Improve polymorphic loads on single slots.

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 985cdc6d98e49c6fd377a9b94edc97e1d3bbbe15..05094783abdf709c90fecf9065b7cd404c8d1b8f 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -3848,12 +3848,21 @@ void LCodeGen::DoCheckMapCommon(Register reg,
 }
 
 
-void LCodeGen::DoCheckMap(LCheckMap* instr) {
+void LCodeGen::DoCheckMaps(LCheckMaps* instr) {
   LOperand* input = instr->InputAt(0);
   ASSERT(input->IsRegister());
   Register reg = ToRegister(input);
-  Handle<Map> map = instr->hydrogen()->map();
-  DoCheckMapCommon(reg, map, instr->hydrogen()->mode(), instr->environment());
+
+  Label success;
+  SmallMapList* map_set = instr->hydrogen()->map_set();
+  for (int i = 0; i < map_set->length() - 1; i++) {
+    Handle<Map> map = map_set->at(i);
+    __ CompareMap(reg, map, &success, REQUIRE_EXACT_MAP);
+    __ j(equal, &success);
+  }
+  Handle<Map> map = map_set->last();
+  DoCheckMapCommon(reg, map, REQUIRE_EXACT_MAP, instr->environment());
+  __ bind(&success);
 }