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

src/mips64/lithium-codegen-mips64.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"
 
 #include "src/code-factory.h"
 #include "src/code-stubs.h"
 #include "src/cpu-profiler.h"
 #include "src/hydrogen-osr.h"
@@ skipping 3315 matching lines @@
   // Check for the hole value.
   if (hinstr->RequiresHoleCheck()) {
     if (IsFastSmiElementsKind(instr->hydrogen()->elements_kind())) {
       __ SmiTst(result, scratch);
       DeoptimizeIf(ne, instr, Deoptimizer::kNotASmi, scratch,
                    Operand(zero_reg));
     } else {
       __ LoadRoot(scratch, Heap::kTheHoleValueRootIndex);
       DeoptimizeIf(eq, instr, Deoptimizer::kHole, result, Operand(scratch));
     }
+  } else if (instr->hydrogen()->hole_mode() == CONVERT_HOLE_TO_UNDEFINED) {
+    DCHECK(instr->hydrogen()->elements_kind() == FAST_HOLEY_ELEMENTS);
+    Label done;
+    __ LoadRoot(scratch, Heap::kTheHoleValueRootIndex);
+    __ Branch(&done, ne, result, Operand(scratch));
+    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(scratch, Heap::kArrayProtectorRootIndex);

[balazs.kilvady, 2015/04/23 16:24:37]

The result register should be used instead of scratch in LoadRoot like in
src/arm64/lithium-codegen-arm64.cc.

[mvstanton, 2015/04/27 07:57:13]

Done.

+      __ lw(scratch, FieldMemOperand(result, Cell::kValueOffset));

[paul.l..., 2015/04/23 15:34:23]

nit: This should probably be 'ld'. I say 'probably' since in general values are
64-bit. But you are using just a bit flag here, so it's fine (as long as we
never go to big-endian mips64, which we have no plans for :) 

It might be worth a comment here that you are using just LS bit(s) of value.

[mvstanton, 2015/04/27 07:57:13]

Done.

+      DeoptimizeIf(ne, instr, Deoptimizer::kHole, scratch,
+                   Operand(Smi::FromInt(1)));
+    }
+    __ LoadRoot(result, Heap::kUndefinedValueRootIndex);
+    __ bind(&done);
   }
 }
 
 
 void LCodeGen::DoLoadKeyed(LLoadKeyed* instr) {
   if (instr->is_typed_elements()) {
     DoLoadKeyedExternalArray(instr);
   } else if (instr->hydrogen()->representation().IsDouble()) {
     DoLoadKeyedFixedDoubleArray(instr);
   } else {
@@ skipping 2678 matching lines @@
   __ li(at, scope_info);
   __ Push(at, ToRegister(instr->function()));
   CallRuntime(Runtime::kPushBlockContext, 2, instr);
   RecordSafepoint(Safepoint::kNoLazyDeopt);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal