[Bug math/17834] New: [ldbl-96] scalblnl incorrect for subnormal arguments

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

https://sourceware.org/bugzilla/show_bug.cgi?id=17834

            Bug ID: 17834
           Summary: [ldbl-96] scalblnl incorrect for subnormal arguments
           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

The ldbl-96 implementation of scalblnl (used for x86_64 and ia64) is incorrect
for subnormal arguments (this is a separate bug from bug 17803, which is about
underflowing results).  There are two problems with the adjustments of
subnormal arguments: the "two63" variable multiplied by is actually 0x1p52L not
0x1p63L, so is insufficient to make values normal, and then
GET_LDOUBLE_EXP(es,x), used to extract the new exponent, extracts it into a
variable that isn't used, while the value taken to by the new exponent is
wrongly taken from the high part of the mantissa before the adjustment (hx). 
Thus, for example, scalblnl (0x1p-16383L, 0) returns 0 with underflow, and
scalblnl (0x1.ffffp-16383L, 0) returns 0xf.fff8p-65.

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