[llvm] [flang] [compiler-rt] [clang] [clang-tools-extra] [Flang][OpenMP] Restructure recursive lowering in `createBodyOfOp` (PR #77761)

[Kiran Chandramohan via cfe-commits]

================
@@ -2223,37 +2222,100 @@ static void createBodyOfOp(
                                             mlir::omp::YieldOp>(
         firOpBuilder, eval.getNestedEvaluations());
 
-  // Insert the terminator.
-  Fortran::lower::genOpenMPTerminator(firOpBuilder, op.getOperation(), loc);
-  // Reset the insert point to before the terminator.
-  resetBeforeTerminator(firOpBuilder, storeOp, block);
+  // Start with privatization, so that the lowering of the nested
+  // code will use the right symbols.
+  constexpr bool isLoop = std::is_same_v<Op, mlir::omp::WsLoopOp> ||
+                          std::is_same_v<Op, mlir::omp::SimdLoopOp>;
+  bool privatize = clauses && !outerCombined;
 
-  // Handle privatization. Do not privatize if this is the outer operation.
-  if (clauses && !outerCombined) {
-    constexpr bool isLoop = std::is_same_v<Op, mlir::omp::WsLoopOp> ||
-                            std::is_same_v<Op, mlir::omp::SimdLoopOp>;
+  firOpBuilder.setInsertionPoint(marker);
+  std::optional<DataSharingProcessor> tempDsp;
+  if (privatize) {
     if (!dsp) {
-      DataSharingProcessor proc(converter, *clauses, eval);
-      proc.processStep1();
-      proc.processStep2(op, isLoop);
-    } else {
-      if (isLoop && args.size() > 0)
-        dsp->setLoopIV(converter.getSymbolAddress(*args[0]));
-      dsp->processStep2(op, isLoop);
+      tempDsp.emplace(converter, *clauses, eval);
+      tempDsp->processStep1();
     }
-
-    if (storeOp)
-      firOpBuilder.setInsertionPointAfter(storeOp);
   }
 
   if constexpr (std::is_same_v<Op, mlir::omp::ParallelOp>) {
     threadPrivatizeVars(converter, eval);
-    if (clauses)
+    if (clauses) {
+      firOpBuilder.setInsertionPoint(marker);
       ClauseProcessor(converter, *clauses).processCopyin();
+    }
   }
 
-  if (genNested)
+  if (genNested) {
+    // genFIR(Evaluation&) tries to patch up unterminated blocks, causing
+    // a lot of trouble if the terminator generation is delayed past this
+    // point. Insert a temporary terminator here, then delete it.
+    firOpBuilder.setInsertionPointToEnd(&op.getRegion().back());
+    auto *temp = Fortran::lower::genOpenMPTerminator(firOpBuilder,
+                                                     op.getOperation(), loc);
+    firOpBuilder.setInsertionPointAfter(marker);
     genNestedEvaluations(converter, eval);
+    temp->erase();
+  }
+
+  // Get or create a unique exiting block from the given region, or
+  // return nullptr if there is no exiting block.
+  auto getUniqueExit = [&](mlir::Region &region) -> mlir::Block * {
+    // Find the blocks where the OMP terminator should go. In simple cases
+    // it is the single block in the operation's region. When the region
+    // is more complicated, especially with unstructured control flow, there
+    // may be multiple blocks, and some of them may have non-OMP terminators
+    // resulting from lowering of the code contained within the operation.
+    // All the remaining blocks are potential exit points from the op's region.
+    //
+    // Explicit control flow cannot exit any OpenMP region (other than via
+    // STOP), and that is enforced by semantic checks prior to lowering. STOP
+    // statements are lowered to a function call.
+
+    // Collect unterminated blocks.
+    llvm::SmallVector<mlir::Block *> exits;
+    for (mlir::Block &b : region) {
+      if (b.empty() || !b.back().hasTrait<mlir::OpTrait::IsTerminator>())
+        exits.push_back(&b);
+    }
+
+    if (exits.empty())
+      return nullptr;
+    // If there already is a unique exiting block, do not create another one.
+    // Additionally, some ops (e.g. omp.sections) require onlt 1 block in
----------------
kiranchandramohan wrote:

```suggestion
    // Additionally, some ops (e.g. omp.sections) require only 1 block in
```

https://github.com/llvm/llvm-project/pull/77761