Eliminate AArch64-specific code paths from LowLevelHash

After internal investigation, it’s no longer clear that the alternative
LowLevelHash mixer committed in a05366d8
unequivocally improves performance on AArch64. It unnecessarily reduces
performance on Apple Silicon and the AWS Graviton. It also lowers hash
quality, which offsets much of the performance gain it provides on the
Arm Neoverse N1 (see https://github.com/abseil/abseil-cpp/issues/1093).
Switch back to the original mixer.

Closes: https://github.com/abseil/abseil-cpp/issues/1093
PiperOrigin-RevId: 493941913
Change-Id: I84c789b2f88c91dec22f6f0f6e8c5129d2939a6f

absl/hash/BUILD.bazel

@@ -157,7 +157,6 @@ cc_library(
     deps = [
     deps = [
         "//absl/base:config",
         "//absl/base:config",
         "//absl/base:endian",
         "//absl/base:endian",
-        "//absl/numeric:bits",
         "//absl/numeric:int128",
         "//absl/numeric:int128",
     ],
     ],
 )
 )

absl/hash/CMakeLists.txt

@@ -140,7 +140,6 @@ absl_cc_library(
   COPTS
   COPTS
     ${ABSL_DEFAULT_COPTS}
     ${ABSL_DEFAULT_COPTS}
   DEPS
   DEPS
-    absl::bits
     absl::config
     absl::config
     absl::endian
     absl::endian
     absl::int128
     absl::int128

absl/hash/internal/low_level_hash.cc

@@ -15,7 +15,6 @@
 #include "absl/hash/internal/low_level_hash.h"
 #include "absl/hash/internal/low_level_hash.h"
 
 
 #include "absl/base/internal/unaligned_access.h"
 #include "absl/base/internal/unaligned_access.h"
-#include "absl/numeric/bits.h"
 #include "absl/numeric/int128.h"
 #include "absl/numeric/int128.h"
 
 
 namespace absl {
 namespace absl {
@@ -23,20 +22,9 @@ ABSL_NAMESPACE_BEGIN
 namespace hash_internal {
 namespace hash_internal {
 
 
 static uint64_t Mix(uint64_t v0, uint64_t v1) {
 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;
   absl::uint128 p = v0;
   p *= v1;
   p *= v1;
   return absl::Uint128Low64(p) ^ absl::Uint128High64(p);
   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,
 uint64_t LowLevelHash(const void* data, size_t len, uint64_t seed,

absl/hash/internal/low_level_hash_test.cc

@@ -452,54 +452,6 @@ TEST(LowLevelHashTest, VerifyGolden) {
       0xdd497891465a2cc1, 0x6f1fe8c57a33072e, 0x2c9f4ec078c460c0,
       0xdd497891465a2cc1, 0x6f1fe8c57a33072e, 0x2c9f4ec078c460c0,
       0x9a725bde8f6a1437, 0x6ce545fa3ef61e4d,
       0x9a725bde8f6a1437, 0x6ce545fa3ef61e4d,
   };
   };
-#elif defined(__aarch64__)
-  constexpr uint64_t kGolden[kNumGoldenOutputs] = {
-      0x45c0aadee165dcbe, 0x25ed8587f6f20d06, 0x5f23ae668ce7926d,
-      0xfef74d1da0846719, 0x54478408e68cb7d4, 0xee27ddaf88c6fe68,
-      0xb7ac7031e81867ca, 0xf1168f818ec6c36d, 0x1dd0b734a83b019a,
-      0xd6ae30d4142b54fe, 0xcd860c721ccb80fb, 0x068acf8493794756,
-      0xd4ada0be58681307, 0x13ffe0f64ca540ed, 0xffc1d7a3401aec02,
-      0xd81c4d865cf95fb9, 0x1dd0793acede62e0, 0xa6722abbca8fe4cf,
-      0x5453d3e4111a7e40, 0xf29b3e3204c9dcd2, 0x23be2980e43117f7,
-      0x74e2ccbc286f08eb, 0x19ef7c0f9496003a, 0xbfbf1c3e49b27987,
-      0x6e6c179eb4a82c70, 0x07f4e184216bc4fc, 0xf17fbc4254927554,
-      0xe57696b70a45b1b6, 0x6d3b144631b320e8, 0xccf8729792c75a2d,
-      0xe832495b41fa980b, 0x5c96cfdc7b227d34, 0xc4dca234ef4e43f4,
-      0x5fc801abf9abe307, 0xe41e3c5076d88f4d, 0x522346200ddec3c3,
-      0x72bed1946fd7aaa4, 0x0ac1f84dcc335f96, 0x3af78db5e0a47670,
-      0x6100ebf1481f1caf, 0xf5fd10037fc651a3, 0xa01227d8944665f3,
-      0x7217681c4bbc9420, 0x4adee538e3eb10d1, 0x35e1761ad96de9a7,
-      0x8b370aef9613bfba, 0x824506f749eeaf59, 0x85e805fa04423991,
-      0xb61e9c33283c3de7, 0xc79721bbcb039ed6, 0x04e1c19a3a1e6639,
-      0x6aaf6346b68dd638, 0x601a4b496be6d0c4, 0x3ece355f91c41787,
-      0xd2fc8998448d7888, 0xd7529804f843efa9, 0xabdcc38a288536aa,
-      0xdd323e48a9718648, 0x2090279c0030a52a, 0xe2f90faca88a3cd1,
-      0x3e0c4e92fc50e4aa, 0xa26d308798e801dd, 0x432eefeedee8c02e,
-      0xca4ce494614b77df, 0xbba82911e838066d, 0x4b00821016adee4b,
-      0x4cf6e526dfb5a20f, 0x5b8466495142cba2, 0xe28ac1406e88a68c,
-      0x8511e5f9d3100999, 0x05acbfe02798890b, 0x74c249c7ce4a8425,
-      0xdbe7468d09bc34bc, 0x11079ab10e3b9b58, 0xb7788dec9032035a,
-      0xb7e8daa786513f80, 0x34c3288831f46b45, 0x014cce5f0c21ecc6,
-      0xc6a8f7b024551a28, 0x49784e902e207fd8, 0x4720d32af0b55158,
-      0x8df3ec5de0c1da00, 0xf4db677b2c9e6853, 0xaa419abea78d312d,
-      0x181e0f91bd757443, 0xa8c45136fada083b, 0x91303b93f5f0582c,
-      0x883b95c6ddc62a08, 0x93186a8875fe952b, 0xd94f533928e957e2,
-      0x6ba343003e10c172, 0xc8623b620c715d6a, 0x8ca0c512e180e244,
-      0xdc9b74c2536b6216, 0x8eb5fdc61b295d96, 0x2ad83966b37c95ba,
-      0xb90bf154ac5edec9, 0x902cf847b326cfb3, 0x7b02d0c0ca7808ca,
-      0x492f310d003ea15f, 0x3eb6497a47c95990, 0x5d46b0ced31428b7,
-      0x081afa67d1986157, 0x043482ec286b20eb, 0xc103c8f18c1a2a53,
-      0xe8e9995a81481e83, 0x6bb3295822bc90b5, 0xeec75297a3fa5672,
-      0x591c8440c4857412, 0x74947f455aaf24ad, 0xcf0e571586ec77a9,
-      0x0c2553ea8c0400ad, 0x380219118066255f, 0x7595adb88b15ebe2,
-      0xb33c00696c64ae23, 0xa143516ddd7c9857, 0x39179af229248d26,
-      0x65d387a6f2ee2079, 0x89f8a9b21cd2f195, 0xbfef032d25df92e6,
-      0x6b7e18a36c69da71, 0x4b3b15f6c28974e6, 0x032a75917f6c544c,
-      0xe3b97ecca6d287cd, 0xa4a563110d3cda81, 0x35e09e8134f4e7f1,
-      0xc9419dd03a9a390e, 0x7b86fae9000fd329, 0x1e044f8d54fe74c3,
-      0x9c4991d7a47e9666, 0xfb485f3a1df4fdb6, 0xb11519969eeb94ff,
-      0x3224ea1c44caeb8d, 0x86570bbd7cc6b80d,
-  };
 #else
 #else
   constexpr uint64_t kGolden[kNumGoldenOutputs] = {
   constexpr uint64_t kGolden[kNumGoldenOutputs] = {
       0xe5a40d39ab796423, 0x1766974bf7527d81, 0x5c3bbbe230db17a8,
       0xe5a40d39ab796423, 0x1766974bf7527d81, 0x5c3bbbe230db17a8,