Fix the logic that should ensure that a string cannot have

a hash key of zero.
Review URL: http://codereview.chromium.org/9113006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10338 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/objects-inl.h

@@ -4381,7 +4381,7 @@ uint32_t StringHasher::GetHash() {
   result += (result << 3);
   result ^= (result >> 11);
   result += (result << 15);
-  if (result == 0) {
+  if ((result & String::kHashBitMask) == 0) {
     result = 27;
   }
   return result;

src/objects.cc

@@ -11444,7 +11444,7 @@ class TwoCharHashTableKey : public HashTableKey {
     hash += hash << 3;
     hash ^= hash >> 11;
     hash += hash << 15;
-    if (hash == 0) hash = 27;
+    if ((hash & String::kHashBitMask) == 0) hash = 27;
 #ifdef DEBUG
     StringHasher hasher(2, seed);
     hasher.AddCharacter(c1);

src/objects.h

@@ -6459,6 +6459,10 @@ class String: public HeapObject {
   // Shift constant retrieving hash code from hash field.
   static const int kHashShift = kNofHashBitFields;
 
+  // Only these bits are relevant in the hash, since the top two are shifted
+  // out.
+  static const uint32_t kHashBitMask = 0xffffffffu >> kHashShift;
+
   // Array index strings this short can keep their index in the hash
   // field.
   static const int kMaxCachedArrayIndexLength = 7;