Improve polymorphic loads on single slots.

src/ia32/lithium-codegen-ia32.cc

Index: src/ia32/lithium-codegen-ia32.cc
diff --git a/src/ia32/lithium-codegen-ia32.cc b/src/ia32/lithium-codegen-ia32.cc
index 6ddc024258940f2776c0b400105a6dd9b56dddfe..9ef7781253965fcb114ac603c96271146e2b506d 100644
--- a/src/ia32/lithium-codegen-ia32.cc
+++ b/src/ia32/lithium-codegen-ia32.cc
@@ -4123,8 +4123,17 @@ void LCodeGen::DoCheckMap(LCheckMap* 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);
 }