[Bug math/13472] New: PPC64 hypot() is "inf" for very large values that are non-infinite on other architectures

["dmalcolm at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=13472

             Bug #: 13472
           Summary: PPC64 hypot() is "inf" for very large values that are
                    non-infinite on other architectures
           Product: glibc
           Version: 2.14
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
        AssignedTo: aj@suse.de
        ReportedBy: dmalcolm@redhat.com
    Classification: Unclassified


ppc64's glibc appears to have an optimized implementation of "hypot" (in libm):
  sysdeps/powerpc/fpu/e_hypot.c

This seems to have different behavior near infinity when compared to other
architectures.

This leads to CPython's test suite failing on ppc64 with glibc-2.14.90-19.ppc64
(albeit with Fedora's build of glibc) within the "cmath" module (math on
complex numbers):

  https://bugzilla.redhat.com/show_bug.cgi?id=750811

  http://bugs.python.org/issue13534

CPython's "cmath" module internally uses hypot() in various cases of very large
numbers in order to avoid overflow.  All of the failing test cases turned out
to involve (at the C layer) a call to hypot() with a pair of very large finite
values, where the result is very large but finite on other architectures, but
is "inf" on ppc64, leading to various unexpected values, and an
architecture-specific failure of this test.

"man hypot" says:
> The calculation is performed without undue overflow or underflow during
> the intermediate steps of the calculation.

Test case:
Python's math.hypot() is a thin wrapper around hypot(3); a Python "float" uses
a C "double" internally.

https://bugzilla.redhat.com/attachment.cgi?id=541097 is a Python script to
exercise hypot(3).  It calls hypot() for all of the x,y pairs that fail within
the Python test case on ppc64.

On x86_64, with glibc-2.14-5.x86_64 (and python-2.7.1-7.fc15.x86_64), all
results are non-infinite, with exponent e+307 or e+308 (see:
  https://bugzilla.redhat.com/attachment.cgi?id=541102
for the results on this box).

On ppc64, with glibc-2.14.90-19.ppc64 (and python-2.7.2-4.2.fc16.ppc64), all
results are "inf".

(hopefully the attachments on that other bugzilla instance are readable).

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