Fix spurious overflow exceptions from x86/x86_64 powl (bug 13872)

[Andreas Jaeger]

On 04/09/2012 07:32 PM, Joseph S. Myers wrote:
> Bug 13872 is a problem with overflow exceptions for certain underflow
> cases of x86 and x86_64 powl with very large exponents.  The problem
> is that the y*log2(x) computation overflows (to minus infinity), and
> while there is code to check for such overflow (to avoid NaNs
> appearing in the computation of the fractional part), this is too late
> to avoid overflow exceptions from underflow cases.  I propose this
> patch to fix this problem by saturating the exponents to ± 2**78
> (powl(1-2**-64, 2**64) is about 1/e, powl(1+2**-63, 2**64) is about
> e^2, and in either case raising to the power 2**14 results in
> underflow to 0 / overflow to +infinity).  This issue does not apply to
> pow or powf because DBL_MAX * log2(DBL_TRUE_MIN) does not overflow the
> range of long double and all these intermediate computations are in
> long double

Thanks, this is fine,
Andreas
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