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Subject: [Bug 237800] pow(3) returns inaccurate results
Date: Wed, 08 May 2019 16:53:46 +0000
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D237800

            Bug ID: 237800
           Summary: pow(3) returns inaccurate results
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: misc
          Assignee: bugs@FreeBSD.org
          Reporter: khw@cpan.org
 Attachment #204271 text/plain
         mime type:

Created attachment 204271
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=3D204271&action=
=3Dedit
Reproducer .c

The attached essentially one-liner, program prints 0x1.431e0fae6d722p+96
on FreeBSD 13 clang version 8.0.0 (tags/RELEASE_800/final 356365) (based on
LLVM 8.0.0)

On Linux, it prints 0x1.431e0fae6d721p+96
a difference of 1 in the final hex digit of the mantissa.  The Linux versio=
n is
the correct value.
We are getting identical failures on 11.2, 12.0, and Open BSD 6.4

http://www.cpantesters.org/distro/M/Math-Clipper.html?grade=3D3&perlmat=3D1=
&patches=3D2&oncpan=3D2&distmat=3D2&perlver=3DALL&osname=3DALL&version=3D1.=
27

I work on maintaining and extending the Perl 5 programming language.  We ma=
de a
change earlier in our current development cycle that improved the accuracy =
of
converting a string representing a floating point number into an equivalent
double precision C value.  We now use strtod() instead of atof().  That bro=
ke
the module Math::Clipper.  It turns out it was relying on the imprecision of
atof() to work around this pow() bug.  That work around was added in May 20=
18.

The above smoke reports rely on a volunteer network of people to do the
testing.  That means the coverage of a module such as Math::Clipper is spot=
ty.=20
Probably it's failing Open BSD in other versions of Perl, but those just di=
dn't
get tested.

--=20
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