[PATCH] Fix expf overflow limit

[Paul Zimmermann]

       Dear Andoni,

I confirm that this fixes the expf large errors. With commit 8f2bd6f I now get
a maximal error of 0.911 ulps:

Checking exp with newlib-4.2.0.20211231
Using RedHat newlib
MPFR library: 4.1.0       
MPFR header:  4.1.0 (based on 4.1.0)
Checking function expf with MPFR_RNDN
libm wrong by up to 9.11e-01 ulp(s) [1] for x=-0x1.2e35b6p+5
exp      gives 0x1.6a2544p-55
mpfr_exp gives 0x1.6a2546p-55
Total: errors=17982848 (0.42%) errors2=0 maxerr=9.11e-01 ulp(s)

Thank you,
Paul

PS: in this case and in the two other issues I reported (about lgammaf and tgammaf),
I have to apologize. This was not a regression in Newlib 4.2.0, but a bug in our
checking program which did not report such issues. With the new version of our
program, we get similar results with Newlib 4.1.0 and Newlib 4.2.0.

> From: Andoni Arregi <andoni.arregui@gtd-gmbh.de>
> Cc: Paul Zimmermann <Paul.Zimmermann@inria.fr>
> Date: Wed, 09 Feb 2022 14:52:07 +0100
> Organization: GTD GmbH
> User-Agent: Evolution 3.42.3 
> 
> Correct the overflow limit in the variable o_threshold to be consistent
> with the FLT_UWORD_LOG_MAX variable used by the internal implementation
> of the expf algorithm itself.
> The u_threshold variable has also been modified to be written in the
> same format.
> 
> Note that this fix improves the situation but does not completely
> correct the inconsistencies regarding the overflow and underflow limits
> between the expf wrapper (wf_exp.c) and the expf algorithm itself
> (ef_exp.c).
> 
> Currently these limits are different for the
> _FLT_LARGEST_EXPONENT_IS_NORMAL and _FLT_NO_DENORMALS cases as well as
> for the case where __OBSOLETE_MATH is not defined (only for the
> underflow limit in this case).
> ---
>  newlib/libm/math/wf_exp.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/newlib/libm/math/wf_exp.c b/newlib/libm/math/wf_exp.c
> index 38cacd95a..075b5bb0a 100644
> --- a/newlib/libm/math/wf_exp.c
> +++ b/newlib/libm/math/wf_exp.c
> @@ -26,8 +26,8 @@ static const float
>  #else
>  static float
>  #endif
> -o_threshold=  8.8721679688e+01,  /* 0x42b17180 */
> -u_threshold= -1.0397208405e+02;  /* 0xc2cff1b5 */
> +o_threshold=  0x1.62e42ep+06,  /* 0x42b17217 */
> +u_threshold= -0x1.9fe36ap+06;  /* 0xc2cff1b5 */
>  
>  #ifdef __STDC__
>  	float expf(float x)		/* wrapper expf */
> -- 
> 2.35.1
> 
>