[turbofan] Preserve cached code across GCs.

src/heap/objects-visiting-inl.h

Index: src/heap/objects-visiting-inl.h
diff --git a/src/heap/objects-visiting-inl.h b/src/heap/objects-visiting-inl.h
index e3e8e710203ea03bdb376d97a03d6ba6722326bb..05aa375bcdb042f30e1d9c672e6c3876283f30f5 100644
--- a/src/heap/objects-visiting-inl.h
+++ b/src/heap/objects-visiting-inl.h
@@ -419,10 +419,10 @@ void StaticMarkingVisitor<StaticVisitor>::VisitSharedFunctionInfo(
       // Add the shared function info holding an optimized code map to
       // the code flusher for processing of code maps after marking.
       collector->code_flusher()->AddOptimizedCodeMap(shared);
-      // Treat all references within the code map weakly by marking the
+      // Treat some references within the code map weakly by marking the
       // code map itself but not pushing it onto the marking deque.
       FixedArray* code_map = FixedArray::cast(shared->optimized_code_map());
-      StaticVisitor::MarkObjectWithoutPush(heap, code_map);
+      MarkOptimizedCodeMap(heap, code_map);
     }
     if (IsFlushable(heap, shared)) {
       // This function's code looks flushable. But we have to postpone
@@ -589,6 +589,23 @@ void StaticMarkingVisitor<StaticVisitor>::MarkTransitionArray(
 
 
 template <typename StaticVisitor>
+void StaticMarkingVisitor<StaticVisitor>::MarkOptimizedCodeMap(
+    Heap* heap, FixedArray* code_map) {
+  if (!StaticVisitor::MarkObjectWithoutPush(heap, code_map)) return;
+
+  // Mark the context-independent entry in the optimized code map. Depending on
+  // the age of the code object, we treat it as a strong or a weak reference.
+  Object* shared_object = code_map->get(SharedFunctionInfo::kSharedCodeIndex);
+  if (FLAG_turbo_preserve_shared_code && shared_object->IsCode() &&
+      FLAG_age_code && !Code::cast(shared_object)->IsOld()) {
+    StaticVisitor::VisitPointer(
+        heap,
+        code_map->RawFieldOfElementAt(SharedFunctionInfo::kSharedCodeIndex));
+  }
+}
+
+
+template <typename StaticVisitor>
 void StaticMarkingVisitor<StaticVisitor>::MarkInlinedFunctionsCode(Heap* heap,
                                                                    Code* code) {
   // For optimized functions we should retain both non-optimized version