[llvm] [Hashing] Replace CityHash mixers with xxh3 (PR #194567)

[Jakub Kuderski via llvm-commits]

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@@ -589,10 +314,14 @@ struct hash_combine_recursive_helper {
 /// The result is suitable for returning from a user's hash_value
 /// *implementation* for their user-defined type. Consumers of a type should
 /// *not* call this routine, they should instead call 'hash_value'.
-template <typename ...Ts> hash_code hash_combine(const Ts &...args) {
-  // Recursively hash each argument using a helper class.
-  ::llvm::hashing::detail::hash_combine_recursive_helper helper;
-  return helper.combine(0, helper.buffer, helper.buffer + 64, args...);
+template <typename... Ts> hash_code hash_combine(const Ts &...args) {
+  constexpr size_t Total = hashing::detail::total_hashable_size<Ts...>();
+  // Round up so `data()` is non-null when Total == 0; combine_bytes won't
+  // read the buffer in that case (len=0 short-circuits in xxh3_64bits).
+  std::array<char, Total == 0 ? 1 : Total> buf;
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kuhar wrote:

nit/optional: this could be `std::max(1, Total)`

https://github.com/llvm/llvm-project/pull/194567