[ADT] Make BitmaskEnum operations constant expressions

[labath]

Summary

This avoids runtime initialization (a global constructor) whenever they appear
in the initializer.

The patch just adds the constexpr keyword to a couple of functions.

[shafik]

This is just prep work for another change then?

[labath]

In D121281#3373818, @shafik wrote:

This is just prep work for another change then?

Nope. This is the real thing. The "just" in the description is there because sprinkling the constexprs is literally the only thing I had to do to make it work.

llvm/include/llvm/ADT/BitmaskEnum.h

 template <typename E>
 struct is_bitmask_enum<
     E, std::enable_if_t<sizeof(E::LLVM_BITMASK_LARGEST_ENUMERATOR) >= 0>>
     : std::true_type {};
 namespace BitmaskEnumDetail {
 
 /// Get a bitmask with 1s in all places up to the high-order bit of E's largest
 /// value.
-template <typename E> std::underlying_type_t<E> Mask() {
+template <typename E> constexpr std::underlying_type_t<E> Mask() {
   // On overflow, NextPowerOf2 returns zero with the type uint64_t, so
   // subtracting 1 gives us the mask with all bits set, like we want.
   return NextPowerOf2(static_cast<std::underlying_type_t<E>>(
              E::LLVM_BITMASK_LARGEST_ENUMERATOR)) -
          1;
 }
 
 /// Check that Val is in range for E, and return Val cast to E's underlying
 /// type.
-template <typename E> std::underlying_type_t<E> Underlying(E Val) {
+template <typename E> constexpr std::underlying_type_t<E> Underlying(E Val) {
   auto U = static_cast<std::underlying_type_t<E>>(Val);
   assert(U >= 0 && "Negative enum values are not allowed.");
   assert(U <= Mask<E>() && "Enum value too large (or largest val too small?)");
   return U;
 }
 
 constexpr unsigned bitWidth(uint64_t Value) {
   return Value ? 1 + bitWidth(Value >> 1) : 0;
 }
 
 template <typename E, typename = std::enable_if_t<is_bitmask_enum<E>::value>>
-E operator~(E Val) {
+constexpr E operator~(E Val) {
   return static_cast<E>(~Underlying(Val) & Mask<E>());
 }
 
 template <typename E, typename = std::enable_if_t<is_bitmask_enum<E>::value>>
-E operator|(E LHS, E RHS) {
+constexpr E operator|(E LHS, E RHS) {
   return static_cast<E>(Underlying(LHS) | Underlying(RHS));
 }
 
 template <typename E, typename = std::enable_if_t<is_bitmask_enum<E>::value>>
-E operator&(E LHS, E RHS) {
+constexpr E operator&(E LHS, E RHS) {
   return static_cast<E>(Underlying(LHS) & Underlying(RHS));
 }
 
 template <typename E, typename = std::enable_if_t<is_bitmask_enum<E>::value>>
-E operator^(E LHS, E RHS) {
+constexpr E operator^(E LHS, E RHS) {
   return static_cast<E>(Underlying(LHS) ^ Underlying(RHS));
 }
 
 // |=, &=, and ^= return a reference to LHS, to match the behavior of the
 // operators on builtin types.
 
 template <typename E, typename = std::enable_if_t<is_bitmask_enum<E>::value>>
 E &operator|=(E &LHS, E RHS) {

llvm/include/llvm/Support/MathExtras.h

constexpr inline uint64_t MinAlign(uint64_t A, uint64_t B) {
   // Replace "-Value" by "1+~Value" in the following commented code to avoid
   // MSVC warning C4146
   //    return (A | B) & -(A | B);
   return (A | B) & (1 + ~(A | B));
 }
 
 /// Returns the next power of two (in 64-bits) that is strictly greater than A.
 /// Returns zero on overflow.
-inline uint64_t NextPowerOf2(uint64_t A) {
+constexpr inline uint64_t NextPowerOf2(uint64_t A) {
   A |= (A >> 1);
   A |= (A >> 2);
   A |= (A >> 4);
   A |= (A >> 8);
   A |= (A >> 16);
   A |= (A >> 32);
   return A + 1;
 }

llvm/unittests/ADT/BitmaskEnumTest.cpp

TEST(BitmaskEnumTest, BitwiseXorEquals) {
   f ^= (F2 | F4);
   EXPECT_EQ(9, f);
 
   // ^= should return a reference to the LHS.
   (f ^= F4) = F3;
   EXPECT_EQ(F3, f);
 }
 
+TEST(BitmaskEnumTest, ConstantExpression) {
+  constexpr Flags f1 = ~F1;
+  constexpr Flags f2 = F1 | F2;
+  constexpr Flags f3 = F1 & F2;
+  constexpr Flags f4 = F1 ^ F2;
+  EXPECT_EQ(f1, ~F1);
+  EXPECT_EQ(f2, F1 | F2);
+  EXPECT_EQ(f3, F1 & F2);
+  EXPECT_EQ(f4, F1 ^ F2);
+}
+
 TEST(BitmaskEnumTest, BitwiseNot) {
   Flags f = ~F1;
   EXPECT_EQ(14, f); // Largest value for f is 15.
   EXPECT_EQ(15, ~F0);
 }
 
 enum class FlagsClass {
   F0 = 0,