Index: llvm/trunk/include/llvm/Support/MathExtras.h
===================================================================
--- llvm/trunk/include/llvm/Support/MathExtras.h
+++ llvm/trunk/include/llvm/Support/MathExtras.h
@@ -316,6 +316,9 @@
 inline uint64_t maxUIntN(uint64_t N) {
   assert(N > 0 && N <= 64 && "integer width out of range");
 
+  // uint64_t(1) << 64 is undefined behavior.
+  if (N == 64)
+    return std::numeric_limits<uint64_t>::max();
   return (UINT64_C(1) << N) - 1;
 }
 
Index: llvm/trunk/unittests/Support/MathExtrasTest.cpp
===================================================================
--- llvm/trunk/unittests/Support/MathExtrasTest.cpp
+++ llvm/trunk/unittests/Support/MathExtrasTest.cpp
@@ -131,6 +131,7 @@
 TEST(MathExtras, maxUIntN) {
   EXPECT_EQ(0xffffULL, maxUIntN(16));
   EXPECT_EQ(0xffffffffULL, maxUIntN(32));
+  EXPECT_EQ(0xffffffffffffffffULL, maxUIntN(64));
   EXPECT_EQ(1ULL, maxUIntN(1));
   EXPECT_EQ(0x0fULL, maxUIntN(4));
 }