From 567b1d75f7da75d86af20f3cfcda4ec20cc7aafe Mon Sep 17 00:00:00 2001
From: Robert Jackson <rjackson@linkedin.com>
Date: Wed, 31 May 2017 19:23:57 -0400
Subject: [PATCH] [BUGFIX release] Avoid re-freezing already frozen objects.

When a helper is invoked we pass in the arguments, those arguments
are frozen (via Object.freeze) so that helpers can't mutate them
(and cause issues in the rendering engine itself). When a helper
doesn't have hash arguments, we use a shared `EMPTY_HASH` empty
object to avoid allocating a bunch of empty objects for no reason
(we do roughly the same thing when no positional params are
passed). Since these objects are shared they are being frozen
over and over again (throughout the lifetime of the running
application). Turns out that there is what we think is almost
certainly a bug in Chrome, where re-freezing the same object many
times starts taking significantly more time upon each freeze
attempt.

This change introduces a guard to ensure we do not re-freeze repeatedly.

Thanks to @krisselden for identifying the root cause and reporting
the upstream Chrome bug: https://bugs.chromium.org/p/v8/issues/detail?id=6450
---
 .../ember-glimmer/lib/utils/references.js     | 34 ++++++++++++-------
 1 file changed, 22 insertions(+), 12 deletions(-)

diff --git a/packages/ember-glimmer/lib/utils/references.js b/packages/ember-glimmer/lib/utils/references.js
index bb0d7a37f1a..75ac4b368e0 100644
--- a/packages/ember-glimmer/lib/utils/references.js
+++ b/packages/ember-glimmer/lib/utils/references.js
@@ -38,6 +38,22 @@ export const UPDATE = symbol('UPDATE');
 
 export { NULL_REFERENCE, UNDEFINED_REFERENCE } from '@glimmer/runtime';
 
+let maybeFreeze;
+if (DEBUG) {
+  // gaurding this in a DEBUG gaurd (as well as all invocations)
+  // so that it is properly stripped during the minification's
+  // dead code elimination
+  maybeFreeze = (obj) => {
+    // re-freezing an already frozen object introduces a significant
+    // performance penalty on Chrome (tested through 59).
+    //
+    // See: https://bugs.chromium.org/p/v8/issues/detail?id=6450
+    if (!Object.isFrozen(obj) && HAS_NATIVE_WEAKMAP) {
+      Object.freeze(obj);
+    }
+  }
+}
+
 // @abstract
 // @implements PathReference
 class EmberPathReference {
@@ -296,10 +312,8 @@ export class SimpleHelperReference extends CachedReference {
       let namedValue = named.value();
 
       if (DEBUG) {
-        if (HAS_NATIVE_WEAKMAP) {
-          Object.freeze(positionalValue);
-          Object.freeze(namedValue);
-        }
+        maybeFreeze(positionalValue);
+        maybeFreeze(namedValue);
       }
 
       let result = helper(positionalValue, namedValue);
@@ -333,10 +347,8 @@ export class SimpleHelperReference extends CachedReference {
     let namedValue = named.value();
 
     if (DEBUG) {
-      if (HAS_NATIVE_WEAKMAP) {
-        Object.freeze(positionalValue);
-        Object.freeze(namedValue);
-      }
+      maybeFreeze(positionalValue);
+      maybeFreeze(namedValue);
     }
 
     return helper(positionalValue, namedValue);
@@ -365,10 +377,8 @@ export class ClassBasedHelperReference extends CachedReference {
     let namedValue = named.value();
 
     if (DEBUG) {
-      if (HAS_NATIVE_WEAKMAP) {
-        Object.freeze(positionalValue);
-        Object.freeze(namedValue);
-      }
+      maybeFreeze(positionalValue);
+      maybeFreeze(namedValue);
     }
 
     return instance.compute(positionalValue, namedValue);