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..b98e23d0382336068af685a6bb8c67edddcf9aaf 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;
   }
@@ -1629,7 +1618,11 @@ void JitOptimizer::VisitInstanceCall(InstanceCallInstr* instr) {
   }
 
   bool call_with_checks;
-  if (has_one_target && FLAG_polymorphic_with_deopt) {
+  const int max_cids = (op_kind == Token::kEQ)

[Vyacheslav Egorov (Google), 2017/04/19 08:24:00]

It might be good to put the reasoning here to inform future generations

[erikcorry, 2017/04/19 13:15:41]

Done.

+                           ? FLAG_max_equality_polymorphic_checks
+                           : FLAG_max_polymorphic_checks;
+  if (has_one_target && FLAG_polymorphic_with_deopt &&
+      unary_checks.NumberOfChecks() <= max_cids) {
     // Type propagation has not run yet, we cannot eliminate the check.
     AddReceiverCheck(instr);
     // Call can still deoptimize, do not detach environment from instr.