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

[Michael Kruse via llvm-commits]

================
@@ -675,6 +675,60 @@ OpenMPIRBuilder::getOrCreateRuntimeFunction(Module &M, RuntimeFunction FnID) {
   return {FnTy, Fn};
 }
 
+Expected<BasicBlock *>
+OpenMPIRBuilder::FinalizationInfo::getFiniBB(IRBuilderBase &Builder) {
+  if (!FiniBB) {
+    Function *ParentFunc = Builder.GetInsertBlock()->getParent();
+    IRBuilderBase::InsertPointGuard Guard(Builder);
+    FiniBB = BasicBlock::Create(Builder.getContext(), ".fini", ParentFunc);
+    Builder.SetInsertPoint(FiniBB);
+    // FiniCB adds the branch to the exit stub.
+    if (Error Err = FiniCB(Builder.saveIP()))
+      return Err;
+  }
+  return FiniBB;
+}
+
+Error OpenMPIRBuilder::FinalizationInfo::mergeFiniBB(IRBuilderBase &Builder,
+                                                     BasicBlock *OtherFiniBB) {
+  // Simple case: FiniBB does not exist yet: re-use OtherFiniBB.
+  if (!FiniBB) {
+    FiniBB = OtherFiniBB;
+    IRBuilderBase::InsertPointGuard Guard(Builder);
+    Builder.SetInsertPoint(FiniBB->getFirstNonPHIIt());
+    if (Error Err = FiniCB(Builder.saveIP()))
+      return Err;
+
+    return Error::success();
+  }
+
+  // Move instructions from FiniBB to the start of OtherFiniBB.
+  auto InsertAfter = OtherFiniBB->getFirstNonPHIIt();
+  for (Instruction &I : make_early_inc_range(*FiniBB)) {
+    if (I.isTerminator()) {
+      I.eraseFromParent();
+      break;
+    }
+    I.insertAfter(InsertAfter);
+    InsertAfter = BasicBlock::iterator(&I);
+  }
+
+  // Replace FiniBB with OtherFiniBB.
+  for (User *U : make_early_inc_range(FiniBB->users())) {
+    if (auto *Instr = dyn_cast<Instruction>(U)) {
----------------
Meinersbur wrote:

```suggestion
    auto *Instr = dyn_cast<Instruction>(U);
    if (!Instr)
      continue;
```
[style] [Use Early Exits and continue to Simplify Code](https://llvm.org/docs/CodingStandards.html#use-early-exits-and-continue-to-simplify-code)


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