[Bug testsuite/32057] Random failure on gfortran.dg/secnds.f

["dominiq at lps dot ens dot fr" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #1 from dominiq at lps dot ens dot fr  2007-05-23 15:34 -------
> I assume the dummy loop is used for delay. I don't think it is that reliable.

I think you are right about the delay, but not about the real problem: you have
the same with secnds-1.f which does not cantain the loop. The goal of the tests
is not to measure some time, but to check that intervals are properly ordered,
i.e., t1<=dat1<=t1a and t2a<=dat2-dat1<= t2.

I have started to investigate the problem, see the thread:

http://gcc.gnu.org/ml/fortran/2007-05/msg00216.html

I have started to prepare a report of my findings, but I got distracted because
gcc is no longer building on Darwin. 

The main cause of failures is because the tests do not account for round off
errors.  This can easily handled for first test by comparing to the nearest
values from below and above: the failure rate seems below a couple ppb (part
per billion) measured on secnds-1.f, using a naive way to measure it. I think
this is due to the fact that t1 and t1a are absolute times.  For the second
test I had to use wider tolerances (such as spacing(24.0*3600.0)) and even so I
get some errors on Pentium machines ~1/10000.

After having removed this main source of failure I have found three more:
(1) the tests do not handle timing around midnight when the counts go from
86400.0 to 0.0,
(2) there is a rounding problem in secnds such that secnds(86400.0) returns
86400.0 during the 3ms before midnight,
(3) on some machine the clock synchronization put the tests in jeopardy when
the clock has a negative "lag": the value returned "after" is before the value
returned "before"! I think this is a detectable but unrecoverable error and,
when detected, the tests should be skipped and repeated.  I have a fix for the
two other cases, although (2) should probably handled at the secnds level.

In top of the report, I have started to prepare a program to do some testing on
platforms I have no access to (I have used PPC Darwin, and AMD64 and Pentium
Linux). I was planning to join it to the report (Encouragements are welcome !-)


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32057