[vm,aot] Improve performance of certain 'is' operations

runtime/vm/aot_optimizer.cc

Index: runtime/vm/aot_optimizer.cc
diff --git a/runtime/vm/aot_optimizer.cc b/runtime/vm/aot_optimizer.cc
index 31897ebdd07c4b9b21ee6078db9a2dc75e44f63d..684c64a7f3a54e8d84ea066f1a8837dd6bd45402 100644
--- a/runtime/vm/aot_optimizer.cc
+++ b/runtime/vm/aot_optimizer.cc
@@ -1354,6 +1354,16 @@ void AotOptimizer::ReplaceWithInstanceOf(InstanceCallInstr* call) {
       InsertBefore(call, left_cid, NULL, FlowGraph::kValue);
       ConstantInstr* lower_cid =
           flow_graph()->GetConstant(Smi::Handle(Z, Smi::New(lower_limit)));
+
+      if (lower_limit == upper_limit) {

[Vyacheslav Egorov (Google), 2017/08/25 07:59:15]

Does it make sense to do a similar optimization for `as` Checks?

There is a code below that handles as checks.

Maybe we should unify handling of is and as more? It does not make sense that
they are so duplicated right now.

[alexmarkov, 2017/08/29 22:45:32]

I think you're right - 'as' and 'is' checks should be unified, preferably
lowered to the same kind of type test.

Without this unification, it doesn't look right to duplicate this optimization
for 'as' checks, because 'as' checks, unlike 'is' checks,  are implemented with
more heavyweight calls to helper functions and won't benefit much from this
particular optimization.

I'd like to consider unification of 'is' and 'as' type checks as a separate task
and approach it after implementing proper lowering pass.

Created issue: https://github.com/dart-lang/sdk/issues/30571.

+        StrictCompareInstr* check_cid = new (Z) StrictCompareInstr(
+            call->token_pos(), Token::kEQ_STRICT, new (Z) Value(left_cid),
+            new (Z) Value(lower_cid), /* number_check = */ false,
+            Thread::kNoDeoptId);
+        ReplaceCall(call, check_cid);
+        return;
+      }
+
       ConstantInstr* upper_cid =
           flow_graph()->GetConstant(Smi::Handle(Z, Smi::New(upper_limit)));