[Patch, Fortran] PR42517: Bogus runtime error with -fopenmp -fcheck=recursion

[Janus Weil]

>> I recently noticed that -fcheck=recursion does not work together with
>> OpenMP. This is not surprising, since all that the recursion check
>> does is setting some internal logical variable to true when entering
>> the procedure and setting it back to false when leaving. So that, if
>> the procedure is already being executed, a second instance of that
>> procedure can detect this situation and throw a runtime error.
>>
>> Now, in an OpenMP program you typically have several parallel threads,
>> and any procedure can simultaneously be executed in multiple threads
>> without being called recursively. Therefore the recursion check simply
>> does not work for this case and throws false errors galore.
>
>> Surely there are ways to make the check work also for parallel
>> programs.
>
>  Yes, that's what TLS was made for; I hesitate to use the word "trivial"
> since it won't be, but conceptually it is: just create a __thread variable
> instead of an ordinary static one for the recursion check guard var.

Thanks for the hint. I have no experience with TLS, so I wouldn't know
how to do that right away. The biggest hurdle would probably be the
fact that TLS is not supported in the Fortran front end. Or is it? I
mean, is it possible to declare a TLS variable in a Fortran program?

Btw, since I just had a look at it: There seem to be some funny
artifacts in the chapters on TLS in the manual, stuff like "Add new
text after paragraph ..." etc (chapter 6.56.1 and 6.56.2).

Cheers,
Janus