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

[Jakub Kuderski via llvm-commits]

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@@ -320,6 +170,13 @@ inline uint64_t get_execution_seed() {
 #endif
 }
 
+/// Hash a contiguous byte buffer to a hash_code. The execution seed is XORed
+/// into the result (not propagated through the avalanche), so a given byte
+/// stream produces the same xxh3 output modulo the per-process seed.
+inline hash_code combine_bytes(const char *data, size_t len) {
+  return xxh3_64bits(reinterpret_cast<const uint8_t *>(data), len) ^
+         get_execution_seed();
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kuhar wrote:

Could this use a seeded `xxh3` entry point instead of XORing `get_execution_seed()` after the avalanche? With post-XOR seeding, `hash_combine(x) ^ hash_combine(y)` cancels the process seed, which seems to weaken the ABI-breaking hash perturbation this seed is meant to provide.

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