Merged: [runtime] Set instance prototypes directly on maps

Bug: chromium:1452137
(cherry picked from commit c7c4477)

Change-Id: I022863daf8da14feb68862b45bf3d3504a25540c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/4637890
Reviewed-by: Toon Verwaest <[email protected]>
Auto-Submit: Igor Sheludko <[email protected]>
Commit-Queue: Toon Verwaest <[email protected]>
Commit-Queue: Igor Sheludko <[email protected]>
Cr-Commit-Position: refs/branch-heads/11.5@{v8#20}
Cr-Branched-From: 0c4044b-refs/heads/11.5.150@{#1}
Cr-Branched-From: b71d303-refs/heads/main@{#87781}

src/objects/js-function.cc

@@ -676,6 +676,10 @@ void SetInstancePrototype(Isolate* isolate, Handle<JSFunction> function,
       // At that point, a new initial map is created and the prototype is put
       // into the initial map where it belongs.
       function->set_prototype_or_initial_map(*value, kReleaseStore);
+      if (value->IsJSObjectThatCanBeTrackedAsPrototype()) {
+        // Optimize as prototype to detach it from its transition tree.
+        JSObject::OptimizeAsPrototype(Handle<JSObject>::cast(value));
+      }
     } else {
       Handle<Map> new_map =
           Map::Copy(isolate, initial_map, "SetInstancePrototype");
@@ -808,8 +812,10 @@ void JSFunction::EnsureHasInitialMap(Handle<JSFunction> function) {
   Handle<HeapObject> prototype;
   if (function->has_instance_prototype()) {
     prototype = handle(function->instance_prototype(), isolate);
+    map->set_prototype(*prototype);
   } else {
     prototype = isolate->factory()->NewFunctionPrototype(function);
+    Map::SetPrototype(isolate, map, prototype);
   }
   DCHECK(map->has_fast_object_elements());