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digitalmars.D.bugs - [Issue 5565] New: [64-bit] Wrong Floating Point Results, Related to Mixing With size_t

reply d-bugmail puremagic.com writes:
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           Summary: [64-bit] Wrong Floating Point Results, Related to
                    Mixing With size_t
           Product: D
           Version: D2
          Platform: x86_64
        OS/Version: Windows
            Status: NEW
          Keywords: wrong-code
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: dsimcha yahoo.com


--- Comment #0 from David Simcha <dsimcha yahoo.com> 2011-02-11 22:06:07 PST ---
The following code produces erratic results.  I don't know where exactly the
culprit is, but I'm hoping I've reduced it enough that someone with a decent
mental model of how the compiler works will be able to finish the job.

import std.stdio;

double foo(int[] data, double q = 0.25) {
    immutable double N = data.length;

    // This actually affects the result, making it more severe and obvious
    // in addition to showing that the value is correct.
    stderr.writeln(N);  // Prints 8.  Correct.

    // lowEnd should be floor(7 * 0.25) == 1.
    immutable lowEnd = cast(size_t) ((N - 1) * q);

    // highEnd should be floor(7 * 0.75) - 1 = 4.
    immutable highEnd = cast(size_t) ((N - 1) * (1.0 - q) - lowEnd);

    // highFract should be 7 * 0.75 - 4 - 1 = 0.25
    immutable highFract = (N - 1) * (1.0 - q) - lowEnd - highEnd;

    stderr.writeln(lowEnd, '\t', highEnd, '\t', highFract);
    return highFract;
}

void main() {
    writeln(foo([1,2,3,4,5,6,7,8]));
}

Prints:

8
0    4545916    -4.54591e+06
-4.54591e+06

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--- Comment #1 from David Simcha <dsimcha yahoo.com> 2011-02-11 22:08:48 PST ---
Forgot to mention:  This test case doesn't reproduce the bug if -O is enabled,
though the test case I reduced it from did, I think.  (I'm not 100% sure that I
wasn't just seeing two different bugs simultaneously.)

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |bugzilla digitalmars.com
         Resolution|                            |FIXED


--- Comment #2 from Walter Bright <bugzilla digitalmars.com> 2011-02-12
15:04:22 PST ---
https://github.com/D-Programming-Language/dmd/commit/95b70d83ac51496eb171c1b408b5781eeee1a29d

https://github.com/D-Programming-Language/dmd/commit/fe147641a0334bca8d5ac3def7a0aba8529a0a66

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David Simcha <dsimcha yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|FIXED                       |


--- Comment #3 from David Simcha <dsimcha yahoo.com> 2011-02-12 15:37:05 PST ---
I think this was actually two bugs that I thought had a single root cause.  You
only got one of them.  Now, the result from running this code with -m64 and
without -O on linux is:

8
1    4545912    -4.54591e+06
-4.54591e+06

On the line with the three columns, the first one is now correct.  The other
two are still wrong, unless -O is enabled.

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prev sibling parent d-bugmail puremagic.com writes:
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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
         Resolution|                            |FIXED


--- Comment #4 from Walter Bright <bugzilla digitalmars.com> 2011-02-12
21:57:02 PST ---
https://github.com/D-Programming-Language/dmd/commit/2421fd325b8e7528f5bafd2c4a4d6fd5008f3ab1

https://github.com/D-Programming-Language/dmd/commit/c51de3ab026f811744a4dc81b6f50c2525fc07a5

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