diff options
Diffstat (limited to 'absl/crc/internal/crc_x86_arm_combined.cc')
| -rw-r--r-- | absl/crc/internal/crc_x86_arm_combined.cc | 10 |
1 files changed, 7 insertions, 3 deletions
diff --git a/absl/crc/internal/crc_x86_arm_combined.cc b/absl/crc/internal/crc_x86_arm_combined.cc index 20dd3e01..79dace34 100644 --- a/absl/crc/internal/crc_x86_arm_combined.cc +++ b/absl/crc/internal/crc_x86_arm_combined.cc @@ -101,13 +101,17 @@ constexpr size_t kMediumCutoff = 2048; namespace { uint32_t multiply(uint32_t a, uint32_t b) { - V128 shifts = V128_From64WithZeroFill(1); V128 power = V128_From64WithZeroFill(a); V128 crc = V128_From64WithZeroFill(b); V128 res = V128_PMulLow(power, crc); - // Combine crc values - res = V128_ShiftLeft64(res, shifts); + // Combine crc values. + // + // Adding res to itself is equivalent to multiplying by 2, + // or shifting left by 1. Addition is used as not all compilers + // are able to generate optimal code without this hint. + // https://godbolt.org/z/rr3fMnf39 + res = V128_Add64(res, res); return static_cast<uint32_t>(V128_Extract32<1>(res)) ^ CRC32_u32(0, static_cast<uint32_t>(V128_Low64(res))); } |