[clang] [llvm] [mlir] [OMPIRBuilder] always leave PARALLEL via the same barrier (PR #164586)

[Tom Eccles via cfe-commits]

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@@ -1800,8 +1779,18 @@ OpenMPIRBuilder::InsertPointOrErrorTy OpenMPIRBuilder::createParallel(
   Instruction *PRegPreFiniTI = PRegPreFiniBB->getTerminator();
 
   InsertPointTy PreFiniIP(PRegPreFiniBB, PRegPreFiniTI->getIterator());
-  if (Error Err = FiniCB(PreFiniIP))
-    return Err;
+  if (!FiniInfo.FiniBB) {
+    if (Error Err = FiniCB(PreFiniIP))
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tblah wrote:

The finalisation basic block could have already been created if the body of the parallel operation contained a cancellation point or cancel. In that case we should just branch straight to the block created previously. I agree the control flow with all of the callbacks and the cancellation stack are a bit hard to follow. This is not new with this patch.

In most cases, no cancellation will have already created a finalisation block so finalisation should be generated right here as was done before this patch.

The intention here is to only run the finalisation callback once and have all exists branch to that one instance (and also to include the barrier in that unique exit so that all threads block on the same barrier).

https://github.com/llvm/llvm-project/pull/164586