[ADT] Compare strings' hashes first before comparing their values.

Summary:
We already have the hashes in hand, and comparing hashes should be much
more discriminatory than comparing the StringRefs' sizes.

Reviewers: rafael

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25705

llvm-svn: 284872

llvm/include/llvm/ADT/CachedHashString.h

@@ -61,7 +61,8 @@ template <> struct DenseMapInfo<CachedHashStringRef> {
   }
   static bool isEqual(const CachedHashStringRef &LHS,
                       const CachedHashStringRef &RHS) {
-    return DenseMapInfo<StringRef>::isEqual(LHS.val(), RHS.val());
+    return LHS.hash() == RHS.hash() &&
+           DenseMapInfo<StringRef>::isEqual(LHS.val(), RHS.val());
   }
 };