[flang-commits] [flang] [flang][Lower] Always generate namelist group locally (PR #109303)

[Tom Eccles via flang-commits]

tblah wrote:

Thanks for taking a look. 

> Could we look at compilation options to inform this decision, either now or in the long run?

I was originally thinking about doing this only when `-fopenmp` is passed, but there is `do concurrent` as well.

> Is this really legal OpenMP?

I agree this probably shouldn't be legal OpenMP, but so far as I can tell, restrictions on privatisation do not apply to reductions. The upstream ticket indicates that this is supported by both ifort and gfortran. But I would be very happy to instead add a semantic check preventing this if sufficient consensus could be reached to deviate from OpenMP and F23.

I agree that creating a very large namelist could be a performance hit. Do those programs declare all variables inside of the namelist inside of a common block (i.e. is this an optimization that users expect)?. If some of the variables are local then this optimization could not be performed anyway.

https://github.com/llvm/llvm-project/pull/109303