2 files changed, 4 insertions, 0 deletions
diff --git a/llvm/include/llvm/Support/MathExtras.h b/llvm/include/llvm/Support/MathExtras.h
index f44fd19c544..b60732a9b34 100644
--- a/llvm/include/llvm/Support/MathExtras.h
+++ b/llvm/include/llvm/Support/MathExtras.h
@@ -316,6 +316,9 @@ inline bool isShiftedUInt(uint64_t x) {
 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;
 }
 
diff --git a/llvm/unittests/Support/MathExtrasTest.cpp b/llvm/unittests/Support/MathExtrasTest.cpp
index 04e16628e64..ef46311665d 100644
--- a/llvm/unittests/Support/MathExtrasTest.cpp
+++ b/llvm/unittests/Support/MathExtrasTest.cpp
@@ -131,6 +131,7 @@ TEST(MathExtras, minIntN) {
 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));
 }