[flang][OpenMP] Avoid unnecessary parsing of OpenMP constructs (#148629)

When parsing a specification part, the parser will look ahead to see if
the next construct is an executable construct. In doing so it will
invoke OpenMPConstruct parser, whereas the only necessary thing to check
would be the directive alone.

Author: kparzysz

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

@@ -3481,6 +3481,9 @@ struct OmpDirectiveName {
   // This allows "construct<OmpDirectiveName>(Verbatim("<name>"))".
   OmpDirectiveName(const Verbatim &name);
   using WrapperTrait = std::true_type;
+
+  bool IsExecutionPart() const; // Is allowed in the execution part
+
   CharBlock source;
   llvm::omp::Directive v{llvm::omp::Directive::OMPD_unknown};
 };
@@ -5028,6 +5031,13 @@ struct OpenMPLoopConstruct {
       t;
 };
 
+// Lookahead class to identify execution-part OpenMP constructs without
+// parsing the entire OpenMP construct.
+struct OpenMPExecDirective {
+  WRAPPER_CLASS_BOILERPLATE(OpenMPExecDirective, OmpDirectiveName);
+  CharBlock source;
+};
+
 struct OpenMPConstruct {
   UNION_CLASS_BOILERPLATE(OpenMPConstruct);
   std::variant<OpenMPStandaloneConstruct, OpenMPSectionsConstruct,

flang/lib/Parser/openmp-parsers.cpp

@@ -912,7 +912,7 @@ TYPE_PARSER(construct<OmpNumTasksClause>(
     scalarIntExpr))
 
 TYPE_PARSER(
-    construct<OmpObject>(designator) || construct<OmpObject>("/" >> name / "/"))
+    construct<OmpObject>(designator) || "/" >> construct<OmpObject>(name) / "/")
 
 // OMP 5.0 2.19.4.5 LASTPRIVATE ([lastprivate-modifier :] list)
 TYPE_PARSER(construct<OmpLastprivateClause>(
@@ -1757,6 +1757,13 @@ TYPE_PARSER(construct<OpenMPSectionsConstruct>(
     Parser<OmpBeginSectionsDirective>{} / endOmpLine,
     Parser<OmpSectionBlocks>{}, Parser<OmpEndSectionsDirective>{} / endOmpLine))
 
+static bool IsExecutionPart(const OmpDirectiveName &name) {
+  return name.IsExecutionPart();
+}
+
+TYPE_PARSER(construct<OpenMPExecDirective>(
+    startOmpLine >> predicated(Parser<OmpDirectiveName>{}, IsExecutionPart)))
+
 TYPE_CONTEXT_PARSER("OpenMP construct"_en_US,
     startOmpLine >>
         withMessage("expected OpenMP construct"_err_en_US,

flang/lib/Parser/parse-tree.cpp

@@ -368,6 +368,53 @@ llvm::omp::Clause OmpClause::Id() const {
   return std::visit([](auto &&s) { return getClauseIdForClass(s); }, u);
 }
 
+bool OmpDirectiveName::IsExecutionPart() const {
+  // Can the directive appear in the execution part of the program.
+  llvm::omp::Directive id{v};
+  switch (llvm::omp::getDirectiveCategory(id)) {
+  case llvm::omp::Category::Executable:
+    return true;
+  case llvm::omp::Category::Declarative:
+    switch (id) {
+    case llvm::omp::Directive::OMPD_allocate:
+      return true;
+    default:
+      return false;
+    }
+    break;
+  case llvm::omp::Category::Informational:
+    switch (id) {
+    case llvm::omp::Directive::OMPD_assume:
+      return true;
+    default:
+      return false;
+    }
+    break;
+  case llvm::omp::Category::Meta:
+    return true;
+  case llvm::omp::Category::Subsidiary:
+    switch (id) {
+    // TODO: case llvm::omp::Directive::OMPD_task_iteration:
+    case llvm::omp::Directive::OMPD_section:
+    case llvm::omp::Directive::OMPD_scan:
+      return true;
+    default:
+      return false;
+    }
+    break;
+  case llvm::omp::Category::Utility:
+    switch (id) {
+    case llvm::omp::Directive::OMPD_error:
+    case llvm::omp::Directive::OMPD_nothing:
+      return true;
+    default:
+      return false;
+    }
+    break;
+  }
+  return false;
+}
+
 const OmpArgumentList &OmpDirectiveSpecification::Arguments() const {
   static OmpArgumentList empty{decltype(OmpArgumentList::v){}};
   if (auto &arguments = std::get<std::optional<OmpArgumentList>>(t)) {

flang/lib/Parser/program-parsers.cpp

@@ -108,7 +108,7 @@ constexpr auto actionStmtLookAhead{first(actionStmt >> ok,
     "ALLOCATE ("_tok, "CALL" >> name >> "("_tok, "GO TO"_tok, "OPEN ("_tok,
     "PRINT"_tok / space / !"("_tok, "READ ("_tok, "WRITE ("_tok)};
 constexpr auto execPartLookAhead{first(actionStmtLookAhead >> ok,
-    openaccConstruct >> ok, openmpConstruct >> ok, "ASSOCIATE ("_tok,
+    openaccConstruct >> ok, openmpExecDirective >> ok, "ASSOCIATE ("_tok,
     "BLOCK"_tok, "SELECT"_tok, "CHANGE TEAM"_sptok, "CRITICAL"_tok, "DO"_tok,
     "IF ("_tok, "WHERE ("_tok, "FORALL ("_tok, "!$CUF"_tok)};
 constexpr auto declErrorRecovery{

flang/lib/Parser/type-parsers.h

@@ -137,6 +137,7 @@ constexpr Parser<CompilerDirective> compilerDirective;
 constexpr Parser<OpenACCConstruct> openaccConstruct;
 constexpr Parser<OpenACCDeclarativeConstruct> openaccDeclarativeConstruct;
 constexpr Parser<OpenMPConstruct> openmpConstruct;
+constexpr Parser<OpenMPExecDirective> openmpExecDirective;
 constexpr Parser<OpenMPDeclarativeConstruct> openmpDeclarativeConstruct;
 constexpr Parser<OmpEndLoopDirective> ompEndLoopDirective;
 constexpr Parser<IntrinsicVectorTypeSpec> intrinsicVectorTypeSpec; // Extension

flang/test/Parser/OpenMP/fail-construct1.f90

@@ -1,5 +1,5 @@
 ! RUN: not %flang_fc1 -fsyntax-only -fopenmp %s 2>&1 | FileCheck %s
 
-! CHECK: error: expected OpenMP construct
 !$omp  parallel
+! CHECK: error: expected '!$OMP '
 end