Re: [COMMITTED] arm: update libm test ulps

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 13/04/2021 06:52, Szabolcs Nagy wrote:
> The 04/11/2021 18:04, Vineet Gupta wrote:
>> On 4/8/21 1:58 AM, Szabolcs Nagy via Libc-alpha wrote:
>>> Updated after commits 9acda61d94acc5348c2330f2519a14d1a4a37e73
>>> and 43576de04afc6a0896a3ecc094e1581069a0652a.
>>
>> Is there a better way for this to be done at the time of original change 
>> other than running the tests on all affected architectures. arches 
>> always seem to be chasing these kind of changes ...
> 
> i think there are only small variations between targets for
> float and double, so the ulp error limits for those could be
> shared (unless a target has its own implementation with
> different quality requirement for some reason).
> 
> long double is more target specific. i think the tests could
> be organised such that each sysdeps/ieee754/* directory has
> its own ulp limits file and the target picks up the right one.
> 
> and libmvec needs target specific ulp limits.
> 
> i don't know how hard it would be to implement this.
> 

The math/libm-test-support.c imposes maximum ulp limits depending of 
the underling type:

 228   if (testing_ibm128)
 229     /* The documented accuracy of IBM long double division is 3ulp
 230        (see libgcc/config/rs6000/ibm-ldouble-format), so do not
 231        require better accuracy for libm functions that are exactly
 232        defined for other formats.  */
 233     max_valid_error = exact ? 3 : 16;
 234   else
 235     max_valid_error = exact ? 0 : 9;

And if I recall correctly there was a suggestion to consolidate and/or
remove the ulps file altogether and use the maximum valid error as the
threshold to report regressions.  The only drawback I see of moving
towards it is each architecture won't see if some change has degraded
the function precision.

But I tend to agree that using per-architecture ulp files do not add
much, since arch maintainer tend todig into the precision issues only
when it is higher the the tests threshold.