[Flang][openmp][2/5] Make Default clause part of OmpClause

After discussion in `D93482` we found that the some of the clauses were not
following the common OmpClause convention.

The benefits of using OmpClause:
- Functionalities from structure checker are mostly aligned to work with
  `llvm::omp::Clause`.
- The unparsing as well can take advantage.
- Homogeneity with OpenACC and rest of the clauses in OpenMP.
- Could even generate the parser with TableGen, when there is homogeneity.
- It becomes confusing when to use `flangClass` and `flangClassValue` inside
  TableGen, if incase we generate parser using TableGen we could have only a
  single `let expression`.

This patch makes `OmpDefaultClause` clause part of `OmpClause`.
The unparse function is dropped as the unparsing is done by `WALK_NESTED_ENUM`
for `OmpDefaultClause`.

Reviewed By: clementval, kiranktp

Differential Revision: https://reviews.llvm.org/D93641

flang/lib/Lower/OpenMP.cpp

@@ -191,8 +191,9 @@ genOMP(Fortran::lower::AbstractConverter &converter,
     // Handle attribute based clauses.
     for (const auto &clause : parallelOpClauseList.v) {
       if (const auto &defaultClause =
-              std::get_if<Fortran::parser::OmpDefaultClause>(&clause.u)) {
-        switch (defaultClause->v) {
+              std::get_if<Fortran::parser::OmpClause::Default>(&clause.u)) {
+        const auto &ompDefaultClause{defaultClause->v};
+        switch (ompDefaultClause.v) {
         case Fortran::parser::OmpDefaultClause::Type::Private:
           parallelOp.default_valAttr(firOpBuilder.getStringAttr(
               omp::stringifyClauseDefault(omp::ClauseDefault::defprivate)));

flang/lib/Parser/openmp-parsers.cpp

@@ -167,8 +167,8 @@ TYPE_PARSER(
                     parenthesized(Parser<OmpObjectList>{}))) ||
     "COPYPRIVATE" >> construct<OmpClause>(construct<OmpClause::Copyprivate>(
                          (parenthesized(Parser<OmpObjectList>{})))) ||
-    "DEFAULT"_id >>
-        construct<OmpClause>(parenthesized(Parser<OmpDefaultClause>{})) ||
+    "DEFAULT"_id >> construct<OmpClause>(construct<OmpClause::Default>(
+                        parenthesized(Parser<OmpDefaultClause>{}))) ||
     "DEFAULTMAP" >>
         construct<OmpClause>(parenthesized(Parser<OmpDefaultmapClause>{})) ||
     "DEPEND" >>

flang/lib/Parser/unparse.cpp

@@ -2058,11 +2058,6 @@ public:
                       },
         x.u);
   }
-  bool Pre(const OmpDefaultClause &) {
-    Word("DEFAULT(");
-    return true;
-  }
-  void Post(const OmpDefaultClause &) { Put(")"); }
   bool Pre(const OmpProcBindClause &) {
     Word("PROC_BIND(");
     return true;

flang/lib/Semantics/check-omp-structure.cpp

@@ -406,6 +406,7 @@ void OmpStructureChecker::Enter(const parser::OmpClause &x) {
 CHECK_SIMPLE_CLAUSE(Allocate, OMPC_allocate)
 CHECK_SIMPLE_CLAUSE(Copyin, OMPC_copyin)
 CHECK_SIMPLE_CLAUSE(Copyprivate, OMPC_copyprivate)
+CHECK_SIMPLE_CLAUSE(Default, OMPC_default)
 CHECK_SIMPLE_CLAUSE(Device, OMPC_device)
 CHECK_SIMPLE_CLAUSE(Final, OMPC_final)
 CHECK_SIMPLE_CLAUSE(Firstprivate, OMPC_firstprivate)
@@ -490,7 +491,6 @@ void OmpStructureChecker::CheckIsVarPartOfAnotherVar(
   }
 }
 // Following clauses have a seperate node in parse-tree.h.
-CHECK_SIMPLE_PARSER_CLAUSE(OmpDefaultClause, OMPC_default)
 CHECK_SIMPLE_PARSER_CLAUSE(OmpDistScheduleClause, OMPC_dist_schedule)
 CHECK_SIMPLE_PARSER_CLAUSE(OmpNowait, OMPC_nowait)
 CHECK_SIMPLE_PARSER_CLAUSE(OmpProcBindClause, OMPC_proc_bind)

flang/lib/Semantics/check-omp-structure.h

@@ -137,6 +137,7 @@ public:
   void Enter(const parser::OmpClause::Collapse &);
   void Enter(const parser::OmpClause::Copyin &);
   void Enter(const parser::OmpClause::Copyprivate &);
+  void Enter(const parser::OmpClause::Default &);
   void Enter(const parser::OmpClause::Device &);
   void Enter(const parser::OmpClause::Final &);
   void Enter(const parser::OmpClause::Firstprivate &);
@@ -175,7 +176,6 @@ public:
   void Enter(const parser::OmpAtomicCapture &);
   void Leave(const parser::OmpAtomic &);
   void Enter(const parser::OmpAlignedClause &);
-  void Enter(const parser::OmpDefaultClause &);
   void Enter(const parser::OmpDefaultmapClause &);
   void Enter(const parser::OmpDependClause &);
   void Enter(const parser::OmpDistScheduleClause &);

llvm/include/llvm/Frontend/OpenMP/OMP.td

@@ -62,7 +62,7 @@ def OMPC_Collapse : Clause<"collapse"> {
 }
 def OMPC_Default : Clause<"default"> {
   let clangClass = "OMPDefaultClause";
-  let flangClass = "OmpDefaultClause";
+  let flangClassValue = "OmpDefaultClause";
 }
 def OMPC_Private : Clause<"private"> {
   let clangClass = "OMPPrivateClause";