Fix catan, catanh inaccuracy through use of log (bug 15394)

[Andreas Jaeger]

On 04/24/2013 06:42 PM, Joseph S. Myers wrote:
> Bug 15394 is inaccuracy of catan and catanh for certain arguments
> where they involve computing log of a number close to 1 (or in the
> case of catanh, subtracting logs of similar arguments, which could
> lead to worse results in some cases than computing the log of their
> ratio, through large cancellation even when the ratio isn't that close
> to 1).
>
> This patch fixes this inaccuracy by using log1p except in the cases
> where log of a number close to 0 is involved, when use of log1p of a
> number close to -1 would yield worse results than log and so log of a
> ratio is used instead.
>
> Tested x86_64 and x86 and ulps updated accordingly.

Thanks, this is ok,
Andreas
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