Fall back to megamorphic stub when dispatching calls to rare classes.

runtime/vm/jit_optimizer.cc

Index: runtime/vm/jit_optimizer.cc
diff --git a/runtime/vm/jit_optimizer.cc b/runtime/vm/jit_optimizer.cc
index 9e71b2728a12de19a082ce4d0ae07d76ab7f6825..bdfcddcf09c00fd4e391dfcdc6dc4debc786c0ba 100644
--- a/runtime/vm/jit_optimizer.cc
+++ b/runtime/vm/jit_optimizer.cc
@@ -1549,17 +1549,6 @@ void JitOptimizer::VisitInstanceCall(InstanceCallInstr* instr) {
   const ICData& unary_checks =
       ICData::ZoneHandle(Z, instr->ic_data()->AsUnaryClassChecks());
 
-  const bool is_dense = CheckClassInstr::IsDenseCidRange(unary_checks);
-  const intptr_t number_of_checks = unary_checks.NumberOfChecks();
-  if (op_kind == Token::kEQ &&
-      number_of_checks > FLAG_max_equality_polymorphic_checks && !is_dense &&
-      flow_graph()->InstanceCallNeedsClassCheck(
-          instr, RawFunction::kRegularFunction)) {
-    // Too many checks, it will be megamorphic which needs unary checks.
-    instr->set_ic_data(&unary_checks);
-    return;
-  }
-
   if ((op_kind == Token::kASSIGN_INDEX) && TryReplaceWithIndexedOp(instr)) {
     return;
   }
@@ -1628,8 +1617,20 @@ void JitOptimizer::VisitInstanceCall(InstanceCallInstr* instr) {
     }
   }
 
+  // If there is only one target we can make this into a deopting class check,
+  // followed by a call instruction that does not check the class of the
+  // receiver.  This enables a lot of optimizations because after the class
+  // check we can probably inline the call and not worry about side effects.
+  // However, this can fall down if new receiver classes arrive at this call
+  // site after we generated optimized code.  This causes a deopt, and after a
+  // few deopts we won't optimize this function any more at all.  Therefore for
+  // very polymorphic sites we don't make this optimization, keeping it as a
+  // regular checked PolymorphicInstanceCall, which falls back to the slow but
+  // non-deopting megamorphic call stub when it sees new receiver classes.
   bool call_with_checks;
-  if (has_one_target && FLAG_polymorphic_with_deopt) {
+  if (has_one_target && FLAG_polymorphic_with_deopt &&
+      (!instr->ic_data()->HasDeoptReason(ICData::kDeoptCheckClass) ||
+       unary_checks.NumberOfChecks() <= FLAG_max_polymorphic_checks)) {
     // Type propagation has not run yet, we cannot eliminate the check.
     AddReceiverCheck(instr);
     // Call can still deoptimize, do not detach environment from instr.