Improve polymorphic loads on single slots.

src/ia32/lithium-codegen-ia32.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 4101 matching lines @@
                                 Handle<Map> map,
                                 CompareMapMode mode,
                                 LEnvironment* env) {
   Label success;
   __ CompareMap(reg, map, &success, mode);
   DeoptimizeIf(not_equal, env);
   __ bind(&success);
 }
 
 
-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);
 }
 
 
 void LCodeGen::DoClampDToUint8(LClampDToUint8* instr) {
   XMMRegister value_reg = ToDoubleRegister(instr->unclamped());
   Register result_reg = ToRegister(instr->result());
   __ ClampDoubleToUint8(value_reg, xmm0, result_reg);
 }
 
 
@@ skipping 763 matching lines @@
                               FixedArray::kHeaderSize - kPointerSize));
   __ bind(&done);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32