diff --git a/include/llvm/ADT/StringRef.h b/include/llvm/ADT/StringRef.h
index 1766d2b..adff42f 100644
--- a/include/llvm/ADT/StringRef.h
+++ b/include/llvm/ADT/StringRef.h
@@ -125,6 +125,12 @@
       return Length < RHS.Length ? -1 : 1;
     }
 
+    /// compare - Compare two strings; the result is -1, 0, or 1 if this string
+    /// is lexicographically less than, equal to, or greater than the \arg RHS.
+    /// This is different than compare with no size specified as it only 
+    /// compares at most the first n bytes.
+    int compare(StringRef RHS, size_t n) const;
+
     /// compare_lower - Compare two strings, ignoring case.
     int compare_lower(StringRef RHS) const;
 
diff --git a/lib/Support/StringRef.cpp b/lib/Support/StringRef.cpp
index 8c3fc09..066e774 100644
--- a/lib/Support/StringRef.cpp
+++ b/lib/Support/StringRef.cpp
@@ -29,6 +29,27 @@
   return x >= '0' && x <= '9';
 }
 
+/// compare - Compare two strings; the result is -1, 0, or 1 if this string
+/// is lexicographically less than, equal to, or greater than the \arg RHS.
+/// This is different than compare with no size specified as it only 
+/// compares at most the first n bytes.
+int StringRef::compare(StringRef RHS, size_t n) const {
+  // Check the prefix for a mismatch.
+  size_t maxToCmp = min(Length, RHS.Length);
+  maxToCmp = min(maxToCmp, n);
+  if (int Res = memcmp(Data, RHS.Data, maxToCmp))
+    return Res < 0 ? -1 : 1;
+  
+  // Otherwise the prefixes match, so we only need to check the lengths.
+  // Be mindful that if the n is less than or equal to the length of either
+  // string, that is the same as the strings matching because in that case
+  // we only care about the prefix.
+  if (((n <= Length) && (n <= RHS.Length)) || 
+      (Length == RHS.Length))
+    return 0;
+  return Length < RHS.Length ? -1 : 1;
+}
+
 /// compare_lower - Compare strings, ignoring case.
 int StringRef::compare_lower(StringRef RHS) const {
   for (size_t I = 0, E = min(Length, RHS.Length); I != E; ++I) {
diff --git a/unittests/ADT/StringRefTest.cpp b/unittests/ADT/StringRefTest.cpp
index 5731e4a..633fcc8 100644
--- a/unittests/ADT/StringRefTest.cpp
+++ b/unittests/ADT/StringRefTest.cpp
@@ -55,6 +55,19 @@
   EXPECT_EQ( 1, StringRef("aab").compare("aa"));
   EXPECT_EQ( 1, StringRef("\xFF").compare("\1"));
 
+  EXPECT_EQ(-1, StringRef("aab").compare("aad", 3));
+  EXPECT_EQ( 0, StringRef("aab").compare("aab", 3));
+  EXPECT_EQ( 1, StringRef("aab").compare("aaa", 3));
+  EXPECT_EQ(-1, StringRef("aab").compare("aabb", 4));
+  EXPECT_EQ( 1, StringRef("aab").compare("aa", 3));
+  EXPECT_EQ( 1, StringRef("\xFF").compare("\1", 3));
+  EXPECT_EQ( 0, StringRef("aab").compare("aad", 2));
+  EXPECT_EQ( 0, StringRef("aab").compare("aab", 2));
+  EXPECT_EQ( 0, StringRef("aab").compare("aab", 4));
+  EXPECT_EQ( 0, StringRef("aab").compare("aaa", 2));
+  EXPECT_EQ( 0, StringRef("aab").compare("aabb", 3));
+  EXPECT_EQ( 0, StringRef("aab").compare("aa", 2));
+
   EXPECT_EQ(-1, StringRef("AaB").compare_lower("aAd"));
   EXPECT_EQ( 0, StringRef("AaB").compare_lower("aab"));
   EXPECT_EQ( 1, StringRef("AaB").compare_lower("AAA"));