Fixing performance regression in issue 1787.

src/x64/code-stubs-x64.cc

Index: src/x64/code-stubs-x64.cc
diff --git a/src/x64/code-stubs-x64.cc b/src/x64/code-stubs-x64.cc
index 3dfebeec41123ae0fbc149714c02b3be35647603..f62c5172ebf8560f42bf8795cfcea71031d00395 100644
--- a/src/x64/code-stubs-x64.cc
+++ b/src/x64/code-stubs-x64.cc
@@ -5595,6 +5595,11 @@ void StringDictionaryLookupStub::GeneratePositiveLookup(MacroAssembler* masm,
                                                         Register name,
                                                         Register r0,
                                                         Register r1) {
+  ASSERT(!elements.is(r0));
+  ASSERT(!elements.is(r1));
+  ASSERT(!name.is(r0));
+  ASSERT(!name.is(r1));
+
   // Assert that name contains a string.
   if (FLAG_debug_code) __ AbortIfNotString(name);