diff options
Diffstat (limited to 'absl/hash/internal/low_level_hash.cc')
| -rw-r--r-- | absl/hash/internal/low_level_hash.cc | 12 |
1 files changed, 12 insertions, 0 deletions
diff --git a/absl/hash/internal/low_level_hash.cc b/absl/hash/internal/low_level_hash.cc index 856bbd9b..6f9cb9c7 100644 --- a/absl/hash/internal/low_level_hash.cc +++ b/absl/hash/internal/low_level_hash.cc @@ -15,6 +15,7 @@ #include "absl/hash/internal/low_level_hash.h" #include "absl/base/internal/unaligned_access.h" +#include "absl/numeric/bits.h" #include "absl/numeric/int128.h" namespace absl { @@ -22,9 +23,20 @@ ABSL_NAMESPACE_BEGIN namespace hash_internal { static uint64_t Mix(uint64_t v0, uint64_t v1) { +#if !defined(__aarch64__) + // The default bit-mixer uses 64x64->128-bit multiplication. absl::uint128 p = v0; p *= v1; return absl::Uint128Low64(p) ^ absl::Uint128High64(p); +#else + // The default bit-mixer above would perform poorly on some ARM microarchs, + // where calculating a 128-bit product requires a sequence of two + // instructions with a high combined latency and poor throughput. + // Instead, we mix bits using only 64-bit arithmetic, which is faster. + uint64_t p = v0 ^ absl::rotl(v1, 40); + p *= v1 ^ absl::rotl(v0, 39); + return p ^ (p >> 11); +#endif } uint64_t LowLevelHash(const void* data, size_t len, uint64_t seed, |