Refactor ObjectVisitor functions to not mutate the slot.

BUG=chromium:612847
LOG=N

Review-Url: https://codereview.chromium.org/1989973002
Cr-Commit-Position: refs/heads/master@{#36337}

src/heap/mark-compact.cc

@@ -2892,6 +2892,16 @@ class PointersUpdatingVisitor : public ObjectVisitor {
     }
   }
 
+  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());

src/objects.cc

@@ -12965,63 +12965,66 @@ const char* const VisitorSynchronization::kTagNames[
 
 void ObjectVisitor::VisitCodeTarget(RelocInfo* rinfo) {
   DCHECK(RelocInfo::IsCodeTarget(rinfo->rmode()));
-  Object* target = Code::GetCodeFromTargetAddress(rinfo->target_address());
-  Object* old_target = target;
-  VisitPointer(&target);
-  CHECK_EQ(target, old_target);  // VisitPointer doesn't change Code* *target.
+  Object* old_pointer = Code::GetCodeFromTargetAddress(rinfo->target_address());
+  Object* new_pointer = old_pointer;
+  VisitPointer(&new_pointer);
+  DCHECK_EQ(old_pointer, new_pointer);
 }
 
 
 void ObjectVisitor::VisitCodeAgeSequence(RelocInfo* rinfo) {
   DCHECK(RelocInfo::IsCodeAgeSequence(rinfo->rmode()));
-  Object* stub = rinfo->code_age_stub();
-  if (stub) {
-    VisitPointer(&stub);
+  Object* old_pointer = rinfo->code_age_stub();
+  Object* new_pointer = old_pointer;
+  if (old_pointer != nullptr) {
+    VisitPointer(&new_pointer);
+    DCHECK_EQ(old_pointer, new_pointer);
   }
 }
 
 
 void ObjectVisitor::VisitCodeEntry(Address entry_address) {
-  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();
-  }
+  Object* old_pointer = Code::GetObjectFromEntryAddress(entry_address);
+  Object* new_pointer = old_pointer;
+  VisitPointer(&new_pointer);
+  DCHECK_EQ(old_pointer, new_pointer);
 }
 
 
 void ObjectVisitor::VisitCell(RelocInfo* rinfo) {
   DCHECK(rinfo->rmode() == RelocInfo::CELL);
-  Object* cell = rinfo->target_cell();
-  Object* old_cell = cell;
-  VisitPointer(&cell);
-  if (cell != old_cell) {
-    rinfo->set_target_cell(reinterpret_cast<Cell*>(cell));
-  }
+  Object* old_pointer = rinfo->target_cell();
+  Object* new_pointer = old_pointer;
+  VisitPointer(&new_pointer);
+  DCHECK_EQ(old_pointer, new_pointer);
 }
 
 
 void ObjectVisitor::VisitDebugTarget(RelocInfo* rinfo) {
   DCHECK(RelocInfo::IsDebugBreakSlot(rinfo->rmode()) &&
          rinfo->IsPatchedDebugBreakSlotSequence());
-  Object* target = Code::GetCodeFromTargetAddress(rinfo->debug_call_address());
-  Object* old_target = target;
-  VisitPointer(&target);
-  CHECK_EQ(target, old_target);  // VisitPointer doesn't change Code* *target.
+  Object* old_pointer =
+      Code::GetCodeFromTargetAddress(rinfo->debug_call_address());
+  Object* new_pointer = old_pointer;
+  VisitPointer(&new_pointer);
+  DCHECK_EQ(old_pointer, new_pointer);
 }
 
 
 void ObjectVisitor::VisitEmbeddedPointer(RelocInfo* rinfo) {
   DCHECK(rinfo->rmode() == RelocInfo::EMBEDDED_OBJECT);
-  Object* p = rinfo->target_object();
-  VisitPointer(&p);
+  Object* old_pointer = rinfo->target_object();
+  Object* new_pointer = old_pointer;
+  VisitPointer(&new_pointer);
+  DCHECK_EQ(old_pointer, new_pointer);
 }
 
 
 void ObjectVisitor::VisitExternalReference(RelocInfo* rinfo) {
-  Address p = rinfo->target_external_reference();
-  VisitExternalReference(&p);
+  Address old_reference = rinfo->target_external_reference();
+  Address new_reference = old_reference;
+  VisitExternalReference(&new_reference);
+  DCHECK_EQ(old_reference, new_reference);
 }