Basic array capacity feedback via allocation sites.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index cff5b61b527bec83db46b2efbc99d85898e0a1ae..c4f386eb1bf2c80872e7d5be40fba475ad2eb856 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1704,6 +1704,38 @@ inline bool AllocationSite::MakePretenureDecision(
 }
 
 
+int AllocationSite::GetInitialElementsCapacity() {
+  if (FLAG_initial_capacity_mode != 2) {
+    return JSArray::PreallocatedArrayElements();
+  }
+
+  if (SitePointsToLiteral()) return 0;
+  int value = Smi::cast(transition_info())->value();
+  return InitialElementsCapacity::decode(value);
+}
+
+
+void AllocationSite::DigestInitialElementsCapacityFeedback(int new_length) {
+  if (FLAG_initial_capacity_mode != 2) return;
+
+  if (new_length > 0 && !SitePointsToLiteral()) {
+    int to_store = (new_length + (new_length >> 1) + 16) / 4;
+    int old_capacity = GetInitialElementsCapacity();
+    if (to_store > old_capacity &&
+        InitialElementsCapacity::is_valid(to_store)) {
+      int value = Smi::cast(transition_info())->value();
+      set_transition_info(
+          Smi::FromInt(InitialElementsCapacity::update(value, to_store)),
+          SKIP_WRITE_BARRIER);
+      if (FLAG_trace_track_allocation_sites) {
+        PrintF("Initial elements capacity for site %p, updated to %d.\n",
+               static_cast<void*>(this), to_store);
+      }
+    }
+  }
+}
+
+
 inline bool AllocationSite::DigestPretenuringFeedback(
     bool maximum_size_scavenge) {
   bool deopt = false;