[flang] Add lowering for basic empty SUBROUTINE

[clementval]

Summary

This patch adds the ability to lower an empty subroutine.

[kiranchandramohan]

LGTM.

I have added entries to the tracker https://github.com/flang-compiler/f18-llvm-project/projects/6

flang/lib/Lower/Bridge.cpp

248–252

Nit: Are these required for this Patch?

[clementval]

flang/lib/Lower/Bridge.cpp

248–252

Not necessary right. I'll remove it.

flang/lib/Lower/Bridge.cpp

return currentBlock->empty() ||
            !currentBlock->back().hasTrait<mlir::OpTrait::IsTerminator>();
   }
 
   /// Emit return and cleanup after the function has been translated.
   void endNewFunction(Fortran::lower::pft::FunctionLikeUnit &funit) {
     setCurrentPosition(Fortran::lower::pft::stmtSourceLoc(funit.endStmt));
     if (funit.isMainProgram())
       genExitRoutine();
+    else
+      genFIRProcedureExit(funit, funit.getSubprogramSymbol());
     funit.finalBlock = nullptr;
     LLVM_DEBUG(llvm::dbgs() << "*** Lowering result:\n\n"
                             << *builder->getFunction() << '\n');
     delete builder;
     builder = nullptr;
     localSymbols.clear();
   }
 
private:
   ///
   /// Generate the cleanup block before the program exits
   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::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) {
     TODO(toLocation(), "CallStmt lowering");
   }
 
   void genFIR(const Fortran::parser::ComputedGotoStmt &stmt) {
     TODO(toLocation(), "ComputedGotoStmt lowering");
   }
 
private:
   void genFIR(const Fortran::parser::EndMpSubprogramStmt &) {
     TODO(toLocation(), "EndMpSubprogramStmt lowering");
   }
 
   void genFIR(const Fortran::parser::EndSelectStmt &) {
     TODO(toLocation(), "EndSelectStmt lowering");
   }
 
-  void genFIR(const Fortran::parser::EndSubroutineStmt &) {
-    TODO(toLocation(), "EndSubroutineStmt lowering");
-  }
+  // Nop statements - No code, or code is generated at the construct level.
+  void genFIR(const Fortran::parser::EndSubroutineStmt &) {} // nop
 
   void genFIR(const Fortran::parser::EntryStmt &) {
     TODO(toLocation(), "EntryStmt lowering");
   }
 
   void genFIR(const Fortran::parser::IfStmt &) {
     TODO(toLocation(), "IfStmt lowering");
   }

flang/test/Lower/basic-subroutine.f90

+! RUN: bbc %s --pft-test | FileCheck %s
+! RUN: bbc %s -o "-" -emit-fir | FileCheck %s --check-prefix=FIR
+
+subroutine sub1()
+end subroutine
+
+! CHECK: 1 Subroutine sub1: subroutine sub1()
+! CHECK:   1 EndSubroutineStmt: end subroutine
+! CHECK: End Subroutine sub1
+
+! FIR-LABEL: func @_QPsub1() {
+! FIR:         return
+! FIR:       }