D needs to also inherit C's fmod behaviour. So D should stay with FPREM. But we need to document it properly. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Jul 14 2009
http://d.puremagic.com/issues/show_bug.cgi?id=3171 15:15:10 PDT --- We're not breaking with C because C has no % operator for floats. But I agree we should match C99's fmod behavior, which is its current behavior. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Jul 14 2009
http://d.puremagic.com/issues/show_bug.cgi?id=3171 Walter Bright <bugzilla digitalmars.com> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|NEW |RESOLVED Resolution| |FIXED 00:46:11 PST --- Fixed dmd 1.053 and 2.037 -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Dec 06 2009