regression in Newlib 4.2.0

[Andoni Arregi]

Dear Paul,

although we could fix the problem you spotted, we don't really got to
the root cause of the regression, i.e., this is not something that was
introduced in Newlib 4.2.0 and was not present in 4.1.0. wf_expf.c
didn't change in the meantime at all.

The only explanation I have is that it may have to do with the fact
that what you observed happens with the default compilation of Newlib.
If you compile with -D_IEEE_LIBM, then you will not see this
regression, as the wrappers for error handling disappear and with them
the problem.
It is a pity that for such a detailed analysis as the one you do, such
a small change in the build mechanism yields you completely different
implementations of the mathematical functions. I don't know whether
Newlib 4.1.0 was built by default with _IEEE_LIBM defined.

By the way, as we finished the work we have been carrying out for the
European Space Agency to qualify a mathematical library for critical
systems (which we based on the Newlib libm), we will now start
submitting a bunch of improvement patches for the Newlib libm.

We presented the LibmCS for the first time during the current Flight
Software Workshop 2022
(http://flightsoftware.jhuapl.edu/workshop/FSW2022)
https://www.youtube.com/watch?v=UcRefVlVL_A&list=PLK-T7jljJ6zbk0PshIwofgE2hiakBufJF&index=21

Best regards
  
On Fri, 2022-02-04 at 05:36 +0100, Paul Zimmermann wrote:
>        Hi,
> 
> I noticed a regression in Newlib 4.2.0:
> 
> Checking exp with newlib-4.2.0.20211231
> MPFR library: 4.1.0
> MPFR header:  4.1.0 (based on 4.1.0)
> libm wrong by up to 1.94e+04 ulp(s) for x=0x1.62e302p+6
> exp      gives inf
> mpfr_exp gives 0x1.ff691ep+127
> Total: errors=17982998 (0.42%) errors2=151 maxerr=1.94e+04 ulp(s)
> 
> The largest error for expf in Newlib 4.1.0 was 0.911 ulps
> (https://members.loria.fr/PZimmermann/papers/accuracy.pdf).
> 
> Best regards,
> Paul

-- 
Andoni Arregi
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