[flang][OpenMP] Allow UPDATE clause to not have any arguments

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

@@ -4501,6 +4501,11 @@ struct OmpToClause {
 
 // Ref: [5.0:254-255], [5.1:287-288], [5.2:321-322]
 //
+// In ATOMIC construct
+// update-clause ->
+//    UPDATE                                        // Since 4.5
+//
+// In DEPOBJ construct
 // update-clause ->
 //    UPDATE(dependence-type)                       // since 5.0, until 5.1
 // update-clause ->

flang/lib/Lower/OpenMP/Clauses.cpp

@@ -1400,9 +1400,13 @@ Uniform make(const parser::OmpClause::Uniform &inp,
 Update make(const parser::OmpClause::Update &inp,
             semantics::SemanticsContext &semaCtx) {
   // inp.v -> parser::OmpUpdateClause
-  auto depType =
-      common::visit([](auto &&s) { return makeDepType(s); }, inp.v.u);
-  return Update{/*DependenceType=*/depType};
+  if (inp.v) {
+    return common::visit(
+        [](auto &&s) { return Update{/*DependenceType=*/makeDepType(s)}; },
+        inp.v->u);
+  } else {
+    return Update{/*DependenceType=*/std::nullopt};
+  }
 }
 
 Use make(const parser::OmpClause::Use &inp,

flang/lib/Parser/openmp-parsers.cpp

@@ -836,9 +836,9 @@ TYPE_PARSER(construct<OmpInitClause>(
 TYPE_PARSER(construct<OmpAlignedClause>(Parser<OmpObjectList>{},
     maybe(":" >> nonemptyList(Parser<OmpAlignedClause::Modifier>{}))))
 
-TYPE_PARSER(construct<OmpUpdateClause>(
-    construct<OmpUpdateClause>(Parser<OmpDependenceType>{}) ||
-    construct<OmpUpdateClause>(Parser<OmpTaskDependenceType>{})))
+TYPE_PARSER( //
+    construct<OmpUpdateClause>(parenthesized(Parser<OmpDependenceType>{})) ||
+    construct<OmpUpdateClause>(parenthesized(Parser<OmpTaskDependenceType>{})))
 
 TYPE_PARSER(construct<OmpOrderClause>(
     maybe(nonemptyList(Parser<OmpOrderClause::Modifier>{}) / ":"),
@@ -1079,7 +1079,7 @@ TYPE_PARSER( //
                      parenthesized(nonemptyList(name)))) ||
     "UNTIED" >> construct<OmpClause>(construct<OmpClause::Untied>()) ||
     "UPDATE" >> construct<OmpClause>(construct<OmpClause::Update>(
-                    parenthesized(Parser<OmpUpdateClause>{}))) ||
+                    maybe(Parser<OmpUpdateClause>{}))) ||
     "WHEN" >> construct<OmpClause>(construct<OmpClause::When>(
                   parenthesized(Parser<OmpWhenClause>{}))) ||
     // Cancellable constructs
@@ -1313,24 +1313,30 @@ TYPE_PARSER(
     endOfLine)
 
 // Directives enclosing structured-block
-TYPE_PARSER(construct<OmpBlockDirective>(first(
-    "MASKED" >> pure(llvm::omp::Directive::OMPD_masked),
-    "MASTER" >> pure(llvm::omp::Directive::OMPD_master),
-    "ORDERED" >> pure(llvm::omp::Directive::OMPD_ordered),
-    "PARALLEL MASKED" >> pure(llvm::omp::Directive::OMPD_parallel_masked),
-    "PARALLEL MASTER" >> pure(llvm::omp::Directive::OMPD_parallel_master),
-    "PARALLEL WORKSHARE" >> pure(llvm::omp::Directive::OMPD_parallel_workshare),
-    "PARALLEL" >> pure(llvm::omp::Directive::OMPD_parallel),
-    "SCOPE" >> pure(llvm::omp::Directive::OMPD_scope),
-    "SINGLE" >> pure(llvm::omp::Directive::OMPD_single),
-    "TARGET DATA" >> pure(llvm::omp::Directive::OMPD_target_data),
-    "TARGET PARALLEL" >> pure(llvm::omp::Directive::OMPD_target_parallel),
-    "TARGET TEAMS" >> pure(llvm::omp::Directive::OMPD_target_teams),
-    "TARGET" >> pure(llvm::omp::Directive::OMPD_target),
-    "TASK"_id >> pure(llvm::omp::Directive::OMPD_task),
-    "TASKGROUP" >> pure(llvm::omp::Directive::OMPD_taskgroup),
-    "TEAMS" >> pure(llvm::omp::Directive::OMPD_teams),
-    "WORKSHARE" >> pure(llvm::omp::Directive::OMPD_workshare))))
+TYPE_PARSER(
+    // In this context "TARGET UPDATE" can be parsed as a TARGET directive
+    // followed by an UPDATE clause. This is the only combination at the
+    // moment, exclude it explicitly.
+    (!"TARGET UPDATE"_sptok) >=
+    construct<OmpBlockDirective>(first(
+        "MASKED" >> pure(llvm::omp::Directive::OMPD_masked),
+        "MASTER" >> pure(llvm::omp::Directive::OMPD_master),
+        "ORDERED" >> pure(llvm::omp::Directive::OMPD_ordered),
+        "PARALLEL MASKED" >> pure(llvm::omp::Directive::OMPD_parallel_masked),
+        "PARALLEL MASTER" >> pure(llvm::omp::Directive::OMPD_parallel_master),
+        "PARALLEL WORKSHARE" >>
+            pure(llvm::omp::Directive::OMPD_parallel_workshare),
+        "PARALLEL" >> pure(llvm::omp::Directive::OMPD_parallel),
+        "SCOPE" >> pure(llvm::omp::Directive::OMPD_scope),
+        "SINGLE" >> pure(llvm::omp::Directive::OMPD_single),
+        "TARGET DATA" >> pure(llvm::omp::Directive::OMPD_target_data),
+        "TARGET PARALLEL" >> pure(llvm::omp::Directive::OMPD_target_parallel),
+        "TARGET TEAMS" >> pure(llvm::omp::Directive::OMPD_target_teams),
+        "TARGET" >> pure(llvm::omp::Directive::OMPD_target),
+        "TASK"_id >> pure(llvm::omp::Directive::OMPD_task),
+        "TASKGROUP" >> pure(llvm::omp::Directive::OMPD_taskgroup),
+        "TEAMS" >> pure(llvm::omp::Directive::OMPD_teams),
+        "WORKSHARE" >> pure(llvm::omp::Directive::OMPD_workshare))))
 
 TYPE_PARSER(sourced(construct<OmpBeginBlockDirective>(
     sourced(Parser<OmpBlockDirective>{}), Parser<OmpClauseList>{})))

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

@@ -4571,10 +4571,18 @@ void OmpStructureChecker::Enter(const parser::OmpClause::Update &x) {
   llvm::omp::Directive dir{GetContext().directive};
   unsigned version{context_.langOptions().OpenMPVersion};
 
-  auto *depType{std::get_if<parser::OmpDependenceType>(&x.v.u)};
-  auto *taskType{std::get_if<parser::OmpTaskDependenceType>(&x.v.u)};
-  assert(((depType == nullptr) != (taskType == nullptr)) &&
-      "Unexpected alternative in update clause");
+  const parser::OmpDependenceType *depType{nullptr};
+  const parser::OmpTaskDependenceType *taskType{nullptr};
+  if (auto &maybeUpdate{x.v}) {
+    depType = std::get_if<parser::OmpDependenceType>(&maybeUpdate->u);
+    taskType = std::get_if<parser::OmpTaskDependenceType>(&maybeUpdate->u);
+  }
+
+  if (!depType && !taskType) {
+    assert(dir == llvm::omp::Directive::OMPD_atomic &&
+        "Unexpected alternative in update clause");
+    return;
+  }
 
   if (depType) {
     CheckDependenceType(depType->v);

llvm/include/llvm/Frontend/OpenMP/OMP.td

@@ -525,6 +525,7 @@ def OMPC_Untied : Clause<"untied"> {
 def OMPC_Update : Clause<"update"> {
   let clangClass = "OMPUpdateClause";
   let flangClass = "OmpUpdateClause";
+  let isValueOptional = true;
 }
 def OMPC_Use : Clause<"use"> {
   let clangClass = "OMPUseClause";