Don't do uint64_t(1) << 64 in maxUIntN.

[jlebar]

Summary

This shift is undefined behavior (and, as compiled by clang, gives the
wrong answer for maxUIntN(64)).

[mkuper]

LGTM.
I'm not sure there isn't a better way to do this ( :-) ), but let's get correctness right first.

[jlebar]

Yeah, I have a faster way, but I agree, correctness first.

include/llvm/Support/MathExtras.h

 inline bool isShiftedUInt(uint64_t x) {
   return isUInt<N+S>(x) && (x % (1<<S) == 0);
 }
 
 /// Gets the maximum value for a N-bit unsigned integer.
 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;
 }
 
 /// Gets the minimum value for a N-bit signed integer.
 inline int64_t minIntN(int64_t N) {
   assert(N > 0 && N <= 64 && "integer width out of range");
 
   return -(INT64_C(1)<<(N-1));

unittests/Support/MathExtrasTest.cpp

 TEST(MathExtras, minIntN) {
   EXPECT_EQ(-32768LL, minIntN(16));
   EXPECT_EQ(-64LL, minIntN(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));
 }
 
 TEST(MathExtras, reverseBits) {
   uint8_t NZ8 = 42;
   uint16_t NZ16 = 42;
   uint32_t NZ32 = 42;