[flang] Lower simple RETURN statement

[clementval]

Summary

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.

Co-authored-by: V Donaldson <[e-mail]>
Co-authored-by: Eric Schweitz <[e-mail]>
Co-authored-by: Jean Perier <[e-mail]>

[kiranchandramohan]

flang/lib/Lower/Bridge.cpp

679

Is this branch necessary? The branch seems to be added in 343 of genProcedureExit.

flang/test/Lower/return-statement.f90

16–19

Different kinds of comments in the same test (// vs !). Using ! helps to get the test passing.

[kiranchandramohan]

flang/lib/Lower/Bridge.cpp

679

Scratch that. It is probably needed for situations like the following code. Can you have a look and add this test instead?

subroutine sub2()
  goto 3
  2 return
  3 goto 2
end

[clementval]

Address review comment

flang/test/Lower/return-statement.f90

16–19

My bad ...

[kiranchandramohan]

LGTM

flang/lib/Lower/Bridge.cpp

private:
   void genExitRoutine() {
     if (blockIsUnterminated())
       builder->create<mlir::ReturnOp>(toLocation());
   }
   void genFIR(const Fortran::parser::EndProgramStmt &) { genExitRoutine(); }
 
   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 {
       genExitRoutine();
     }
   }
 
   void genFIR(const Fortran::parser::CallStmt &stmt) {
private:
   }
 
   // call STOP, ERROR STOP in runtime
   void genFIR(const Fortran::parser::StopStmt &stmt) {
     genStopStatement(*this, stmt);
   }
 
   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 &) {
     TODO(toLocation(), "CycleStmt lowering");
   }
 
   void genFIR(const Fortran::parser::ExitStmt &) {
     TODO(toLocation(), "ExitStmt lowering");

flang/test/Lower/return-statement.f90

+! 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
+