Use a special EnumLength field to indicate number of valid enum cache values.

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index df2dd859aa28a39cd88477f8c4e3a5dd7671255d..27bfe15e3378e93db232e51441784415a41e7a63 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -1285,6 +1285,13 @@ void LCodeGen::DoFixedArrayBaseLength(LFixedArrayBaseLength* instr) {
 }
 
 
+void LCodeGen::DoMapEnumLength(LMapEnumLength* instr) {
+  Register result = ToRegister(instr->result());
+  Register map = ToRegister(instr->InputAt(0));
+  __ EnumLength(result, map);
+}
+
+
 void LCodeGen::DoElementsKind(LElementsKind* instr) {
   Register result = ToRegister(instr->result());
   Register input = ToRegister(instr->InputAt(0));
@@ -5217,11 +5224,19 @@ void LCodeGen::DoForInPrepareMap(LForInPrepareMap* instr) {
 void LCodeGen::DoForInCacheArray(LForInCacheArray* instr) {
   Register map = ToRegister(instr->map());
   Register result = ToRegister(instr->result());
+  Label load_cache, done;
+  __ EnumLength(result, map);
+  __ Cmp(result, Smi::FromInt(0));
+  __ j(not_equal, &load_cache);
+  __ LoadRoot(result, Heap::kEmptyFixedArrayRootIndex);
+  __ jmp(&done);
+  __ bind(&load_cache);
   __ LoadInstanceDescriptors(map, result);
   __ movq(result,
           FieldOperand(result, DescriptorArray::kEnumCacheOffset));
   __ movq(result,
           FieldOperand(result, FixedArray::SizeFor(instr->idx())));
+  __ bind(&done);
   Condition cc = masm()->CheckSmi(result);
   DeoptimizeIf(cc, instr->environment());
 }