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

src/x64/macro-assembler-x64.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/base/bits.h"
 #include "src/base/division-by-constant.h"
@@ skipping 3547 matching lines @@
 Condition MacroAssembler::IsObjectNameType(Register heap_object,
                                            Register map,
                                            Register instance_type) {
   movp(map, FieldOperand(heap_object, HeapObject::kMapOffset));
   movzxbl(instance_type, FieldOperand(map, Map::kInstanceTypeOffset));
   cmpb(instance_type, Immediate(static_cast<uint8_t>(LAST_NAME_TYPE)));
   return below_equal;
 }
 
 
+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,
                                              bool miss_on_bound_function) {
   Label non_instance;
   if (miss_on_bound_function) {
     // Check that the receiver isn't a smi.
     testl(function, Immediate(kSmiTagMask));
     j(zero, miss);
 
@@ skipping 33 matching lines @@
 
   // Get the prototype from the initial map.
   movp(result, FieldOperand(result, Map::kPrototypeOffset));
 
   if (miss_on_bound_function) {
     jmp(&done, Label::kNear);
 
     // 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.
   bind(&done);
 }
 
 
 void MacroAssembler::SetCounter(StatsCounter* counter, int value) {
   if (FLAG_native_code_counters && counter->Enabled()) {
     Operand counter_operand = ExternalOperand(ExternalReference(counter));
@@ skipping 1564 matching lines @@
   if (mag.shift > 0) sarl(rdx, Immediate(mag.shift));
   movl(rax, dividend);
   shrl(rax, Immediate(31));
   addl(rdx, rax);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64