[Clang][Sema][OpenMP] Sema support for atomic compare

This patch adds the Sema support for `atomic compare`.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D116637

Author: shiltian

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -10516,6 +10516,15 @@ def err_omp_atomic_capture_not_compound_statement : Error<
   " where x is an lvalue expression with scalar type">;
 def note_omp_atomic_capture: Note<
   "%select{expected assignment expression|expected compound statement|expected exactly two expression statements|expected in right hand side of the first expression}0">;
+def err_omp_atomic_compare : Error<
+  "the statement for 'atomic compare' must be a compound statement of form '{x = expr ordop x ? expr : x;}', '{x = x ordop expr? expr : x;}',"
+  " '{x = x == e ? d : x;}', '{x = e == x ? d : x;}', or 'if(expr ordop x) {x = expr;}', 'if(x ordop expr) {x = expr;}', 'if(x == e) {x = d;}',"
+  " 'if(e == x) {x = d;}' where 'x' is an lvalue expression with scalar type, 'expr', 'e', and 'd' are expressions with scalar type,"
+  " and 'ordop' is one of '<' or '>'.">;
+def note_omp_atomic_compare: Note<
+  "%select{expected compound statement|expected exactly one expression statement|expected assignment statement|expected conditional operator|expect result value to be at false expression|"
+  "expect binary operator in conditional expression|expect '<', '>' or '==' as order operator|expect comparison in a form of 'x == e', 'e == x', 'x ordop expr', or 'expr ordop x'|"
+  "expect lvalue for result value|expect scalar value|expect integer value}0">;
 def err_omp_atomic_several_clauses : Error<
   "directive '#pragma omp atomic' cannot contain more than one 'read', 'write', 'update', 'capture', or 'compare' clause">;
 def err_omp_several_mem_order_clauses : Error<

clang/lib/CodeGen/CGStmtOpenMP.cpp

@@ -6031,9 +6031,13 @@ static void emitOMPAtomicExpr(CodeGenFunction &CGF, OpenMPClauseKind Kind,
     emitOMPAtomicCaptureExpr(CGF, AO, IsPostfixUpdate, V, X, E, UE,
                              IsXLHSInRHSPart, Loc);
     break;
-  case OMPC_compare:
-    // Do nothing here as we already emit an error.
+  case OMPC_compare: {
+    // Emit an error here.
+    unsigned DiagID = CGF.CGM.getDiags().getCustomDiagID(
+        DiagnosticsEngine::Error, "'atomic compare' is not supported for now");
+    CGF.CGM.getDiags().Report(DiagID);
     break;
+  }
   case OMPC_if:
   case OMPC_final:
   case OMPC_num_threads: