[Openmp-commits] [openmp] Update OpenMP runtime to adopt taskgraph clause from 6.0 Specs (PR #130751)

[Josep Pinot via Openmp-commits]

jpinot wrote:

> > I was wondering, would it be necessary to safeguard the allocate and free functions to make them threadsafe? It's a random thought, I've not given it a full analysis to see if that makes sense.
> 
> The kmp_allocate and kmp_free functions are threadsafe. The whole taskgraph implementation is not.
> 
> A compliant implementation will need to handle concurrent taskgraph execution. This means we cannot have a global `__kmp_curr_tdg` variable. Making the variable thread-private (i.e., moving it into `kmp_info_t` accessed as `__kmp_threads[(__kmp_get_gtid())]->th.th_curr_tdg`) might be a good first step. Additionally access `__kmp_global_tdgs` needs locks.

Thanks, @josemonsalve2 and @jprotze, for the review. I completely agree, taskgraph should be threadsafe. There are multiple points where global variables are used without any safeguards (e.g., `__kmp_global_tdgs`, `__kmp_num_tdg`). I plan to work on a threadsafe implementation once `firstprivate(saved)` and `replayable` logic are added(I'm currently working on), as tasks with `saved` vars will need to access data stored inside the taskgraph region, potentially requiring additional work in that regard.

Regarding this PR, @jprotze's suggestion has been implemented [here](https://github.com/jpinot/llvm-project/commit/46d35ff35cf66108bc6d04a3f6df156120083f93). To make it work, I [had to modify](https://github.com/jpinot/llvm-project/commit/46d35ff35cf66108bc6d04a3f6df156120083f93#diff-8402e656316eb873d5db4dea7f697406d15ae4197dcc60d88b3d9fc252fcb69aR2582-R2659) the `if` statement inside `__kmpc_task_reduction_get_th_data` to ensure that tasksloops with reduction vars access the correct data when executing a recorded TDG. However, the resulting statement looks hacky and misleading, which is why I think is a good idea to wait until have full taskgraph functionality and decrease refactoring.

https://github.com/llvm/llvm-project/pull/130751