Re: [PATCH] Update powerpc libm-test-ulps

[Carlos O'Donell <carlos@redhat.com>]

On 8/20/20 3:44 PM, Tulio Magno Quites Machado Filho wrote:
> Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org> writes:
> 
>> On 20/08/2020 15:39, Carlos O'Donell wrote:
>>> On 8/20/20 2:37 PM, Matheus Castanho via Libc-alpha wrote:
>>>> Before this patch, the following tests were failing:
>>>>
>>>> ppc and ppc64:
>>>>     FAIL: math/test-ldouble-j0
>>>>
>>>> ppc64le:
>>>>     FAIL: math/test-ibm128-j0
>>>> ---
>>>>  sysdeps/powerpc/fpu/libm-test-ulps | 4 ++--
>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/sysdeps/powerpc/fpu/libm-test-ulps b/sysdeps/powerpc/fpu/libm-test-ulps
>>>> index cd2a5fed45..0b82c3f107 100644
>>>> --- a/sysdeps/powerpc/fpu/libm-test-ulps
>>>> +++ b/sysdeps/powerpc/fpu/libm-test-ulps
>>>> @@ -1317,13 +1317,13 @@ Function: "j0_downward":
>>>>  double: 2
>>>>  float: 4
>>>>  float128: 4
>>>> -ldouble: 11
>>>> +ldouble: 12
>>>>  
>>>>  Function: "j0_towardzero":
>>>>  double: 5
>>>>  float: 6
>>>>  float128: 2
>>>> -ldouble: 8
>>>> +ldouble: 16
>>>
>>> We should not have ULPs higher than 9.
>>>
>>> I see Adhemerval added some 11 ULPs here for cexp.
>>>
>>> We should be able to achieve <= 9 ULPs on these algorithms, otherwise there are
>>> compiler problems that need fixing?
>>
>> We are more forgiving for IBM long double due its inherent precision issues:
>>
>> math/libm-test-support.c
>>
>>  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;

Thanks. I didn't know that.

>>
>> And jN implementation also has low precision for some inputs.  With both constraints
>> I think 16ulps should be ok.
> 
> There is also a loss of precision with different rounding modes in libgcc.
> 
> There are currently 30 entries for ibm128 with ULP between 10 and 16 (without
> counting this patch).  Maybe some of these should actually be marked as
> xfail-rounding:ibm128-libgcc instead.

If the loss of precision is due to the implementation then it seems like using
an XFAIL is not an accurate representation of the state.

> The only way to validate this is by compiling glibc with a libgcc that has a
> patch from Joseph.  I have an up-to-date version of that patch in
> https://github.com/tuliom/gcc/commit/ca42479cae3c2b56651c3e97bb5eeaf24ca4bb61
 
Interesting patch, it looks similar to what we do in glibc for some of the
libm functions.

-- 
Cheers,
Carlos.