[flang] Lower alternate return

[clementval]

Summary

This patch adds the lowering infrastructure for the lowering of
alternat returns.

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

Depends on D121698

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Lower/Bridge.cpp

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");
+      // Alternate return statement - If this is a subroutine where some
+      // alternate entries have alternate returns, but the active entry point
+      // does not, ignore the alternate return value.  Otherwise, assign it
+      // to the compiler-generated result variable.
+      const Fortran::semantics::Symbol &symbol = funit->getSubprogramSymbol();
+      if (Fortran::semantics::HasAlternateReturns(symbol)) {
+        Fortran::lower::StatementContext stmtCtx;
+        const Fortran::lower::SomeExpr *expr =
+            Fortran::semantics::GetExpr(*stmt.v);
+        assert(expr && "missing alternate return expression");
+        mlir::Value altReturnIndex = builder->createConvert(
+            loc, builder->getIndexType(), createFIRExpr(loc, expr, stmtCtx));
+        builder->create<fir::StoreOp>(loc, altReturnIndex,
+                                      getAltReturnResult(symbol));
+      }
     }
     // 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);

flang/test/Lower/alternate-return.f90

+! RUN: bbc -emit-fir -o - %s | FileCheck %s
+
+! CHECK-LABEL: func @_QPss
+subroutine ss(n)
+    print*, n
+    ! CHECK: return{{$}}
+    return
+  ! CHECK-LABEL: func @_QPee
+  entry ee(n,*)
+    print*, n
+    ! CHECK: return %{{.}} : index
+    return 1
+  end
+  
+  ! CHECK-LABEL: func @_QQmain
+    call ss(7)
+    call ee(2, *3)
+    print*, 'default'
+  3 print*, 3
+  
+    print*, k(10,20)
+    print*, k(15,15)
+    print*, k(20,10)
+  end
+  
+  ! CHECK-LABEL: func @_QPk
+  function k(n1, n2)
+    ! CHECK-NOT: ^bb
+    ! CHECK: [[selector:%[0-9]+]] = fir.call @_QPs
+    ! CHECK-NEXT: fir.select [[selector]] : index [1, ^[[block1:bb[0-9]+]], 2, ^[[block2:bb[0-9]+]], unit, ^[[blockunit:bb[0-9]+]]
+    call s(n1, *5, n2, *7)
+    ! CHECK: ^[[blockunit]]: // pred: ^bb0
+    k =  0; return;
+    ! CHECK: ^[[block1]]: // pred: ^bb0
+  5 k = -1; return;
+    ! CHECK: ^[[block2]]: // pred: ^bb0
+  7 k =  1; return
+  end
+  
+  ! CHECK-LABEL: func @_QPs
+  subroutine s(n1, *, n2, *)
+    ! CHECK: [[retval:%[0-9]+]] = fir.alloca index {{{.*}}bindc_name = "s"}
+    ! CHECK-COUNT-3: fir.store {{.*}} to [[retval]] : !fir.ref<index>
+    if (n1 < n2) return 1
+    if (n1 > n2) return 2
+    ! CHECK: {{.*}} = fir.load [[retval]] : !fir.ref<index>
+    ! CHECK-NEXT: return {{.*}} : index
+    return
+  end