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

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Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
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                 CC|                            |bugzilla@digitalmars.com




--- Comment #1 from Walter Bright <bugzilla@digitalmars.com>  2009-07-13 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.

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Don <clugdbug@yahoo.com.au> changed:

           What    |Removed                     |Added
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                 CC|                            |clugdbug@yahoo.com.au




--- Comment #2 from Don <clugdbug@yahoo.com.au>  2009-07-14 01:49:30 PDT ---
(In reply to comment #1)
> 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.

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.

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--- Comment #3 from Walter Bright <bugzilla@digitalmars.com>  2009-07-14 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 ?

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--- Comment #4 from Don <clugdbug@yahoo.com.au>  2009-07-14 04:03:19 PDT ---
(In reply to comment #3)
> Thanks 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.

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--- Comment #5 from Walter Bright <bugzilla@digitalmars.com>  2009-07-14 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.

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Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
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             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


--- Comment #6 from Walter Bright <bugzilla@digitalmars.com> 2009-12-06 00:46:11 PST ---
Fixed dmd 1.053 and 2.037

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