[GiNaC-devel] sporadic make check failure in exam_inifcns_nstdsums

[Stefan Weinzierl]

Dear Richy,

currently  prec is set to 5 * pow(10, -Digits) in inifcns_test_HLi,
but this is just some empirical value, which seemed to work fine in the 
last 15 years.

We expect the two values for (H(lst{-2,1,3},numeric(245)/100) to differ in 
the last digits, the main reason being a loss of significant digits when 
subtracting two almost equal numbers.
We are not doing interval arithmetic, so we have to live with this fact.

Your explanation why this even happens on the same machine seems 
convincing.

Clearly the testsuite should pass in 100% of the cases, not just in 98.4%.
Therefore raising prec to 10 * pow(10, -Digits) or 20 * pow(10, -Digits)
in inifcns_test_HLi is fine.

One may discuss if one raises prec just in inifcns_test_HLi, or also in 
other places (like exam_inifcns_elliptic.cpp). I have a slight preference 
for just adjusting it in inifcns_test_HLi.
If you prefer not to be bothered again in a few years with the same issue 
in other places, we may raise it in all places where we used similar 
constructions.

  Best wishes,

     Stefan




On Fri, 14 Jan 2022, Richard B. Kreckel wrote:

> Hi Stefan,
>
> A recent FTBFS for GiNaC-1.8.2 on Debian's mipsel port [1] turned up a
> little issue with the precision goal not being 100% reproducible.
>
> The recursive calls in GiNaC::map_trafo_H_1overx::operator() assemble
> symbolic functions before evalf()'ing and these are not deterministic.
> This makes numeric results slightly differ between calls.
>
> As a result, regression test suite exam_inifcns_nstdsums.cpp fails in
> about 1.6% of all cases. It only ever seems to affect
>    H(lst{-2,1,3},numeric(245)/100) -
> -Li(lst{2,1,3},lst{-numeric(245)/100,-1,1})
> in inifcns_test_HLi(), exam_inifcns_nstdsums.cpp:148.
>
> I tortured my computer with 1365 calls and here is a histogram of
> numerical differences between the H and the Li results:
> [0.00000...1.0e-18[    4
>    okay
> [1.0e-18...1.0e-17[    530
>  okay
> [2.0e-17...3.0e-17[    503
>  okay
> [3.0e-17...4.0e-17[    232
>  okay
> [4.0e-17...5.0e-17[    74
>   okay
> [5.0e-17...6.0e-17[    18   FAILURE!
> [6.0e-17...7.0e-17[    2
>    FAILURE!
> [7.0e-17...8.0e-17[    2
>    FAILURE!
>> = 8e-17               -    N/A
>
> Would it make sense to raise prec to 10*pow(10,-Digits)?
> If so, for which tests?
> Or do you have a better idea?
>
> All my best,
>  -richard.
>
> [1]
> <https://buildd.debian.org/status/fetch.php?pkg=ginac&arch=mipsel&ver=1.8.2-1&stamp=1641065269>
> -- 
> Richard B. Kreckel
> <https://in.terlu.de/~kreckel/>
>