[flang][OpenMP] Implement OpenMP stylized expressions (llvm#165049)

Consider OpenMP stylized expression to be a template to be instantiated
with a series of types listed on the containing directive (currently
DECLARE_REDUCTION). Create a series of instantiations in the parser,
allowing OpenMP special variables to be declared separately for each
type.

---------

Co-authored-by: Tom Eccles <[email protected]>

Author: kparzysz

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

@@ -599,7 +599,7 @@ class ParseTreeDumper {
   NODE(parser, OmpInitClause)
   NODE(OmpInitClause, Modifier)
   NODE(parser, OmpInitializerClause)
-  NODE(parser, OmpInitializerProc)
+  NODE(parser, OmpInitializerExpression)
   NODE(parser, OmpInReductionClause)
   NODE(OmpInReductionClause, Modifier)
   NODE(parser, OmpInteropPreference)
@@ -677,6 +677,10 @@ class ParseTreeDumper {
   NODE_ENUM(OmpSeverityClause, Severity)
   NODE(parser, OmpStepComplexModifier)
   NODE(parser, OmpStepSimpleModifier)
+  NODE(parser, OmpStylizedDeclaration)
+  NODE(parser, OmpStylizedExpression)
+  NODE(parser, OmpStylizedInstance)
+  NODE(OmpStylizedInstance, Instance)
   NODE(parser, OmpTaskDependenceType)
   NODE_ENUM(OmpTaskDependenceType, Value)
   NODE(parser, OmpTaskReductionClause)

flang/include/flang/Parser/openmp-utils.h

@@ -25,6 +25,13 @@
 
 namespace Fortran::parser::omp {
 
+template <typename T> constexpr auto addr_if(std::optional<T> &x) {
+  return x ? &*x : nullptr;
+}
+template <typename T> constexpr auto addr_if(const std::optional<T> &x) {
+  return x ? &*x : nullptr;
+}
+
 namespace detail {
 using D = llvm::omp::Directive;
 
@@ -133,9 +140,24 @@ template <typename T> OmpDirectiveName GetOmpDirectiveName(const T &x) {
 }
 
 const OmpObjectList *GetOmpObjectList(const OmpClause &clause);
+
+template <typename T>
+const T *GetFirstArgument(const OmpDirectiveSpecification &spec) {
+  for (const OmpArgument &arg : spec.Arguments().v) {
+    if (auto *t{std::get_if<T>(&arg.u)}) {
+      return t;
+    }
+  }
+  return nullptr;
+}
+
 const BlockConstruct *GetFortranBlockConstruct(
     const ExecutionPartConstruct &epc);
 
+const OmpCombinerExpression *GetCombinerExpr(
+    const OmpReductionSpecifier &rspec);
+const OmpInitializerExpression *GetInitializerExpr(const OmpClause &init);
+
 } // namespace Fortran::parser::omp
 
 #endif // FORTRAN_PARSER_OPENMP_UTILS_H

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

@@ -24,7 +24,9 @@
 #include "provenance.h"
 #include "flang/Common/idioms.h"
 #include "flang/Common/indirection.h"
+#include "flang/Common/reference.h"
 #include "flang/Support/Fortran.h"
+#include "llvm/ADT/ArrayRef.h"
 #include "llvm/Frontend/OpenACC/ACC.h.inc"
 #include "llvm/Frontend/OpenMP/OMP.h"
 #include "llvm/Frontend/OpenMP/OMPConstants.h"
@@ -3510,6 +3512,8 @@ struct OmpDirectiveName {
 
 // type-name list item
 struct OmpTypeName {
+  CharBlock source;
+  mutable const semantics::DeclTypeSpec *declTypeSpec{nullptr};
   UNION_CLASS_BOILERPLATE(OmpTypeName);
   std::variant<TypeSpec, DeclarationTypeSpec> u;
 };
@@ -3538,6 +3542,39 @@ struct OmpObjectList {
   WRAPPER_CLASS_BOILERPLATE(OmpObjectList, std::list<OmpObject>);
 };
 
+struct OmpStylizedDeclaration {
+  COPY_AND_ASSIGN_BOILERPLATE(OmpStylizedDeclaration);
+  // Since "Reference" isn't handled by parse-tree-visitor, add EmptyTrait,
+  // and visit the members by hand when needed.
+  using EmptyTrait = std::true_type;
+  common::Reference<const OmpTypeName> type;
+  EntityDecl var;
+};
+
+struct OmpStylizedInstance {
+  struct Instance {
+    UNION_CLASS_BOILERPLATE(Instance);
+    std::variant<AssignmentStmt, CallStmt, common::Indirection<Expr>> u;
+  };
+  TUPLE_CLASS_BOILERPLATE(OmpStylizedInstance);
+  std::tuple<std::list<OmpStylizedDeclaration>, Instance> t;
+};
+
+class ParseState;
+
+// Ref: [5.2:76], [6.0:185]
+//
+struct OmpStylizedExpression {
+  CharBlock source;
+  // Pointer to a temporary copy of the ParseState that is used to create
+  // additional parse subtrees for the stylized expression. This is only
+  // used internally during parsing and conveys no information to the
+  // consumers of the AST.
+  const ParseState *state{nullptr};
+  WRAPPER_CLASS_BOILERPLATE(
+      OmpStylizedExpression, std::list<OmpStylizedInstance>);
+};
+
 // Ref: [4.5:201-207], [5.0:293-299], [5.1:325-331], [5.2:124]
 //
 // reduction-identifier ->
@@ -3555,9 +3592,22 @@ struct OmpReductionIdentifier {
 // combiner-expression ->                           // since 4.5
 //    assignment-statement |
 //    function-reference
-struct OmpCombinerExpression {
-  UNION_CLASS_BOILERPLATE(OmpCombinerExpression);
-  std::variant<AssignmentStmt, FunctionReference> u;
+struct OmpCombinerExpression : public OmpStylizedExpression {
+  INHERITED_WRAPPER_CLASS_BOILERPLATE(
+      OmpCombinerExpression, OmpStylizedExpression);
+  static llvm::ArrayRef<CharBlock> Variables();
+};
+
+// Ref: [4.5:222:7-8], [5.0:305:28-29], [5.1:337:20-21], [5.2:127:6-8],
+//      [6.0:242:3-5]
+//
+// initializer-expression ->                        // since 4.5
+//    OMP_PRIV = expression |
+//    subroutine-name(argument-list)
+struct OmpInitializerExpression : public OmpStylizedExpression {
+  INHERITED_WRAPPER_CLASS_BOILERPLATE(
+      OmpInitializerExpression, OmpStylizedExpression);
+  static llvm::ArrayRef<CharBlock> Variables();
 };
 
 inline namespace arguments {
@@ -4558,16 +4608,9 @@ struct OmpInReductionClause {
   std::tuple<MODIFIERS(), OmpObjectList> t;
 };
 
-// declare-reduction -> DECLARE REDUCTION (reduction-identifier : type-list
-//                                              : combiner) [initializer-clause]
-struct OmpInitializerProc {
-  TUPLE_CLASS_BOILERPLATE(OmpInitializerProc);
-  std::tuple<ProcedureDesignator, std::list<ActualArgSpec>> t;
-};
 // Initialization for declare reduction construct
 struct OmpInitializerClause {
-  UNION_CLASS_BOILERPLATE(OmpInitializerClause);
-  std::variant<OmpInitializerProc, AssignmentStmt> u;
+  WRAPPER_CLASS_BOILERPLATE(OmpInitializerClause, OmpInitializerExpression);
 };
 
 // Ref: [4.5:199-201], [5.0:288-290], [5.1:321-322], [5.2:115-117]

flang/include/flang/Semantics/symbol.h

@@ -830,6 +830,8 @@ class Symbol {
       OmpUseDevicePtr, OmpUseDeviceAddr, OmpIsDevicePtr, OmpHasDeviceAddr,
       // OpenMP data-copying attribute
       OmpCopyIn, OmpCopyPrivate,
+      // OpenMP special variables
+      OmpInVar, OmpOrigVar, OmpOutVar, OmpPrivVar,
       // OpenMP miscellaneous flags
       OmpCommonBlock, OmpReduction, OmpInReduction, OmpAligned, OmpNontemporal,
       OmpAllocate, OmpDeclarativeAllocateDirective,