[flang][OpenMP] Make parsing of trait properties more context-sensitive

A trait poperty can be one of serveral alternatives, and each property
in a list was parsed as if it could be any of these alternatives
independently from other properties. This made the parsing vulnerable
to certain ambiguities in the trait grammar (provided in the OpenMP
spec).
At the same time the OpenMP spec gives the expected types of properties
for almost every trait: all properties listed for a given trait are
usually of the same type, e.g. names, clauses, etc.

Incorporate these restrictions into the parser, and additionally use
property extensions as the fallback if the parsing of the expected
property type failed. This is intended to allow the parser to succeed,
and instead let the semantic-checking code emit a more user-friendly
message.

Author: kparzysz

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

@@ -479,7 +479,7 @@ class ParseTreeDumper {
   NODE(parser, OmpTraitPropertyName)
   NODE(parser, OmpTraitScore)
   NODE(parser, OmpTraitPropertyExtension)
-  NODE(OmpTraitPropertyExtension, ExtensionValue)
+  NODE(OmpTraitPropertyExtension, Complex)
   NODE(parser, OmpTraitProperty)
   NODE(parser, OmpTraitSelectorName)
   NODE_ENUM(OmpTraitSelectorName, Value)

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

@@ -3505,37 +3505,22 @@ struct OmpTraitScore {
 };
 
 // trait-property-extension ->
-//    trait-property-name (trait-property-value, ...)
-// trait-property-value ->
 //    trait-property-name |
-//    scalar-integer-expression |
-//    trait-property-extension
-//
-// The grammar in OpenMP 5.2+ spec is ambiguous, the above is a different
-// version (but equivalent) that doesn't have ambiguities.
-// The ambiguity is in
-//   trait-property:
-//      trait-property-name          <- (a)
-//      trait-property-clause
-//      trait-property-expression    <- (b)
-//      trait-property-extension     <- this conflicts with (a) and (b)
-//   trait-property-extension:
-//      trait-property-name          <- conflict with (a)
-//      identifier(trait-property-extension[, trait-property-extension[, ...]])
-//      constant integer expression  <- conflict with (b)
+//    scalar-expr |
+//    trait-property-name (trait-property-extension, ...)
 //
 struct OmpTraitPropertyExtension {
   CharBlock source;
-  TUPLE_CLASS_BOILERPLATE(OmpTraitPropertyExtension);
-  struct ExtensionValue {
+  UNION_CLASS_BOILERPLATE(OmpTraitPropertyExtension);
+  struct Complex {  // name (prop-ext, prop-ext, ...)
     CharBlock source;
-    UNION_CLASS_BOILERPLATE(ExtensionValue);
-    std::variant<OmpTraitPropertyName, ScalarExpr,
-        common::Indirection<OmpTraitPropertyExtension>>
-        u;
+    TUPLE_CLASS_BOILERPLATE(Complex);
+    std::tuple<OmpTraitPropertyName,
+        std::list<common::Indirection<OmpTraitPropertyExtension>>>
+        t;
   };
-  using ExtensionList = std::list<ExtensionValue>;
-  std::tuple<OmpTraitPropertyName, ExtensionList> t;
+
+  std::variant<OmpTraitPropertyName, ScalarExpr, Complex> u;
 };
 
 // trait-property ->
@@ -3568,9 +3553,10 @@ struct OmpTraitProperty {
 //    UID |               T        // unique-string-id /impl-defined
 //    VENDOR |            I        // name-list (vendor-id /add-def-doc)
 //    EXTENSION |         I        // name-list (ext_name /impl-defined)
-//    ATOMIC_DEFAULT_MEM_ORDER I | // value of admo
+//    ATOMIC_DEFAULT_MEM_ORDER I | // clause-list (value of admo)
 //    REQUIRES |          I        // clause-list (from requires)
 //    CONDITION           U        // logical-expr
+//    <other name>        I        // treated as extension
 //
 // Trait-set-selectors:
 //    [D]evice, [T]arget_device, [C]onstruct, [I]mplementation, [U]ser.
@@ -3579,7 +3565,7 @@ struct OmpTraitSelectorName {
   UNION_CLASS_BOILERPLATE(OmpTraitSelectorName);
   ENUM_CLASS(Value, Arch, Atomic_Default_Mem_Order, Condition, Device_Num,
       Extension, Isa, Kind, Requires, Simd, Uid, Vendor)
-  std::variant<Value, llvm::omp::Directive> u;
+  std::variant<Value, llvm::omp::Directive, std::string> u;
 };
 
 // trait-selector ->

flang/lib/Parser/openmp-parsers.cpp

@@ -158,31 +158,23 @@ static TypeDeclarationStmt makeIterSpecDecl(std::list<ObjectName> &&names) {
 static std::string nameToString(Name &&name) { return name.ToString(); }
 
 TYPE_PARSER(sourced(construct<OmpTraitPropertyName>( //
-    (space >> charLiteralConstantWithoutKind) ||
-    applyFunction(nameToString, Parser<Name>{}))))
+    construct<OmpTraitPropertyName>(space >> charLiteralConstantWithoutKind) ||
+    construct<OmpTraitPropertyName>(
+        applyFunction(nameToString, Parser<Name>{})))))
 
 TYPE_PARSER(sourced(construct<OmpTraitScore>( //
-    "SCORE" >> parenthesized(scalarIntExpr))))
+    "SCORE"_tok >> parenthesized(scalarIntExpr))))
 
-TYPE_PARSER(sourced(construct<OmpTraitPropertyExtension::ExtensionValue>(
-    // Parse nested extension first.
-    construct<OmpTraitPropertyExtension::ExtensionValue>(
-        indirect(Parser<OmpTraitPropertyExtension>{})) ||
-    construct<OmpTraitPropertyExtension::ExtensionValue>(
-        Parser<OmpTraitPropertyName>{}) ||
-    construct<OmpTraitPropertyExtension::ExtensionValue>(scalarExpr))))
-
-TYPE_PARSER(sourced(construct<OmpTraitPropertyExtension>( //
+TYPE_PARSER(sourced(construct<OmpTraitPropertyExtension::Complex>(
     Parser<OmpTraitPropertyName>{},
     parenthesized(nonemptySeparated(
-        Parser<OmpTraitPropertyExtension::ExtensionValue>{}, ","_tok)))))
+        indirect(Parser<OmpTraitPropertyExtension>{}), ","_tok)))))
 
-TYPE_PARSER(sourced(construct<OmpTraitProperty>(
-    // Try clause first, then extension before OmpTraitPropertyName.
-    construct<OmpTraitProperty>(indirect(Parser<OmpClause>{})) ||
-    construct<OmpTraitProperty>(Parser<OmpTraitPropertyExtension>{}) ||
-    construct<OmpTraitProperty>(Parser<OmpTraitPropertyName>{}) ||
-    construct<OmpTraitProperty>(scalarExpr))))
+TYPE_PARSER(sourced(construct<OmpTraitPropertyExtension>(
+    construct<OmpTraitPropertyExtension>(
+        Parser<OmpTraitPropertyExtension::Complex>{}) ||
+    construct<OmpTraitPropertyExtension>(Parser<OmpTraitPropertyName>{}) ||
+    construct<OmpTraitPropertyExtension>(scalarExpr))))
 
 TYPE_PARSER(construct<OmpTraitSelectorName::Value>(
     "ARCH" >> pure(OmpTraitSelectorName::Value::Arch) ||
@@ -201,15 +193,96 @@ TYPE_PARSER(construct<OmpTraitSelectorName::Value>(
 TYPE_PARSER(sourced(construct<OmpTraitSelectorName>(
     // Parse predefined names first (because of SIMD).
     construct<OmpTraitSelectorName>(Parser<OmpTraitSelectorName::Value>{}) ||
-    construct<OmpTraitSelectorName>(OmpDirectiveNameParser{}))))
+    construct<OmpTraitSelectorName>(OmpDirectiveNameParser{}) ||
+    // identifier-or-string for extensions
+    construct<OmpTraitSelectorName>(
+        applyFunction(nameToString, Parser<Name>{})) ||
+    construct<OmpTraitSelectorName>(space >> charLiteralConstantWithoutKind))))
+
+// Parser for OmpTraitSelector::Properties
+template <typename... PropParser>
+static constexpr auto propertyListParser(PropParser... pp) {
+  // Parse the property list "(score(expr): item1...)" in three steps:
+  // 1. Parse the "("
+  // 2. Parse the optional "score(expr):"
+  // 3. Parse the "item1, ...)", together with the ")".
+  // The reason for including the ")" in the 3rd step is to force parsing
+  // the entire list in each of the alternative property parsers. Otherwise,
+  // the name parser could stop after "foo" in "(foo, bar(1))", without
+  // allowing the next parser to give the list a try.
+
+  using P = OmpTraitProperty;
+  return maybe("(" >>
+      construct<OmpTraitSelector::Properties>(
+          maybe(Parser<OmpTraitScore>{} / ":"_tok),
+          (attempt(nonemptySeparated(construct<P>(pp), ","_tok) / ")"_tok) ||
+              ...)));
+}
+
+// Parser for OmpTraitSelector
+struct TraitSelectorParser {
+  using resultType = OmpTraitSelector;
+
+  constexpr TraitSelectorParser(Parser<OmpTraitSelectorName> p) : np(p) {}
+
+  std::optional<resultType> Parse(ParseState &state) const {
+    auto name{attempt(np).Parse(state)};
+    if (!name.has_value()) {
+      return std::nullopt;
+    }
+
+    // Default fallback parser for lists that cannot be parser using the
+    // primary property parser.
+    auto extParser{Parser<OmpTraitPropertyExtension>{}};
+
+    if (auto *v{std::get_if<OmpTraitSelectorName::Value>(&name->u)}) {
+      switch (*v) {
+      // name-list properties
+      case OmpTraitSelectorName::Value::Arch: // [6.0:319:18]
+      case OmpTraitSelectorName::Value::Extension: // [6.0:319:30]
+      case OmpTraitSelectorName::Value::Isa:  // [6.0:319:15]
+      case OmpTraitSelectorName::Value::Kind: // [6.0:319:10]
+      case OmpTraitSelectorName::Value::Uid: // [6.0:319:23](*)
+      case OmpTraitSelectorName::Value::Vendor: { // [6.0:319:27]
+        auto pp{propertyListParser(Parser<OmpTraitPropertyName>{}, extParser)};
+        return OmpTraitSelector(std::move(*name), std::move(*pp.Parse(state)));
+      }
+      // clause-list
+      case OmpTraitSelectorName::Value::Atomic_Default_Mem_Order:
+        // [6.0:321:26-29](*)
+      case OmpTraitSelectorName::Value::Requires: // [6.0:319:33]
+      case OmpTraitSelectorName::Value::Simd: { // [6.0:318:31]
+        auto pp{propertyListParser(indirect(Parser<OmpClause>{}), extParser)};
+        return OmpTraitSelector(std::move(*name), std::move(*pp.Parse(state)));
+      }
+      // expr-list
+      case OmpTraitSelectorName::Value::Condition: // [6.0:321:33](*)
+      case OmpTraitSelectorName::Value::Device_Num: { // [6.0:321:23-24](*)
+        auto pp{propertyListParser(scalarExpr, extParser)};
+        return OmpTraitSelector(std::move(*name), std::move(*pp.Parse(state)));
+      }
+      // (*) The spec doesn't assign any list-type to these traits, but for
+      // convenience they can be treated as if they were.
+      } // switch
+    } else {
+      // The other alternatives are `llvm::omp::Directive`, and `std::string`.
+      // The former doesn't take any properties[1], the latter is a name of an
+      // extension[2].
+      // [1] [6.0:319:1-2]
+      // [2] [6.0:319:36-37]
+      auto pp{propertyListParser(extParser)};
+      return OmpTraitSelector(std::move(*name), std::move(*pp.Parse(state)));
+    }
 
-TYPE_PARSER(construct<OmpTraitSelector::Properties>(
-    maybe(Parser<OmpTraitScore>{} / ":"_tok),
-    nonemptySeparated(Parser<OmpTraitProperty>{}, ","_tok)))
+    llvm_unreachable("Unhandled trait name?");
+  }
+
+private:
+  const Parser<OmpTraitSelectorName> np;
+};
 
-TYPE_PARSER(sourced(construct<OmpTraitSelector>( //
-    Parser<OmpTraitSelectorName>{}, //
-    maybe(parenthesized(Parser<OmpTraitSelector::Properties>{})))))
+TYPE_PARSER(construct<OmpTraitSelector>(
+    TraitSelectorParser(Parser<OmpTraitSelectorName>{})))
 
 TYPE_PARSER(construct<OmpTraitSetSelectorName::Value>(
     "CONSTRUCT" >> pure(OmpTraitSetSelectorName::Value::Construct) ||

flang/lib/Parser/unparse.cpp

@@ -2075,10 +2075,11 @@ class UnparseVisitor {
     Walk(x.v);
     Put(")");
   }
-  void Unparse(const OmpTraitPropertyExtension &x) {
+  void Unparse(const OmpTraitPropertyExtension::Complex &x) {
+    using PropList = std::list<common::Indirection<OmpTraitPropertyExtension>>;
     Walk(std::get<OmpTraitPropertyName>(x.t));
     Put("(");
-    Walk(std::get<OmpTraitPropertyExtension::ExtensionList>(x.t), ",");
+    Walk(std::get<PropList>(x.t), ",");
     Put(")");
   }
   void Unparse(const OmpTraitSelector &x) {