Optimize `absl::Hash` by making `LowLevelHash` faster.

Throughput of the 64 byte chunk loop inside `LowLevelHash` (or now in `LowLevelHashLenGt16`) gets limited by the loop carried dependency on `current_state`. By using 4 states instead of 2, we can reduce this duration by 1 cycle. On Skylake, it is reduced from 9 cycles to 8 cycles (12.5% faster asymptotically).

To see the reduction in a simplified version of `LowLevelHash` implementation on Skylake:
* Before: https://godbolt.org/z/Tcj9vsGax, llvm-mca (https://godbolt.org/z/3o78Msr63) shows 9 cycles / iteration.
* After: https://godbolt.org/z/q4GM4EjPr, llvm-mca (https://godbolt.org/z/W5d1KEMzq) shows 8 cycles / iteration.
* This CL is removing 1 xor (1 cycle) per iteration from the critical path.

A block for 32 byte chunk is also added.

Finally, just before returning, `Mix` is called 1 time instead of twice.

PiperOrigin-RevId: 605090653
Change-Id: Ib7517ebb8bef7484066cd14cf41a943953e93377

1 files changed, 4 insertions, 0 deletions

diff --git a/absl/hash/internal/low_level_hash.h b/absl/hash/internal/low_level_hash.h
index 439968aa..d460e351 100644
--- a/absl/hash/internal/low_level_hash.h
+++ b/absl/hash/internal/low_level_hash.h
@@ -43,6 +43,10 @@ namespace hash_internal {
 uint64_t LowLevelHash(const void* data, size_t len, uint64_t seed,
                       const uint64_t salt[5]);
 
+// Same as above except the length must be greater than 16.
+uint64_t LowLevelHashLenGt16(const void* data, size_t len, uint64_t seed,
+                             const uint64_t salt[5]);
+
 }  // namespace hash_internal
 ABSL_NAMESPACE_END
 }  // namespace absl