Move Maps' back pointers from "transitions" to "constructor" field

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index a1172262b06cc82bd8034f5e909f7158c611abcd..295f89106f1dac07c3a62f585c9c922b0502be0c 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -3565,6 +3565,20 @@ Condition MacroAssembler::IsObjectNameType(Register heap_object,
 }
 
 
+void MacroAssembler::GetMapConstructor(Register result, Register map,
+                                       Register temp) {
+  Label done, loop;
+  movp(result, FieldOperand(map, Map::kConstructorOrBackPointerOffset));
+  bind(&loop);
+  JumpIfSmi(result, &done);
+  CmpObjectType(result, MAP_TYPE, temp);
+  j(not_equal, &done);
+  movp(result, FieldOperand(result, Map::kConstructorOrBackPointerOffset));
+  jmp(&loop);
+  bind(&done);
+}
+
+
 void MacroAssembler::TryGetFunctionPrototype(Register function,
                                              Register result,
                                              Label* miss,
@@ -3618,7 +3632,7 @@ void MacroAssembler::TryGetFunctionPrototype(Register function,
     // Non-instance prototype: Fetch prototype from constructor field
     // in initial map.
     bind(&non_instance);
-    movp(result, FieldOperand(result, Map::kConstructorOffset));
+    GetMapConstructor(result, result, kScratchRegister);
   }
 
   // All done.