[Bug math/18346] New: ldbl-128 roundl incorrect for exponents in [31, 47]

["jsm28 at gcc dot gnu.org" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=18346

            Bug ID: 18346
           Summary: ldbl-128 roundl incorrect for exponents in [31, 47]
           Product: glibc
           Version: 2.21
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: jsm28 at gcc dot gnu.org
  Target Milestone: ---

The implementation of roundl for ldbl-128 involves undefined behavior for
arguments with exponents from 31 to 47 inclusive, from the shift:

      u_int64_t i = -1ULL >> (j0 - 48);

For example, on mips64, this means roundl (0xffffffffffff.8p0L) wrongly returns
its argument, which is not an integer.

This is GCC libquadmath bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65757
.

Working on a patch.

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