[Flang][OpenMP] Accept the reduction modifier (#86492)

Accept the reduction modifier in the Flang parser. Issue a TODO message
during lowering.

OpenMP 5.0 introduced the reduction modifier. Details can be seen in
2.19.5.4 reductionClause.
OpenMP 5.2 relevant section is 5.5.8reductionClause.

This will help remove some of the parser errors highlighted in the
following post and also bring it to a well defined behaviour (produce
TODO errors for unsupported features, do not crash).
https://discourse.llvm.org/t/proposal-rename-flang-new-to-flang/69462/60

Author: kiranchandramohan

flang/include/flang/Parser/dump-parse-tree.h

@@ -542,6 +542,7 @@ class ParseTreeDumper {
   NODE_ENUM(OmpOrderModifier, Kind)
   NODE(parser, OmpProcBindClause)
   NODE_ENUM(OmpProcBindClause, Type)
+  NODE_ENUM(OmpReductionClause, ReductionModifier)
   NODE(parser, OmpReductionClause)
   NODE(parser, OmpInReductionClause)
   NODE(parser, OmpReductionCombiner)

flang/include/flang/Parser/parse-tree.h

@@ -3552,7 +3552,10 @@ struct OmpReductionOperator {
 //                                         variable-name-list)
 struct OmpReductionClause {
   TUPLE_CLASS_BOILERPLATE(OmpReductionClause);
-  std::tuple<OmpReductionOperator, OmpObjectList> t;
+  ENUM_CLASS(ReductionModifier, Inscan, Task, Default)
+  std::tuple<std::optional<ReductionModifier>, OmpReductionOperator,
+      OmpObjectList>
+      t;
 };
 
 // OMP 5.0 2.19.5.6 in_reduction-clause -> IN_REDUCTION (reduction-identifier:

flang/lib/Lower/OpenMP/Clauses.cpp

@@ -1005,12 +1005,28 @@ ProcBind make(const parser::OmpClause::ProcBind &inp,
 Reduction make(const parser::OmpClause::Reduction &inp,
                semantics::SemanticsContext &semaCtx) {
   // inp.v -> parser::OmpReductionClause
-  auto &t0 = std::get<parser::OmpReductionOperator>(inp.v.t);
-  auto &t1 = std::get<parser::OmpObjectList>(inp.v.t);
+  using wrapped = parser::OmpReductionClause;
+
+  CLAUSET_ENUM_CONVERT( //
+      convert, wrapped::ReductionModifier, Reduction::ReductionModifier,
+      // clang-format off
+      MS(Inscan,  Inscan)
+      MS(Task,    Task)
+      MS(Default, Default)
+      // clang-format on
+  );
+
+  auto &t0 =
+      std::get<std::optional<parser::OmpReductionClause::ReductionModifier>>(
+          inp.v.t);
+  auto &t1 = std::get<parser::OmpReductionOperator>(inp.v.t);
+  auto &t2 = std::get<parser::OmpObjectList>(inp.v.t);
   return Reduction{
-      {/*ReductionIdentifiers=*/{makeReductionOperator(t0, semaCtx)},
-       /*ReductionModifier=*/std::nullopt,
-       /*List=*/makeObjects(t1, semaCtx)}};
+      {/*ReductionModifier=*/t0
+           ? std::make_optional<Reduction::ReductionModifier>(convert(*t0))
+           : std::nullopt,
+       /*ReductionIdentifiers=*/{makeReductionOperator(t1, semaCtx)},
+       /*List=*/makeObjects(t2, semaCtx)}};
 }
 
 // Relaxed: empty

flang/lib/Lower/OpenMP/ReductionProcessor.cpp

@@ -574,6 +574,11 @@ void ReductionProcessor::addDeclareReduction(
     llvm::SmallVectorImpl<const Fortran::semantics::Symbol *>
         *reductionSymbols) {
   fir::FirOpBuilder &firOpBuilder = converter.getFirOpBuilder();
+
+  if (std::get<std::optional<omp::clause::Reduction::ReductionModifier>>(
+          reduction.t))
+    TODO(currentLocation, "Reduction modifiers are not supported");
+
   mlir::omp::DeclareReductionOp decl;
   const auto &redOperatorList{
       std::get<omp::clause::Reduction::ReductionIdentifiers>(reduction.t)};

flang/lib/Parser/openmp-parsers.cpp

@@ -136,6 +136,11 @@ TYPE_PARSER(construct<OmpReductionOperator>(Parser<DefinedOperator>{}) ||
     construct<OmpReductionOperator>(Parser<ProcedureDesignator>{}))
 
 TYPE_PARSER(construct<OmpReductionClause>(
+    maybe(
+        ("INSCAN" >> pure(OmpReductionClause::ReductionModifier::Inscan) ||
+            "TASK" >> pure(OmpReductionClause::ReductionModifier::Task) ||
+            "DEFAULT" >> pure(OmpReductionClause::ReductionModifier::Default)) /
+        ","),
     Parser<OmpReductionOperator>{} / ":", Parser<OmpObjectList>{}))
 
 // OMP 5.0 2.19.5.6 IN_REDUCTION (reduction-identifier: variable-name-list)

flang/lib/Parser/unparse.cpp

@@ -2090,6 +2090,8 @@ class UnparseVisitor {
     Walk(":", x.step);
   }
   void Unparse(const OmpReductionClause &x) {
+    Walk(std::get<std::optional<OmpReductionClause::ReductionModifier>>(x.t),
+        ",");
     Walk(std::get<OmpReductionOperator>(x.t));
     Put(":");
     Walk(std::get<OmpObjectList>(x.t));
@@ -2727,6 +2729,8 @@ class UnparseVisitor {
   WALK_NESTED_ENUM(OmpScheduleClause, ScheduleType) // OMP schedule-type
   WALK_NESTED_ENUM(OmpDeviceClause, DeviceModifier) // OMP device modifier
   WALK_NESTED_ENUM(OmpDeviceTypeClause, Type) // OMP DEVICE_TYPE
+  WALK_NESTED_ENUM(
+      OmpReductionClause, ReductionModifier) // OMP reduction-modifier
   WALK_NESTED_ENUM(OmpIfClause, DirectiveNameModifier) // OMP directive-modifier
   WALK_NESTED_ENUM(OmpCancelType, Type) // OMP cancel-type
   WALK_NESTED_ENUM(OmpOrderClause, Type) // OMP order-type

flang/lib/Semantics/check-omp-structure.cpp

@@ -2289,7 +2289,7 @@ void OmpStructureChecker::Enter(const parser::OmpClause::Reduction &x) {
 bool OmpStructureChecker::CheckReductionOperators(
     const parser::OmpClause::Reduction &x) {
 
-  const auto &definedOp{std::get<0>(x.v.t)};
+  const auto &definedOp{std::get<parser::OmpReductionOperator>(x.v.t)};
   bool ok = false;
   common::visit(
       common::visitors{

flang/test/Lower/OpenMP/Todo/reduction-modifiers.f90

@@ -0,0 +1,13 @@
+! RUN: %not_todo_cmd bbc -emit-fir -fopenmp -o - %s 2>&1 | FileCheck %s
+! RUN: %not_todo_cmd %flang_fc1 -emit-fir -fopenmp -o - %s 2>&1 | FileCheck %s
+
+! CHECK: not yet implemented: Reduction modifiers are not supported
+
+subroutine foo()
+  integer :: i, j
+  j = 0
+  !$omp do reduction (inscan, *: j)
+  do i = 1, 10
+    j = j + 1
+  end do
+end subroutine

flang/test/Parser/OpenMP/reduction-modifier.f90

@@ -0,0 +1,20 @@
+! RUN: %flang_fc1 -fdebug-unparse-no-sema -fopenmp %s | FileCheck --ignore-case %s
+! RUN: %flang_fc1 -fdebug-dump-parse-tree-no-sema -fopenmp %s | FileCheck --check-prefix="PARSE-TREE" %s
+
+subroutine foo()
+  integer :: i, j
+  j = 0
+! CHECK: !$OMP DO  REDUCTION(TASK,*:j)
+! PARSE-TREE: | | ExecutionPartConstruct -> ExecutableConstruct -> OpenMPConstruct -> OpenMPLoopConstruct
+! PARSE-TREE: | | | OmpBeginLoopDirective
+! PARSE-TREE: | | | | OmpLoopDirective -> llvm::omp::Directive = do
+! PARSE-TREE: | | | | OmpClauseList -> OmpClause -> Reduction -> OmpReductionClause
+! PARSE-TREE: | | | | | ReductionModifier = Task
+! PARSE-TREE: | | | | | OmpReductionOperator -> DefinedOperator -> IntrinsicOperator = Multiply
+! PARSE-TREE: | | | | | OmpObjectList -> OmpObject -> Designator -> DataRef -> Name = 'j
+  !$omp do reduction (task, *: j)
+  do i = 1, 10
+    j = j + 1
+  end do
+  !$omp end do
+end

llvm/include/llvm/Frontend/OpenMP/ClauseT.h

@@ -949,7 +949,7 @@ struct ReductionT {
   using ReductionIdentifiers = ListT<type::ReductionIdentifierT<I, E>>;
   ENUM(ReductionModifier, Default, Inscan, Task);
   using TupleTrait = std::true_type;
-  std::tuple<ReductionIdentifiers, OPT(ReductionModifier), List> t;
+  std::tuple<OPT(ReductionModifier), ReductionIdentifiers, List> t;
 };
 
 // V5.2: [15.8.1] `memory-order` clauses