Fix ldbl-128 lgammal for small negative arguments (bug 16337)

[Andreas Jaeger]

On 12/22/2013 03:20 AM, Joseph S. Myers wrote:
> This patch fixes bug 16337, ldbl-128 lgammal spurious overflows for
> small negative arguments (the arguments in question are already in the
> testsuite).  The implementation uses the reflection formula to compute
> lgamma of negative x from lgamma of -x, effectively resulting in a
> calculation -log(x^2) + log(-x); cancellation isn't problematic in
> this case (bugs for problematic cancellation in lgamma are 2542, 2543,
> 2558), but the x^2 calculation can underflow (in which case there is
> spurious logic to return an overflowing value - lgamma can only ever
> correctly overflow for large positive arguments, though tgamma can
> overflow for small arguments of either sign as well as large positive
> arguments).  The fix is simply to calculate the result directly with
> logl when the argument is a small enough negative number.

ok, thanks,

Andreas
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