Avoid array bounds check when allowed by guarded field.

runtime/vm/intermediate_language_x64.cc

 // Copyright (c) 2013, 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/globals.h"  // Needed here to get TARGET_ARCH_X64.
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 4419 matching lines @@
     }
     ASSERT((Smi::Cast(length_loc.constant()).Value() <=
             Smi::Cast(index_loc.constant()).Value()) ||
            (Smi::Cast(index_loc.constant()).Value() < 0));
     // Unconditionally deoptimize for constant bounds checks because they
     // only occur only when index is out-of-bounds.
     __ jmp(deopt);
     return;
   }
 
+  intptr_t guarded_array_length = -1;
+  if (array_ != NULL) {
+    if (array_->guarded_list_length() >= 0) {
+      guarded_array_length = array_->guarded_list_length();
+    }
+  }
+
+  if (index_loc.IsConstant() && guarded_array_length >= 0) {
+    const Object& index = Smi::Cast(index_loc.constant());
+    int64_t i = reinterpret_cast<int64_t>(index.raw());
+    if (i >= guarded_array_length) {
+      // We know this bounds check will trigger a deoptimization.
+      __ jmp(deopt);
+    }
+    // Nothing to emit.
+    return;
+  }
+
   if (index_loc.IsConstant()) {
     Register length = length_loc.reg();
     const Smi& index = Smi::Cast(index_loc.constant());
     __ cmpq(length, Immediate(reinterpret_cast<int64_t>(index.raw())));
     __ j(BELOW_EQUAL, deopt);
   } else if (length_loc.IsConstant()) {
     const Smi& length = Smi::Cast(length_loc.constant());
     Register index = index_loc.reg();
     __ cmpq(index, Immediate(reinterpret_cast<int64_t>(length.raw())));
     __ j(ABOVE_EQUAL, deopt);
@@ skipping 345 matching lines @@
                          PcDescriptors::kOther,
                          locs());
   __ Drop(2);  // Discard type arguments and receiver.
 }
 
 }  // namespace dart
 
 #undef __
 
 #endif  // defined TARGET_ARCH_X64