Fix x86/x86_64 expm1l spurious underflow exceptions (bug 16539)

[Andreas Jaeger]

On 06/24/2014 10:47 PM, Joseph S. Myers wrote:
> This patch fixes bug 16539, spurious underflow exceptions from x86 /
> x86-64 expm1l.  The problem is that the computation of a base-2
> exponent with extra precision involves spurious underflows for
> arguments that are small but not subnormal, so a check is added to
> just return the argument in those cases.  (If the argument *is*
> subnormal, underflowing is correct and the existing code will always
> underflow, so it suffices to keep using the existing code in that
> case; some expm1 implementations have a bug (bug 16353) with missing
> underflow exceptions, but I don't think there's such a bug in this
> particular version.)

Thanks,
Andreas
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