Optimize SameDomainOrHost via a fast-fail path

net/base/registry_controlled_domains/registry_controlled_domain.cc

Index: net/base/registry_controlled_domains/registry_controlled_domain.cc
diff --git a/net/base/registry_controlled_domains/registry_controlled_domain.cc b/net/base/registry_controlled_domains/registry_controlled_domain.cc
index 5f29f89f27cacdd71154db87177dd39ab6a224da..73d5cc1134fe4e864b4b727e146d15784e7476ff 100644
--- a/net/base/registry_controlled_domains/registry_controlled_domain.cc
+++ b/net/base/registry_controlled_domains/registry_controlled_domain.cc
@@ -321,11 +321,28 @@ bool SameDomainOrHost(base::StringPiece host1,
   if (host1 == host2)
     return true;
 
-  // Check for a domain and registry match.
-  const base::StringPiece& domain1 =
+  const base::StringPiece domain1 =
       GetDomainAndRegistryAsStringPiece(host1, filter);
-  return !domain1.empty() &&
-         (domain1 == GetDomainAndRegistryAsStringPiece(host2, filter));
+
+  // If |domain1| is empty, |host1| either matched no rules in the registry,
+  // or is an IP literal. Since |host2| and |host1| are known to differ,
+  // they can't possibly be same-origin.
+  if (domain1.empty())
+    return false;
+
+  // Fail fast before computing |domain2|, if possible, by checking to see if
+  // |domain1| is a suffix of |host2|. This is a necessary but not sufficient
+  // condition for |domain1| and |domain2| to be equal.
+  if (!base::EndsWith(host2, domain1, base::CompareCase::SENSITIVE))

[Ryan Sleevi, 2017/01/06 00:38:16]

Unclear if you're doing sensitive as an optimization (to force a memcmp vs a
normalization), but the comparison here would be case-insensitive.

However, both |host1| and |host2| should have already been normalized
(punycoded, lowercased, dot stripped) in order to get the domain, so you should
be fine.

+    return false;
+
+  const base::StringPiece domain2 =
+      GetDomainAndRegistryAsStringPiece(host2, filter);
+
+  // We already verified that |domain1| is a suffix of |host2|. And |domain2| is
+  // also a suffix of |host2|, by construction. So if their lengths are equal,
+  // they are identical.
+  return domain2.length() == domain1.length();
 }
 
 }  // namespace