Refactor ObjectVisitor functions to not mutate the slot.

src/heap/mark-compact.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/heap/mark-compact.h"
 
 #include "src/base/atomicops.h"
 #include "src/base/bits.h"
 #include "src/base/sys-info.h"
 #include "src/code-stubs.h"
@@ skipping 2832 matching lines @@
   void VisitCodeAgeSequence(RelocInfo* rinfo) override {
     DCHECK(RelocInfo::IsCodeAgeSequence(rinfo->rmode()));
     Object* stub = rinfo->code_age_stub();
     DCHECK(stub != NULL);
     VisitPointer(&stub);
     if (stub != rinfo->code_age_stub()) {
       rinfo->set_code_age_stub(Code::cast(stub));
     }
   }
 
+  void VisitCodeEntry(Address entry_address) override {
+    Object* code = Code::GetObjectFromEntryAddress(entry_address);
+    Object* old_code = code;
+    VisitPointer(&code);
+    if (code != old_code) {
+      Memory::Address_at(entry_address) =
+          reinterpret_cast<Code*>(code)->entry();
+    }
+  }
+
   void VisitDebugTarget(RelocInfo* rinfo) override {
     DCHECK(RelocInfo::IsDebugBreakSlot(rinfo->rmode()) &&
            rinfo->IsPatchedDebugBreakSlotSequence());
     Object* target =
         Code::GetCodeFromTargetAddress(rinfo->debug_call_address());
     VisitPointer(&target);
     rinfo->set_debug_call_address(Code::cast(target)->instruction_start());
   }
 
   static inline void UpdateSlot(Heap* heap, Object** slot) {
@@ skipping 1084 matching lines @@
     MarkBit mark_bit = Marking::MarkBitFrom(host);
     if (Marking::IsBlack(mark_bit)) {
       RelocInfo rinfo(isolate(), pc, RelocInfo::CODE_TARGET, 0, host);
       RecordRelocSlot(host, &rinfo, target);
     }
   }
 }
 
 }  // namespace internal
 }  // namespace v8