Fix expm1l spurious underflows for ldbl-128ibm

[Andreas Jaeger]

On 11/21/2012 11:25 PM, Joseph S. Myers wrote:
> The ldbl-128ibm exmp1l implementation has the same problem with
> spurious underflow exceptions for large negative inputs that I
> previously fixed for some other versions.  Here, it's the code that
> returns (4.0/big - 1.0L) in such cases, where 4.0/big underflows for
> IBM long double; this patch uses a minimal fix of changing that
> variable so 4.0/big no longer underflows (but big * big still
> overflows, as required for the overflow case).  As with the existing
> code, nothing special is done here for the peculiarities of IBM long
> double (where strictly the discontiguous mantissa would allow results
> of expm1l with sufficiently negative arguments to be represented much
> more exactly than the code tries to do, as long doubles where the high
> part is -1.0 and the low part is the result of expl, to double
> precision and without underflow exceptions).
>
> Tested that this eliminates the underflow exceptions on powerpc.

Thanks, this is fine,
Andreas

> 2012-11-21  Joseph Myers  <joseph@codesourcery.com>
>
> 	[BZ #6778]
> 	* sysdeps/ieee754/ldbl-128ibm/s_expm1l.c (big): Change to 1e290L.
>
> diff --git a/sysdeps/ieee754/ldbl-128ibm/s_expm1l.c b/sysdeps/ieee754/ldbl-128ibm/s_expm1l.c
> index 98ae4e1..8808dcd 100644
> --- a/sysdeps/ieee754/ldbl-128ibm/s_expm1l.c
> +++ b/sysdeps/ieee754/ldbl-128ibm/s_expm1l.c
> @@ -85,7 +85,7 @@ static const long double
>   /* ln (2^16384 * (1 - 2^-113)) */
>     maxlog = 1.1356523406294143949491931077970764891253E4L,
>   /* ln 2^-114 */
> -  minarg = -7.9018778583833765273564461846232128760607E1L, big = 2e307L;
> +  minarg = -7.9018778583833765273564461846232128760607E1L, big = 1e290L;
>
>
>   long double
>


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