Fix missing underflow from cexp (bug 14478)

[Andreas Jaeger]

On 04/03/2013 06:30 PM, Joseph S. Myers wrote:
> Bug 14478 is failure of cexp functions to raise underflow exceptions
> when the final result underflows (in at least one of real and
> imaginary parts) but the multiplications generating that result were
> exact so did not raise the underflow exception themselves.  (The test
> in that bug is incorrect, since 0x1p-120 doesn't underflow for float,
> but the bug itself is valid as shown by the tests this patch adds.)
>
> This patch fixes this bug by checking for tiny results and multiplying
> them by themselves to force underflow.  (If the tiny result is zero,
> the multiplication is still OK: it won't cause underflow, but if it
> wasn't exactly zero there must have been an earlier underflow to get
> that value.)

Ok, thanks,

Andreas
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