[Bug math/18031] New: [ldbl-128ibm] modfl problems

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

https://sourceware.org/bugzilla/show_bug.cgi?id=18031

            Bug ID: 18031
           Summary: [ldbl-128ibm] modfl problems
           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-128ibm implementation of modfl has several issues:

* In the case for high part exponent < 52,
(((i0&i)|(i1&0x7fffffffffffffffLL))==0) is not a correct test for integer
argument; the latter part effectively tests just if the low part is a power of
2.  Example: an argument of 0x1.ffffffffffffffp51L is wrongly treated as being
an integer.

* In that case, the handling of non-integers when detected as such fails to
allow for the low part being of opposite sign to the integer high part. 
Example: 0x1.ffffffffffffff1p51L produces integer part 0x1p+52, fractional part
-0x1.ep-6.

* The case of high part exponent > 103 is treated as meaning the argument is an
integer, but that's not true even when restricted to numbers with 106
contiguous mantissa bits.  Example: 0x1.ffffffffffffffffffffffffff8p104L is
wrongly treated as being an integer.

* In the case for exponents 52 to 103, the test i1&i)==0 for being an integer
is completely bogus; the mask i = -1ULL>>(j0-52) is computed as if this were a
conventional IEEE format.  Example: 0x0.fffffffffffffcp52L is wrongly treated
as being an integer.

* When something is detected as not an integer in that case, the INSERT_WORDS64
(xlo, i1&(~i)) is always inserting a subnormal / zero value in the low part,
its exponent having been masked off earlier.  Example:
0x1.00000000000001111p56L gets integer part 0x1p+56 and fractional part
0x1.111p+0, which is more than 1.

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