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[Bug math/14328] New: ctan, ctanh inaccuracy for subnormals in round-upwards mode


http://sourceware.org/bugzilla/show_bug.cgi?id=14328

             Bug #: 14328
           Summary: ctan, ctanh inaccuracy for subnormals in round-upwards
                    mode
           Product: glibc
           Version: 2.15
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
        AssignedTo: unassigned@sourceware.org
        ReportedBy: jsm28@gcc.gnu.org
                CC: liubov.dmitrieva@gmail.com
    Classification: Unclassified


(This bug report is for an issue identified in test logs sent off-list by
Liubov Dmitrieva.)

ctan and ctanh functions (for all floating-point types) give inaccurate results
(with spurious underflow exceptions), in round-upwards mode only, in some cases
when the imaginary part of the argument (for ctan) or the real part (for ctanh)
is subnormal.  For example, ctan (0x1.921fb54442d18p+0 + 0x1p-1074i) gives (on
x86_64) a result with imaginary part 2.635438829887015752e-291, but it should
be about half that; likewise, ctanh (0x1p-1074 + 0x1.921fb54442d18p+0i) gives
real part 2.635438829887015752e-291, but it should be about half that.

I'm working on a patch for this issue.

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