[OpenACC] Implement 'reduction' Sema on 'loop' construct

The reduction clause has some minor restrictions on the variable
references that are implementable, so this implements those.  Others
require reachability analysis, so this patch documents that we're not
going to do that in the CFE(or at least save it for the MLIR passes).

Author: erichkeane

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -12718,7 +12718,18 @@ def err_acc_num_arg_conflict
 def err_acc_clause_in_clause_region
     : Error<"loop with a '%0' clause may not exist in the region of a '%1' "
             "clause%select{| on a 'kernels' compute construct}2">;
-
+def err_acc_gang_reduction_conflict
+    : Error<"%select{OpenACC 'gang' clause with a 'dim' value greater than "
+            "1|OpenACC 'reduction' clause}0 cannot "
+            "appear on the same 'loop' construct as a %select{'reduction' "
+            "clause|'gang' clause with a 'dim' value greater than 1}0">;
+def err_acc_gang_reduction_numgangs_conflict
+    : Error<"OpenACC '%0' clause cannot appear on the same 'loop' construct "
+            "as a '%1' clause inside a compute construct with a "
+            "'num_gangs' clause with more than one argument">;
+def err_reduction_op_mismatch
+    : Error<"OpenACC 'reduction' variable must have the same operator in all "
+            "nested constructs (%0 vs %1)">;
 // AMDGCN builtins diagnostics
 def err_amdgcn_global_load_lds_size_invalid_value : Error<"invalid size value">;
 def note_amdgcn_global_load_lds_size_valid_value : Note<"size must be 1, 2, or 4">;

clang/include/clang/Sema/SemaOpenACC.h

@@ -68,6 +68,7 @@ class SemaOpenACC : public SemaBase {
     /// used to diagnose if there are multiple 'for' loops at any one level.
     LLVM_PREFERRED_TYPE(bool)
     unsigned CurLevelHasLoopAlready : 1;
+
   } LoopInfo{/*TopLevelLoopSeen=*/false, /*CurLevelHasLoopAlready=*/false};
 
   /// The 'collapse' clause requires quite a bit of checking while
@@ -109,6 +110,14 @@ class SemaOpenACC : public SemaBase {
     bool TileDepthSatisfied = true;
   } TileInfo;
 
+  /// A list of the active reduction clauses, which allows us to check that all
+  /// vars on nested constructs for the same reduction var have the same
+  /// reduction operator. Currently this is enforced against all constructs
+  /// despite the rule being in the 'loop' section. By current reading, this
+  /// should apply to all anyway, but we may need to make this more like the
+  /// 'loop' clause enforcement, where this is 'blocked' by a compute construct.
+  llvm::SmallVector<OpenACCReductionClause *> ActiveReductionClauses;
+
 public:
   ComputeConstructInfo &getActiveComputeConstructInfo() {
     return ActiveComputeConstructInfo;
@@ -615,7 +624,9 @@ class SemaOpenACC : public SemaBase {
 
   /// Called while semantically analyzing the reduction clause, ensuring the var
   /// is the correct kind of reference.
-  ExprResult CheckReductionVar(Expr *VarExpr);
+  ExprResult CheckReductionVar(OpenACCDirectiveKind DirectiveKind,
+                               OpenACCReductionOperator ReductionOp,
+                               Expr *VarExpr);
 
   /// Called to check the 'var' type is a variable of pointer type, necessary
   /// for 'deviceptr' and 'attach' clauses. Returns true on success.
@@ -634,6 +645,22 @@ class SemaOpenACC : public SemaBase {
   // Check a single expression on a gang clause.
   ExprResult CheckGangExpr(OpenACCGangKind GK, Expr *E);
 
+  // Does the checking for a 'gang' clause that needs to be done in dependent
+  // and not dependent cases.
+  OpenACCClause *
+  CheckGangClause(ArrayRef<const OpenACCClause *> ExistingClauses,
+                  SourceLocation BeginLoc, SourceLocation LParenLoc,
+                  ArrayRef<OpenACCGangKind> GangKinds,
+                  ArrayRef<Expr *> IntExprs, SourceLocation EndLoc);
+  // Does the checking for a 'reduction ' clause that needs to be done in
+  // dependent and not dependent cases.
+  OpenACCClause *
+  CheckReductionClause(ArrayRef<const OpenACCClause *> ExistingClauses,
+                       OpenACCDirectiveKind DirectiveKind,
+                       SourceLocation BeginLoc, SourceLocation LParenLoc,
+                       OpenACCReductionOperator ReductionOp,
+                       ArrayRef<Expr *> Vars, SourceLocation EndLoc);
+
   ExprResult BuildOpenACCAsteriskSizeExpr(SourceLocation AsteriskLoc);
   ExprResult ActOnOpenACCAsteriskSizeExpr(SourceLocation AsteriskLoc);
 
@@ -686,6 +713,7 @@ class SemaOpenACC : public SemaBase {
     SourceLocation OldLoopWorkerClauseLoc;
     SourceLocation OldLoopVectorClauseLoc;
     llvm::SmallVector<OpenACCLoopConstruct *> ParentlessLoopConstructs;
+    llvm::SmallVector<OpenACCReductionClause *> ActiveReductionClauses;
     LoopInConstructRAII LoopRAII;
 
   public: