From 5a386c6f89edf011820d60636d16f035ee3a66b0 Mon Sep 17 00:00:00 2001
From: Jan de Mooij <jdemooij@mozilla.com>
Date: Thu, 26 Apr 2018 18:41:30 +0200
Subject: [PATCH] Bug 912079 part 4 - Optimize SuppressDeletedPropertyHelper
 when the property matches the iterator's previous one. r=anba

--HG--
extra : rebase_source : 7c860b0162c165f536e5b316204ea76eebaff59d
---
 js/src/vm/Iteration.cpp | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/js/src/vm/Iteration.cpp b/js/src/vm/Iteration.cpp
index 6bd8788cbaeb..d6ded773c569 100644
--- a/js/src/vm/Iteration.cpp
+++ b/js/src/vm/Iteration.cpp
@@ -1205,6 +1205,17 @@ SuppressDeletedProperty(JSContext* cx, NativeIterator* ni, HandleObject obj,
     if (ni->obj != obj)
         return true;
 
+    // Optimization for the following common case:
+    //
+    //    for (var p in o) {
+    //        delete o[p];
+    //    }
+    //
+    // Note that usually both strings will be atoms so we only check for pointer
+    // equality here.
+    if (ni->props_cursor > ni->begin() && ni->props_cursor[-1] == str)
+        return true;
+
     while (true) {
         bool restart = false;