[flang][OpenMP] Avoid unnecessary parsing of OpenMP constructs

[kparzysz]

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.

[llvmbot]

@llvm/pr-subscribers-flang-openmp

@llvm/pr-subscribers-flang-parser

Author: Krzysztof Parzyszek (kparzysz)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/148629.diff

6 Files Affected:

• (modified) flang/include/flang/Parser/parse-tree.h (+10)
• (modified) flang/lib/Parser/openmp-parsers.cpp (+8-1)
• (modified) flang/lib/Parser/parse-tree.cpp (+47)
• (modified) flang/lib/Parser/program-parsers.cpp (+1-1)
• (modified) flang/lib/Parser/type-parsers.h (+1)
• (modified) flang/test/Parser/OpenMP/fail-construct1.f90 (+1-1)

diff --git a/flang/include/flang/Parser/parse-tree.h b/flang/include/flang/Parser/parse-tree.h
index ab2dde7d5dfbe..cc1d032f94d4a 100644
--- a/flang/include/flang/Parser/parse-tree.h
+++ b/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,
diff --git a/flang/lib/Parser/openmp-parsers.cpp b/flang/lib/Parser/openmp-parsers.cpp
index d70aaab82cbab..76c9499410017 100644
--- a/flang/lib/Parser/openmp-parsers.cpp
+++ b/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,
diff --git a/flang/lib/Parser/parse-tree.cpp b/flang/lib/Parser/parse-tree.cpp
index 824612e49293f..7b46c9f469b06 100644
--- a/flang/lib/Parser/parse-tree.cpp
+++ b/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)) {
diff --git a/flang/lib/Parser/program-parsers.cpp b/flang/lib/Parser/program-parsers.cpp
index d6e7f11297187..5f4e62ffdbbf2 100644
--- a/flang/lib/Parser/program-parsers.cpp
+++ b/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{
diff --git a/flang/lib/Parser/type-parsers.h b/flang/lib/Parser/type-parsers.h
index 8e91ede093fc4..3900c5a86c874 100644
--- a/flang/lib/Parser/type-parsers.h
+++ b/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
diff --git a/flang/test/Parser/OpenMP/fail-construct1.f90 b/flang/test/Parser/OpenMP/fail-construct1.f90
index f0ee22125cee0..f0b3f7438ae58 100644
--- a/flang/test/Parser/OpenMP/fail-construct1.f90
+++ b/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