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

src/ia32/lithium-codegen-ia32.cc

 // Copyright 2012 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"
 
 #if V8_TARGET_ARCH_IA32
 
 #include "src/base/bits.h"
 #include "src/code-factory.h"
@@ skipping 3166 matching lines @@
 
   // Check for the hole value.
   if (instr->hydrogen()->RequiresHoleCheck()) {
     if (IsFastSmiElementsKind(instr->hydrogen()->elements_kind())) {
       __ test(result, Immediate(kSmiTagMask));
       DeoptimizeIf(not_equal, instr, Deoptimizer::kNotASmi);
     } else {
       __ cmp(result, factory()->the_hole_value());
       DeoptimizeIf(equal, instr, Deoptimizer::kHole);
     }
+  } else if (instr->hydrogen()->hole_mode() == CONVERT_HOLE_TO_UNDEFINED) {
+    DCHECK(instr->hydrogen()->elements_kind() == FAST_HOLEY_ELEMENTS);
+    Label done;
+    __ cmp(result, factory()->the_hole_value());
+    __ j(not_equal, &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.
+      __ mov(result, isolate()->factory()->array_protector());

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

You can collapse this into one instruction:

__ cmp(Operand::ForCell(isolate()->factory()->array_protector()),
       Immediate(Smi::FromInt(1)));

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

Actually I can't do that for a PropertyCell, just a Cell. Cool idea though, I
didn't know about that instruction!

+      __ cmp(FieldOperand(result, Cell::kValueOffset),
+             Immediate(Smi::FromInt(1)));

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

Please factor out the magic 0/1 constants as "static const int
kArrayProtectorValid = 1" (in class Isolate) or similar.

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

Done.

+      DeoptimizeIf(not_equal, instr, Deoptimizer::kHole);
+    }
+    __ mov(result, isolate()->factory()->undefined_value());
+    __ bind(&done);
   }
 }
 
 
 void LCodeGen::DoLoadKeyed(LLoadKeyed* instr) {
   if (instr->is_typed_elements()) {
     DoLoadKeyedExternalArray(instr);
   } else if (instr->hydrogen()->representation().IsDouble()) {
     DoLoadKeyedFixedDoubleArray(instr);
   } else {
@@ skipping 2541 matching lines @@
   CallRuntime(Runtime::kPushBlockContext, 2, instr);
   RecordSafepoint(Safepoint::kNoLazyDeopt);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32