Optimizations to IC stub for unoptimized code performance on x64.

runtime/vm/object.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/become.h"
@@ skipping 13447 matching lines @@
       index * TestEntryLength() + CountIndexFor(NumArgsTested());
   data.SetAt(data_pos, Smi::Handle(Smi::New(value)));
 }
 
 
 intptr_t ICData::GetCountAt(intptr_t index) const {
   ASSERT(Isolate::Current()->compilation_allowed());
   const Array& data = Array::Handle(ic_data());
   const intptr_t data_pos =
       index * TestEntryLength() + CountIndexFor(NumArgsTested());
+  if (data_pos < 0) {

[regis, 2017/01/24 18:27:21]

Can you explain with a comment how this is possible?

+    // Overflow.
+    SetCountAt(index, 0);
+    return 0;
+  }
   return Smi::Value(Smi::RawCast(data.At(data_pos)));
 }
 
 
 intptr_t ICData::AggregateCount() const {
   if (IsNull()) return 0;
   const intptr_t len = NumberOfChecks();
   intptr_t count = 0;
   for (intptr_t i = 0; i < len; i++) {
     count += GetCountAt(i);
@@ skipping 9539 matching lines @@
   return UserTag::null();
 }
 
 
 const char* UserTag::ToCString() const {
   const String& tag_label = String::Handle(label());
   return tag_label.ToCString();
 }
 
 }  // namespace dart