Faster code for megamorphic calls fo get:hashCode for Smi and OneByteString

runtime/vm/flow_graph_compiler_x64.cc

Index: runtime/vm/flow_graph_compiler_x64.cc
diff --git a/runtime/vm/flow_graph_compiler_x64.cc b/runtime/vm/flow_graph_compiler_x64.cc
index bdcc3b4d1119eb5d514176cecf5dde82df83903b..def75aa46cf2ef1f6386380f0c21994b414ff86d 100644
--- a/runtime/vm/flow_graph_compiler_x64.cc
+++ b/runtime/vm/flow_graph_compiler_x64.cc
@@ -1328,9 +1328,28 @@ void FlowGraphCompiler::EmitMegamorphicInstanceCall(
   ASSERT(!arguments_descriptor.IsNull() && (arguments_descriptor.Length() > 0));
   const MegamorphicCache& cache = MegamorphicCache::ZoneHandle(zone(),
       MegamorphicCacheTable::Lookup(isolate(), name, arguments_descriptor));
-
   __ Comment("MegamorphicCall");
+  // Load receiver into RDI.
   __ movq(RDI, Address(RSP, (argument_count - 1) * kWordSize));
+  Label done;
+  if (name.raw() == Symbols::hashCode().raw()) {
+    Label try_onebytestring, megamorphic_call;
+    __ Comment("Inlined get:hashCode for Smi and OneByteString");
+    __ testq(RDI, Immediate(kSmiTagMask));
+    __ j(NOT_ZERO, &try_onebytestring, Assembler::kNearJump);  // Non-smi value.
+    __ movq(RAX, RDI);
+    __ jmp(&done, Assembler::kNearJump);
+
+    __ Bind(&try_onebytestring);
+    __ CompareClassId(RDI, kOneByteStringCid);
+    __ j(NOT_EQUAL, &megamorphic_call, Assembler::kNearJump);
+    __ movq(RAX, FieldAddress(RDI, String::hash_offset()));
+    __ cmpq(RAX, Immediate(0));
+    __ j(NOT_EQUAL, &done, Assembler::kNearJump);
+
+    __ Bind(&megamorphic_call);
+    __ Comment("Slow case: megamorphic call");
+  }
   __ LoadObject(RBX, cache);
   if (FLAG_use_megamorphic_stub) {
     __ Call(*StubCode::MegamorphicLookup_entry());
@@ -1339,6 +1358,7 @@ void FlowGraphCompiler::EmitMegamorphicInstanceCall(
   }
   __ call(RCX);
 
+  __ Bind(&done);
   RecordSafepoint(locs, slow_path_argument_count);
   const intptr_t deopt_id_after = Thread::ToDeoptAfter(deopt_id);
   if (FLAG_precompiled_mode) {