Don't MISS if you read the hole from certain FastHoley arrays.

src/arm64/lithium-codegen-arm64.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/arm64/lithium-codegen-arm64.h"
 #include "src/arm64/lithium-gap-resolver-arm64.h"
 #include "src/base/bits.h"
 #include "src/code-factory.h"
@@ skipping 3596 matching lines @@
 
   __ Load(result, mem_op, representation);
 
   if (instr->hydrogen()->RequiresHoleCheck()) {
     if (IsFastSmiElementsKind(instr->hydrogen()->elements_kind())) {
       DeoptimizeIfNotSmi(result, instr, Deoptimizer::kNotASmi);
     } else {
       DeoptimizeIfRoot(result, Heap::kTheHoleValueRootIndex, instr,
                        Deoptimizer::kHole);
     }
+  } else if (instr->hydrogen()->hole_mode() == CONVERT_HOLE_TO_UNDEFINED) {
+    DCHECK(instr->hydrogen()->elements_kind() == FAST_HOLEY_ELEMENTS);
+    Label done;
+    __ CompareRoot(result, Heap::kTheHoleValueRootIndex);
+    __ B(ne, &done);
+    if (info()->IsStub()) {
+      // A stub can safely convert the hole to undefined only if the array
+      // protector cell contains (Smi) 1. Otherwise it needs to bail out.
+      __ LoadRoot(result, Heap::kArrayProtectorRootIndex);
+      __ ldr(result, FieldMemOperand(result, Cell::kValueOffset));

[Jakob Kummerow, 2015/04/23 13:40:00]

nit: s/ldr/Ldr/ (always use MacroAssembler instructions on arm64).

[mvstanton, 2015/04/27 07:57:12]

Done.

+      __ Cmp(result, Operand(Smi::FromInt(1)));
+      DeoptimizeIf(ne, instr, Deoptimizer::kHole);
+    }
+    __ LoadRoot(result, Heap::kUndefinedValueRootIndex);
+    __ bind(&done);

[Jakob Kummerow, 2015/04/23 13:40:00]

nit: s/bind/Bind/

[mvstanton, 2015/04/27 07:57:12]

Done.

   }
 }
 
 
 void LCodeGen::DoLoadKeyedGeneric(LLoadKeyedGeneric* instr) {
   DCHECK(ToRegister(instr->context()).is(cp));
   DCHECK(ToRegister(instr->object()).is(LoadDescriptor::ReceiverRegister()));
   DCHECK(ToRegister(instr->key()).is(LoadDescriptor::NameRegister()));
 
   if (instr->hydrogen()->HasVectorAndSlot()) {
@@ skipping 2384 matching lines @@
   Handle<ScopeInfo> scope_info = instr->scope_info();
   __ Push(scope_info);
   __ Push(ToRegister(instr->function()));
   CallRuntime(Runtime::kPushBlockContext, 2, instr);
   RecordSafepoint(Safepoint::kNoLazyDeopt);
 }
 
 
 
 } }  // namespace v8::internal