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

[Fangrui Song via llvm-commits]

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@@ -432,24 +265,22 @@ hash_code hash_combine_range_impl(InputIteratorT first, InputIteratorT last) {
 template <typename ValueT>
 std::enable_if_t<is_hashable_data<ValueT>::value, hash_code>
 hash_combine_range_impl(ValueT *first, ValueT *last) {
-  const uint64_t seed = get_execution_seed();
-  const char *s_begin = reinterpret_cast<const char *>(first);
-  const char *s_end = reinterpret_cast<const char *>(last);
-  const size_t length = std::distance(s_begin, s_end);
-  if (length <= 64)
-    return hash_short(s_begin, length, seed);
-
-  const char *s_aligned_end = s_begin + (length & ~63);
-  hash_state state = state.create(s_begin, seed);
-  s_begin += 64;
-  while (s_begin != s_aligned_end) {
-    state.mix(s_begin);
-    s_begin += 64;
-  }
-  if (length & 63)
-    state.mix(s_end - 64);
+  return combine_bytes(reinterpret_cast<const char *>(first),
+                       size_t(last - first) * sizeof(ValueT));
+}
+
+/// Sum of `sizeof(get_hashable_data(arg))` across a parameter pack.
+template <typename... Ts> constexpr size_t total_hashable_size() {
+  return (size_t(0) + ... +
+          sizeof(decltype(get_hashable_data(std::declval<Ts>()))));
+}
 
-  return state.finalize(length);
+/// Copy `get_hashable_data(arg)` into `buf` at offset `off`, advancing `off`.
+template <typename T>
+inline void store_hashable_data(char *buf, size_t &off, const T &arg) {
+  auto data = get_hashable_data(arg);
+  std::memcpy(buf + off, &data, sizeof(data));
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MaskRay wrote:

The cross-host instability is probably desired - so that we can more reliably catch misuses.

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