[flang][OpenMP] Introduce variant argument, customize OmpArgument par… (#160372)

…sing

The DECLARE_VARIANT directive takes two names separated by a colon as an
argument: base-name:variant-name. Define OmpBaseVariantNames to
represent this, since no existing argument alternative matches it.

However, there is an issue. The syntax "name1:name2" can be the argument
to DECLARE_VARIANT (if both names are OmpObjects), but it can also be a
reduction-specifier if "name2" is a type. This conflict can only be
resolved once we know what the names are, which is after name resolution
has visited them. The problem is that name resolution has side-effects
that may be (practically) impossible to undo (e.g. creating new symbols,
emitting diagnostic messages).

To avoid this problem this PR makes the parsing of OmpArgument
directive- sensitive: when the directive is DECLARE_VARIANT, don't
attempt to parse a reduction-specifier, consider OmpBaseVariantNames
instead. Otherwise ignore OmpBaseVariantNames in favor of
reduction-specifier.

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

@@ -522,6 +522,7 @@ public:
   NODE(parser, OmpAtomicDefaultMemOrderClause)
   NODE(parser, OmpAutomapModifier)
   NODE_ENUM(OmpAutomapModifier, Value)
+  NODE(parser, OmpBaseVariantNames)
   NODE(parser, OmpBeginDirective)
   NODE(parser, OmpBeginLoopDirective)
   NODE(parser, OmpBeginSectionsDirective)

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

@@ -3555,6 +3555,18 @@ struct OmpLocator {
 
 WRAPPER_CLASS(OmpLocatorList, std::list<OmpLocator>);
 
+// Ref: [4.5:58-60], [5.0:58-60], [5.1:63-68], [5.2:197-198], [6.0:334-336]
+//
+// Argument to DECLARE VARIANT with the base-name present. (When only
+// variant-name is present, it is a simple OmpObject).
+//
+// base-name-variant-name ->                        // since 4.5
+//    base-name : variant-name
+struct OmpBaseVariantNames {
+  TUPLE_CLASS_BOILERPLATE(OmpBaseVariantNames);
+  std::tuple<OmpObject, OmpObject> t;
+};
+
 // Ref: [5.0:326:10-16], [5.1:359:5-11], [5.2:163:2-7], [6.0:293:16-21]
 //
 // mapper-specifier ->
@@ -3584,6 +3596,7 @@ struct OmpArgument {
   CharBlock source;
   UNION_CLASS_BOILERPLATE(OmpArgument);
   std::variant<OmpLocator, // {variable, extended, locator}-list-item
+      OmpBaseVariantNames, // base-name:variant-name
       OmpMapperSpecifier, OmpReductionSpecifier>
       u;
 };

flang/lib/Parser/openmp-parsers.cpp

@@ -315,15 +315,56 @@ TYPE_PARSER( //
     construct<OmpLocator>(Parser<OmpObject>{}) ||
     construct<OmpLocator>(Parser<FunctionReference>{}))
 
-TYPE_PARSER(sourced( //
-    construct<OmpArgument>(Parser<OmpMapperSpecifier>{}) ||
-    construct<OmpArgument>(Parser<OmpReductionSpecifier>{}) ||
-    construct<OmpArgument>(Parser<OmpLocator>{})))
+TYPE_PARSER(construct<OmpBaseVariantNames>(
+    Parser<OmpObject>{} / ":", Parser<OmpObject>{}))
+
+// Make the parsing of OmpArgument directive-sensitive. The issue is that
+// name1:name2 can match either OmpBaseVariantNames or OmpReductionSpecifier.
+// In the former case, "name2" is a name of a function, in the latter, of a
+// type. To resolve the conflict we need information provided by name
+// resolution, but by that time we can't modify the AST anymore, and the
+// name resolution may have implicitly declared a symbol, or issued a message.
+template <llvm::omp::Directive Id = llvm::omp::Directive::OMPD_unknown>
+struct OmpArgumentParser {
+  using resultType = OmpArgument;
+
+  std::optional<resultType> Parse(ParseState &state) const {
+    constexpr auto parser{sourced(first( //
+        construct<OmpArgument>(Parser<OmpMapperSpecifier>{}),
+        // By default, prefer OmpReductionSpecifier over OmpBaseVariantNames.
+        construct<OmpArgument>(Parser<OmpReductionSpecifier>{}),
+        construct<OmpArgument>(Parser<OmpLocator>{})))};
+    return parser.Parse(state);
+  }
+};
+
+template <>
+struct OmpArgumentParser<llvm::omp::Directive::OMPD_declare_variant> {
+  using resultType = OmpArgument;
+
+  std::optional<resultType> Parse(ParseState &state) const {
+    constexpr auto parser{sourced(first( //
+        construct<OmpArgument>(Parser<OmpMapperSpecifier>{}),
+        // In DECLARE_VARIANT parse OmpBaseVariantNames instead of
+        // OmpReductionSpecifier.
+        construct<OmpArgument>(Parser<OmpBaseVariantNames>{}),
+        construct<OmpArgument>(Parser<OmpLocator>{})))};
+    return parser.Parse(state);
+  }
+};
 
 TYPE_PARSER(construct<OmpLocatorList>(nonemptyList(Parser<OmpLocator>{})))
 
-TYPE_PARSER(sourced( //
-    construct<OmpArgumentList>(nonemptyList(Parser<OmpArgument>{}))))
+template <llvm::omp::Directive Id = llvm::omp::Directive::OMPD_unknown>
+struct OmpArgumentListParser {
+  using resultType = OmpArgumentList;
+
+  std::optional<resultType> Parse(ParseState &state) const {
+    return sourced(
+        construct<OmpArgumentList>(nonemptyList(OmpArgumentParser<Id>{})))
+        .Parse(state);
+  }
+};
 
 TYPE_PARSER( //
     construct<OmpTypeSpecifier>(Parser<DeclarationTypeSpec>{}) ||
@@ -1312,12 +1353,23 @@ TYPE_PARSER(
         applyFunction<OmpDirectiveSpecification>(makeFlushFromOldSyntax,
             verbatim("FLUSH"_tok) / !lookAhead("("_tok),
             maybe(Parser<OmpClauseList>{}),
-            maybe(parenthesized(Parser<OmpArgumentList>{})),
+            maybe(parenthesized(
+                OmpArgumentListParser<llvm::omp::Directive::OMPD_flush>{})),
             pure(OmpDirectiveSpecification::Flags::DeprecatedSyntax)))) ||
+    // Parse DECLARE_VARIANT individually, because the "[base:]variant"
+    // argument will conflict with DECLARE_REDUCTION's "ident:types...".
+    predicated(Parser<OmpDirectiveName>{},
+        IsDirective(llvm::omp::Directive::OMPD_declare_variant)) >=
+        sourced(construct<OmpDirectiveSpecification>(
+            sourced(OmpDirectiveNameParser{}),
+            maybe(parenthesized(OmpArgumentListParser<
+                llvm::omp::Directive::OMPD_declare_variant>{})),
+            maybe(Parser<OmpClauseList>{}),
+            pure(OmpDirectiveSpecification::Flags::None))) ||
     // Parse the standard syntax: directive [(arguments)] [clauses]
     sourced(construct<OmpDirectiveSpecification>( //
         sourced(OmpDirectiveNameParser{}),
-        maybe(parenthesized(Parser<OmpArgumentList>{})),
+        maybe(parenthesized(OmpArgumentListParser<>{})),
         maybe(Parser<OmpClauseList>{}),
         pure(OmpDirectiveSpecification::Flags::None))))
 

flang/lib/Parser/unparse.cpp

@@ -2089,6 +2089,11 @@ public:
   // OpenMP Clauses & Directives
   void Unparse(const OmpArgumentList &x) { Walk(x.v, ", "); }
 
+  void Unparse(const OmpBaseVariantNames &x) {
+    Walk(std::get<0>(x.t)); // OmpObject
+    Put(":");
+    Walk(std::get<1>(x.t)); // OmpObject
+  }
   void Unparse(const OmpTypeNameList &x) { //
     Walk(x.v, ",");
   }

flang/lib/Semantics/resolve-names.cpp

@@ -1998,6 +1998,10 @@ bool OmpVisitor::Pre(const parser::OmpDirectiveSpecification &x) {
               ProcessReductionSpecifier(spec, clauses);
               visitClauses = false;
             },
+            [&](const parser::OmpBaseVariantNames &names) {
+              Walk(std::get<0>(names.t));
+              Walk(std::get<1>(names.t));
+            },
             [&](const parser::OmpLocator &locator) {
               // Manually resolve names in CRITICAL directives. This is because
               // these names do not denote Fortran objects, and the CRITICAL