[flang] Lower simple RETURN statement

This patch adds the lowering for the RETURN statement
without alternate returns in the main program or in subroutine
and functions.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D119429

Co-authored-by: V Donaldson <[email protected]>
Co-authored-by: Eric Schweitz <[email protected]>
Co-authored-by: Jean Perier <[email protected]>

Author: 4 people

flang/lib/Lower/Bridge.cpp

@@ -337,6 +337,13 @@ class FirConverter : public Fortran::lower::AbstractConverter {
 
   void genFIRProcedureExit(Fortran::lower::pft::FunctionLikeUnit &funit,
                            const Fortran::semantics::Symbol &symbol) {
+    if (mlir::Block *finalBlock = funit.finalBlock) {
+      // The current block must end with a terminator.
+      if (blockIsUnterminated())
+        builder->create<mlir::cf::BranchOp>(toLocation(), finalBlock);
+      // Set insertion point to final block.
+      builder->setInsertionPoint(finalBlock, finalBlock->end());
+    }
     if (Fortran::semantics::IsFunction(symbol)) {
       TODO(toLocation(), "Function lowering");
     } else {
@@ -652,7 +659,24 @@ class FirConverter : public Fortran::lower::AbstractConverter {
   }
 
   void genFIR(const Fortran::parser::ReturnStmt &stmt) {
-    TODO(toLocation(), "ReturnStmt lowering");
+    Fortran::lower::pft::FunctionLikeUnit *funit =
+        getEval().getOwningProcedure();
+    assert(funit && "not inside main program, function or subroutine");
+    if (funit->isMainProgram()) {
+      genExitRoutine();
+      return;
+    }
+    mlir::Location loc = toLocation();
+    if (stmt.v) {
+      TODO(loc, "Alternate return statement");
+    }
+    // Branch to the last block of the SUBROUTINE, which has the actual return.
+    if (!funit->finalBlock) {
+      mlir::OpBuilder::InsertPoint insPt = builder->saveInsertionPoint();
+      funit->finalBlock = builder->createBlock(&builder->getRegion());
+      builder->restoreInsertionPoint(insPt);
+    }
+    builder->create<mlir::cf::BranchOp>(loc, funit->finalBlock);
   }
 
   void genFIR(const Fortran::parser::CycleStmt &) {

flang/test/Lower/return-statement.f90

@@ -0,0 +1,34 @@
+! RUN: bbc %s -o "-" -emit-fir | FileCheck %s
+
+program basic
+  return
+end program
+
+! CHECK-LABEL: func @_QQmain() {
+! CHECK:         return
+! CHECK:       }
+
+subroutine sub1()
+  return
+end
+
+! CHECK-LABEL: func @_QPsub1() {
+! CHECK:         cf.br ^bb1
+! CHECK:       ^bb1:  // pred: ^bb0
+! CHECK:         return
+
+subroutine sub2()
+  goto 3
+  2 return
+  3 goto 2
+end
+
+! CHECK-LABEL: func @_QPsub2() {
+! CHECK:         cf.br ^bb2
+! CHECK:       ^bb1:  // pred: ^bb2
+! CHECK:         cf.br ^bb3
+! CHECK:       ^bb2:  // pred: ^bb0
+! CHECK:         cf.br ^bb1
+! CHECK:       ^bb3:  // pred: ^bb1
+! CHECK:         return
+