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Introduce a Carbon hashing framework. (#3327)
# Overview This is a latency-optimized hashing framework based on Abseil's and others. At it's core it uses both a normal 64-bit multiply as well as a 64-bit multiply capturing both low and high 64-bit components of the result and XOR-ing them together. These are the primitives used in FxHash and Abseil respectively, although they both appear in others. The implementation has been *substantially* optimized for short inputs and latency over quality. As a result, this function does not remotely pass the SMHasher quality tests. However, basic collisions are rare, and I've included a small subset of the SMHasher collision testing directly to make sure the quality doesn't slip too far inadvertently. The customization framework is roughly similar to Abseil's and LLVM's but has been simplified significantly, inspired in some respects by the AHash API design and in others by my experience of all performance sensitive hashing implementations needing to work at a very low level to hit their performance targets. The abstractions are stripped down to facilitate this. # Details of the performance optimization This function is 2x - 4x faster than LLVM's on small inputs, and up to 2x faster than Abseil. Significant effort has gone into optimizing short strings in particular compared to Abseil. Small integer and pointer hashing is also faster than Abseil's by leveraging a lower quality 64-bit multiply in some cases inspired by FxHash. One consequence is that this routine is particulary fast for 32-bit integers. The short string improvements largely come from packing more of the bytes of string into as few multiplies as possible. While this fails to mix the bits sufficient to hit SMHasher's strict avalanche criteria and does leave some collision windows, it provides dramatic latency improvements. Some of these techniques come from Abseil's own bulk hashing routine but re-applied here. Others are novel, for example using small sizes to sample nicely uniform random data to efficiently handle the very small number of bits of data that need to be hashed. The other observed improvement is diligent handling of pairs and tuples and fairly aggressively turning things into integers. Some of the comparisons with Abseil aren't realistic as the Abseil hash table does some of these mappings before hashing. I've done this directly in the hash function as that seems cleaner. For long strings, the performance is comparable or a bit better than Abseil, and significantly better than LLVM's hash function. Overall, for short inputs this is hoped to be the fastest hash function that still gets "just enough" mixing for modern hash tables to perform well. # Details of the quality vs. latency tradeoff A key insight is that modern hash tables don't need especially high quality hash functions, but do benefit from something beyond the identify function. That isn't the target of SMHasher or other quality assessing tools and has resulted in unnecessarily aggressive hashing for any functions actually evaluated against it. Many hash functions turn off the high quality implementations evaluated with SMHasher for integer or pointer keys to recover latency & performance (AHash for example), but the same performance-oriented design applies beyond these narrow types, for example for short strings. However, a consequence is that there are serious limits to the quality of the hash function. The avalanche test is failed hilariously, etc., but in the exact same ways as Abseil itself fails it for integer keys. There are also real collisions spaces. For example, for 16-byte strings, there is one 64-bit value for the first 8 bytes that will have the same hash regardless of the other 8 bytes of the string. Some minor effort is taken to make this pattern unlikely to be a practical problem, but it is a clear theoretical weakness. It also means that this hash function couldn't be further from providing any hash-flooding DoS attack protection -- I expect it to be trivially easy to attack in this way by a motivated adversary. Defending against these attacks is defined as out-of-scope, in large part because even attempts that have made a compelling effort to address these issues such as HighwayHash have found serious limits. Instead, this takes a principled position that any such defense should be provided entirely at the data structure level with a strong worst-case bound rather than through strengthening the hash function. # Future work A subsequent PR will introduce a hash table inspired very heavily by the design of Abseil's "SwissTable" and using this hash function. The goal is to provide a significant improvement to hot hash tables such as the identifier table in the lexer of Carbon's toolchain. # Detailed benchmark data The benchmarks introduced are heavily inspired by the latency benchmarking of hash functions in Abseil. I've adapted them to fit better into Carbon's coding style and to try to have more stable results with broader coverage of types and string sizes. Running the benchmarks directly gives horizontal comparisons across different hash functions. That can be hard to read, so here is *just* the newly introduced hash function benchmark results on an AMD server: ``` BM_LatencyHash<RandValues<uint8_t>, CarbonHashBench> 3.11ns ± 1% BM_LatencyHash<RandValues<uint16_t>, CarbonHashBench> 3.11ns ± 1% BM_LatencyHash<RandValues<std::pair<uint8_t, uint8_t>>, CarbonHashBench> 4.11ns ± 1% BM_LatencyHash<RandValues<uint32_t>, CarbonHashBench> 3.12ns ± 1% BM_LatencyHash<RandValues<std::pair<uint16_t, uint16_t>>, CarbonHashBench> 4.13ns ± 1% BM_LatencyHash<RandValues<uint64_t>, CarbonHashBench> 3.16ns ± 2% BM_LatencyHash<RandValues<int*>, CarbonHashBench> 3.16ns ± 2% BM_LatencyHash<RandValues<std::pair<uint32_t, uint32_t>>, CarbonHashBench> 4.03ns ± 2% BM_LatencyHash<RandValues<std::pair<uint64_t, uint32_t>>, CarbonHashBench> 4.04ns ± 1% BM_LatencyHash<RandValues<std::pair<uint32_t, uint64_t>>, CarbonHashBench> 4.34ns ± 2% BM_LatencyHash<RandValues<std::pair<int*, uint32_t>>, CarbonHashBench> 4.04ns ± 1% BM_LatencyHash<RandValues<std::pair<uint32_t, int*>>, CarbonHashBench> 4.34ns ± 2% BM_LatencyHash<RandValues<__uint128_t>, CarbonHashBench> 4.33ns ± 1% BM_LatencyHash<RandValues<std::pair<uint64_t, uint64_t>>, CarbonHashBench> 4.33ns ± 1% BM_LatencyHash<RandValues<std::pair<int*, int*>>, CarbonHashBench> 4.34ns ± 1% BM_LatencyHash<RandValues<std::pair<uint64_t, int*>>, CarbonHashBench> 4.33ns ± 1% BM_LatencyHash<RandValues<std::pair<int*, uint64_t>>, CarbonHashBench> 4.33ns ± 1% BM_LatencyHash<RandStrings< true, 4>, CarbonHashBench> 1.95ns ± 4% BM_LatencyHash<RandStrings< true, 8>, CarbonHashBench> 1.70ns ± 3% BM_LatencyHash<RandStrings< true, 16>, CarbonHashBench> 3.52ns ± 3% BM_LatencyHash<RandStrings< true, 32>, CarbonHashBench> 4.46ns ± 2% BM_LatencyHash<RandStrings< true, 64>, CarbonHashBench> 7.69ns ± 1% BM_LatencyHash<RandStrings< true, 256>, CarbonHashBench> 14.8ns ± 1% BM_LatencyHash<RandStrings< true, 512>, CarbonHashBench> 21.5ns ± 1% BM_LatencyHash<RandStrings< true, 1024>, CarbonHashBench> 34.6ns ± 0% BM_LatencyHash<RandStrings< true, 2048>, CarbonHashBench> 63.1ns ± 1% BM_LatencyHash<RandStrings< true, 4096>, CarbonHashBench> 118ns ± 1% BM_LatencyHash<RandStrings< true, 8192>, CarbonHashBench> 225ns ± 1% ``` And on an ARM server: ``` BM_LatencyHash<RandValues<uint8_t>, CarbonHashBench> 5.28ns ± 0% BM_LatencyHash<RandValues<uint16_t>, CarbonHashBench> 5.29ns ± 0% BM_LatencyHash<RandValues<std::pair<uint8_t, uint8_t>>, CarbonHashBench> 7.02ns ± 0% BM_LatencyHash<RandValues<uint32_t>, CarbonHashBench> 5.34ns ± 1% BM_LatencyHash<RandValues<std::pair<uint16_t, uint16_t>>, CarbonHashBench> 7.07ns ± 4% BM_LatencyHash<RandValues<uint64_t>, CarbonHashBench> 5.36ns ± 2% BM_LatencyHash<RandValues<int*>, CarbonHashBench> 5.36ns ± 2% BM_LatencyHash<RandValues<std::pair<uint32_t, uint32_t>>, CarbonHashBench> 7.19ns ± 3% BM_LatencyHash<RandValues<std::pair<uint64_t, uint32_t>>, CarbonHashBench> 7.29ns ± 2% BM_LatencyHash<RandValues<std::pair<uint32_t, uint64_t>>, CarbonHashBench> 7.31ns ± 4% BM_LatencyHash<RandValues<std::pair<int*, uint32_t>>, CarbonHashBench> 7.29ns ± 2% BM_LatencyHash<RandValues<std::pair<uint32_t, int*>>, CarbonHashBench> 7.31ns ± 4% BM_LatencyHash<RandValues<__uint128_t>, CarbonHashBench> 8.69ns ± 3% BM_LatencyHash<RandValues<std::pair<uint64_t, uint64_t>>, CarbonHashBench> 8.69ns ± 3% BM_LatencyHash<RandValues<std::pair<int*, int*>>, CarbonHashBench> 8.69ns ± 3% BM_LatencyHash<RandValues<std::pair<uint64_t, int*>>, CarbonHashBench> 8.69ns ± 3% BM_LatencyHash<RandValues<std::pair<int*, uint64_t>>, CarbonHashBench> 8.69ns ± 3% BM_LatencyHash<RandStrings< true, 4>, CarbonHashBench> 2.64ns ± 2% BM_LatencyHash<RandStrings< true, 8>, CarbonHashBench> 2.90ns ± 4% BM_LatencyHash<RandStrings< true, 16>, CarbonHashBench> 6.14ns ± 1% BM_LatencyHash<RandStrings< true, 32>, CarbonHashBench> 8.27ns ± 1% BM_LatencyHash<RandStrings< true, 64>, CarbonHashBench> 13.8ns ± 0% BM_LatencyHash<RandStrings< true, 256>, CarbonHashBench> 31.2ns ± 0% BM_LatencyHash<RandStrings< true, 512>, CarbonHashBench> 49.9ns ± 0% BM_LatencyHash<RandStrings< true, 1024>, CarbonHashBench> 86.9ns ± 0% BM_LatencyHash<RandStrings< true, 2048>, CarbonHashBench> 163ns ± 0% BM_LatencyHash<RandStrings< true, 4096>, CarbonHashBench> 312ns ± 0% BM_LatencyHash<RandStrings< true, 8192>, CarbonHashBench> 610ns ± 0% ``` I don't have the same nice statistical multi-run error bars, but one run from my M1 MacBook: ``` BM_LatencyHash<RandValues<uint8_t>, CarbonHashBench> 3.89 ns BM_LatencyHash<RandValues<uint16_t>, CarbonHashBench> 3.87 ns BM_LatencyHash<RandValues<std::pair<uint8_t, uint8_t>>, CarbonHashBench> 4.39 ns BM_LatencyHash<RandValues<uint32_t>, CarbonHashBench> 3.93 ns BM_LatencyHash<RandValues<std::pair<uint16_t, uint16_t>>, CarbonHashBench> 4.98 ns BM_LatencyHash<RandValues<uint64_t>, CarbonHashBench> 3.87 ns BM_LatencyHash<RandValues<int*>, CarbonHashBench> 3.87 ns BM_LatencyHash<RandValues<std::pair<uint32_t, uint32_t>>, CarbonHashBench> 4.86 ns BM_LatencyHash<RandValues<std::pair<uint64_t, uint32_t>>, CarbonHashBench> 4.43 ns BM_LatencyHash<RandValues<std::pair<uint32_t, uint64_t>>, CarbonHashBench> 4.41 ns BM_LatencyHash<RandValues<std::pair<int*, uint32_t>>, CarbonHashBench> 4.44 ns BM_LatencyHash<RandValues<std::pair<uint32_t, int*>>, CarbonHashBench> 4.69 ns BM_LatencyHash<RandValues<__uint128_t>, CarbonHashBench> 4.33 ns BM_LatencyHash<RandValues<std::pair<uint64_t, uint64_t>>, CarbonHashBench> 4.38 ns BM_LatencyHash<RandValues<std::pair<int*, int*>>, CarbonHashBench> 4.34 ns BM_LatencyHash<RandValues<std::pair<uint64_t, int*>>, CarbonHashBench> 4.35 ns BM_LatencyHash<RandValues<std::pair<int*, uint64_t>>, CarbonHashBench> 4.38 ns BM_LatencyHash<RandStrings< true, 4>, CarbonHashBench> 1.15 ns BM_LatencyHash<RandStrings< true, 8>, CarbonHashBench> 0.973 ns BM_LatencyHash<RandStrings< true, 16>, CarbonHashBench> 3.03 ns BM_LatencyHash<RandStrings< true, 32>, CarbonHashBench> 3.97 ns BM_LatencyHash<RandStrings< true, 64>, CarbonHashBench> 6.64 ns BM_LatencyHash<RandStrings< true, 256>, CarbonHashBench> 12.5 ns BM_LatencyHash<RandStrings< true, 512>, CarbonHashBench> 17.9 ns BM_LatencyHash<RandStrings< true, 1024>, CarbonHashBench> 27.9 ns BM_LatencyHash<RandStrings< true, 2048>, CarbonHashBench> 48.1 ns BM_LatencyHash<RandStrings< true, 4096>, CarbonHashBench> 87.3 ns BM_LatencyHash<RandStrings< true, 8192>, CarbonHashBench> 166 ns ``` And here I have internally replaced the Carbon hash function with Abseil's hash function for "before" and then restored it in the "after" and computed the delta for each benchmark. This basically shows the speed-up (lower time -> lower latency -> speed-up -> good) over Abseil on an AMD server: ``` BM_LatencyHash<RandValues<uint8_t>, CarbonHashBench> 4.00ns ± 1% 3.10ns ± 0% -22.45% (p=0.000 n=20+15) BM_LatencyHash<RandValues<uint16_t>, CarbonHashBench> 4.01ns ± 1% 3.10ns ± 1% -22.64% (p=0.000 n=19+20) BM_LatencyHash<RandValues<std::pair<uint8_t, uint8_t>>, CarbonHashBench> 6.25ns ± 1% 4.10ns ± 1% -34.30% (p=0.000 n=20+20) BM_LatencyHash<RandValues<uint32_t>, CarbonHashBench> 4.02ns ± 1% 3.12ns ± 1% -22.50% (p=0.000 n=19+19) BM_LatencyHash<RandValues<std::pair<uint16_t, uint16_t>>, CarbonHashBench> 6.25ns ± 1% 4.11ns ± 1% -34.20% (p=0.000 n=20+19) BM_LatencyHash<RandValues<uint64_t>, CarbonHashBench> 4.03ns ± 1% 3.14ns ± 1% -22.17% (p=0.000 n=19+19) BM_LatencyHash<RandValues<int*>, CarbonHashBench> 5.95ns ± 1% 3.14ns ± 1% -47.24% (p=0.000 n=20+18) BM_LatencyHash<RandValues<std::pair<uint32_t, uint32_t>>, CarbonHashBench> 6.04ns ± 1% 4.01ns ± 1% -33.64% (p=0.000 n=20+20) BM_LatencyHash<RandValues<std::pair<uint64_t, uint32_t>>, CarbonHashBench> 5.96ns ± 1% 4.02ns ± 1% -32.51% (p=0.000 n=18+20) BM_LatencyHash<RandValues<std::pair<uint32_t, uint64_t>>, CarbonHashBench> 5.93ns ± 1% 4.30ns ± 1% -27.56% (p=0.000 n=20+17) BM_LatencyHash<RandValues<std::pair<int*, uint32_t>>, CarbonHashBench> 7.97ns ± 1% 4.02ns ± 1% -49.50% (p=0.000 n=20+20) BM_LatencyHash<RandValues<std::pair<uint32_t, int*>>, CarbonHashBench> 7.98ns ± 1% 4.32ns ± 1% -45.88% (p=0.000 n=19+20) BM_LatencyHash<RandValues<__uint128_t>, CarbonHashBench> 4.40ns ± 2% 4.32ns ± 1% -1.81% (p=0.000 n=20+20) BM_LatencyHash<RandValues<std::pair<uint64_t, uint64_t>>, CarbonHashBench> 5.94ns ± 1% 4.32ns ± 1% -27.25% (p=0.000 n=19+20) BM_LatencyHash<RandValues<std::pair<int*, int*>>, CarbonHashBench> 10.0ns ± 1% 4.3ns ± 1% -56.56% (p=0.000 n=20+20) BM_LatencyHash<RandValues<std::pair<uint64_t, int*>>, CarbonHashBench> 8.04ns ± 1% 4.32ns ± 1% -46.29% (p=0.000 n=20+19) BM_LatencyHash<RandValues<std::pair<int*, uint64_t>>, CarbonHashBench> 7.95ns ± 1% 4.33ns ± 1% -45.59% (p=0.000 n=19+20) BM_LatencyHash<RandStrings< true, 4>, CarbonHashBench> 3.28ns ± 3% 1.93ns ± 4% -41.19% (p=0.000 n=18+20) BM_LatencyHash<RandStrings< true, 8>, CarbonHashBench> 3.05ns ± 3% 1.69ns ± 4% -44.52% (p=0.000 n=19+20) BM_LatencyHash<RandStrings< true, 16>, CarbonHashBench> 5.88ns ± 2% 3.50ns ± 3% -40.42% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 32>, CarbonHashBench> 8.92ns ± 1% 4.44ns ± 2% -50.22% (p=0.000 n=19+20) BM_LatencyHash<RandStrings< true, 64>, CarbonHashBench> 12.0ns ± 1% 7.7ns ± 1% -36.16% (p=0.000 n=18+20) BM_LatencyHash<RandStrings< true, 256>, CarbonHashBench> 18.8ns ± 0% 14.7ns ± 1% -21.73% (p=0.000 n=17+20) BM_LatencyHash<RandStrings< true, 512>, CarbonHashBench> 25.5ns ± 1% 21.4ns ± 1% -16.18% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 1024>, CarbonHashBench> 38.7ns ± 2% 34.5ns ± 1% -10.78% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 2048>, CarbonHashBench> 69.7ns ± 1% 62.8ns ± 1% -9.88% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 4096>, CarbonHashBench> 130ns ± 1% 117ns ± 1% -9.45% (p=0.000 n=20+19) BM_LatencyHash<RandStrings< true, 8192>, CarbonHashBench> 244ns ± 0% 225ns ± 1% -8.11% (p=0.000 n=17+20) ``` ... and on an ARM server: ``` BM_LatencyHash<RandValues<uint8_t>, CarbonHashBench> 6.48ns ± 1% 5.28ns ± 0% -18.62% (p=0.000 n=20+20) BM_LatencyHash<RandValues<uint16_t>, CarbonHashBench> 7.40ns ± 1% 5.29ns ± 1% -28.45% (p=0.000 n=20+20) BM_LatencyHash<RandValues<std::pair<uint8_t, uint8_t>>, CarbonHashBench> 10.4ns ± 0% 7.0ns ± 0% -32.34% (p=0.000 n=19+20) BM_LatencyHash<RandValues<uint32_t>, CarbonHashBench> 6.56ns ± 1% 5.32ns ± 1% -18.95% (p=0.000 n=20+19) BM_LatencyHash<RandValues<std::pair<uint16_t, uint16_t>>, CarbonHashBench> 10.8ns ± 2% 7.0ns ± 1% -34.89% (p=0.000 n=20+20) BM_LatencyHash<RandValues<uint64_t>, CarbonHashBench> 6.71ns ± 3% 5.38ns ± 2% -19.84% (p=0.000 n=20+20) BM_LatencyHash<RandValues<int*>, CarbonHashBench> 10.3ns ± 3% 5.4ns ± 2% -47.67% (p=0.000 n=19+20) BM_LatencyHash<RandValues<std::pair<uint32_t, uint32_t>>, CarbonHashBench> 10.9ns ± 2% 7.2ns ± 4% -33.67% (p=0.000 n=19+20) BM_LatencyHash<RandValues<std::pair<uint64_t, uint32_t>>, CarbonHashBench> 10.7ns ± 4% 7.3ns ± 4% -31.66% (p=0.000 n=20+20) BM_LatencyHash<RandValues<std::pair<uint32_t, uint64_t>>, CarbonHashBench> 10.5ns ± 3% 7.3ns ± 4% -30.71% (p=0.000 n=20+19) BM_LatencyHash<RandValues<std::pair<int*, uint32_t>>, CarbonHashBench> 14.1ns ± 3% 7.3ns ± 4% -48.32% (p=0.000 n=19+20) BM_LatencyHash<RandValues<std::pair<uint32_t, int*>>, CarbonHashBench> 14.0ns ± 1% 7.3ns ± 4% -47.95% (p=0.000 n=19+19) BM_LatencyHash<RandValues<__uint128_t>, CarbonHashBench> 9.41ns ± 4% 8.68ns ± 4% -7.71% (p=0.000 n=19+19) BM_LatencyHash<RandValues<std::pair<uint64_t, uint64_t>>, CarbonHashBench> 12.2ns ± 2% 8.7ns ± 4% -28.81% (p=0.000 n=18+19) BM_LatencyHash<RandValues<std::pair<int*, int*>>, CarbonHashBench> 18.9ns ± 2% 8.7ns ± 4% -54.17% (p=0.000 n=17+19) BM_LatencyHash<RandValues<std::pair<uint64_t, int*>>, CarbonHashBench> 15.6ns ± 2% 8.7ns ± 4% -44.37% (p=0.000 n=17+19) BM_LatencyHash<RandValues<std::pair<int*, uint64_t>>, CarbonHashBench> 15.5ns ± 2% 8.7ns ± 4% -44.08% (p=0.000 n=18+19) BM_LatencyHash<RandStrings< true, 4>, CarbonHashBench> 5.89ns ± 2% 2.64ns ± 3% -55.26% (p=0.000 n=19+20) BM_LatencyHash<RandStrings< true, 8>, CarbonHashBench> 5.73ns ± 3% 2.88ns ± 3% -49.71% (p=0.000 n=18+20) BM_LatencyHash<RandStrings< true, 16>, CarbonHashBench> 10.1ns ± 1% 6.1ns ± 2% -39.00% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 32>, CarbonHashBench> 15.7ns ± 0% 8.3ns ± 1% -47.27% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 64>, CarbonHashBench> 21.2ns ± 0% 13.8ns ± 0% -34.81% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 256>, CarbonHashBench> 37.9ns ± 0% 31.2ns ± 0% -17.77% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 512>, CarbonHashBench> 56.8ns ± 0% 49.8ns ± 0% -12.21% (p=0.000 n=20+18) BM_LatencyHash<RandStrings< true, 1024>, CarbonHashBench> 93.8ns ± 0% 86.9ns ± 0% -7.38% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 2048>, CarbonHashBench> 174ns ± 0% 163ns ± 0% -6.03% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 4096>, CarbonHashBench> 330ns ± 0% 312ns ± 0% -5.25% (p=0.000 n=19+20) BM_LatencyHash<RandStrings< true, 8192>, CarbonHashBench> 641ns ± 0% 610ns ± 0% -4.79% (p=0.000 n=19+19) ``` This is the same as the above delta comparison, but with the "before" being LLVM's hash function: ``` BM_LatencyHash<RandValues<uint8_t>, CarbonHashBench> 6.85ns ± 1% 3.10ns ± 1% -54.78% (p=0.000 n=20+19) BM_LatencyHash<RandValues<uint16_t>, CarbonHashBench> 6.85ns ± 1% 3.10ns ± 1% -54.78% (p=0.000 n=20+19) BM_LatencyHash<RandValues<std::pair<uint8_t, uint8_t>>, CarbonHashBench> 6.25ns ± 1% 4.09ns ± 1% -34.58% (p=0.000 n=20+20) BM_LatencyHash<RandValues<uint32_t>, CarbonHashBench> 6.87ns ± 1% 3.12ns ± 2% -54.66% (p=0.000 n=20+20) BM_LatencyHash<RandValues<std::pair<uint16_t, uint16_t>>, CarbonHashBench> 7.35ns ± 1% 4.10ns ± 1% -44.20% (p=0.000 n=20+19) BM_LatencyHash<RandValues<uint64_t>, CarbonHashBench> 7.34ns ± 1% 3.13ns ± 1% -57.34% (p=0.000 n=20+18) BM_LatencyHash<RandValues<int*>, CarbonHashBench> 7.33ns ± 1% 3.13ns ± 2% -57.27% (p=0.000 n=20+18) BM_LatencyHash<RandValues<std::pair<uint32_t, uint32_t>>, CarbonHashBench> 7.27ns ± 1% 3.99ns ± 1% -45.12% (p=0.000 n=20+18) BM_LatencyHash<RandValues<std::pair<uint64_t, uint32_t>>, CarbonHashBench> 14.5ns ± 1% 4.0ns ± 1% -72.23% (p=0.000 n=19+19) BM_LatencyHash<RandValues<std::pair<uint32_t, uint64_t>>, CarbonHashBench> 14.6ns ± 1% 4.3ns ± 2% -70.44% (p=0.000 n=20+20) BM_LatencyHash<RandValues<std::pair<int*, uint32_t>>, CarbonHashBench> 14.5ns ± 1% 4.0ns ± 1% -72.21% (p=0.000 n=20+19) BM_LatencyHash<RandValues<std::pair<uint32_t, int*>>, CarbonHashBench> 14.6ns ± 1% 4.3ns ± 1% -70.46% (p=0.000 n=20+18) BM_LatencyHash<RandValues<__uint128_t>, CarbonHashBench> 7.31ns ± 1% 4.33ns ± 1% -40.81% (p=0.000 n=18+20) BM_LatencyHash<RandValues<std::pair<uint64_t, uint64_t>>, CarbonHashBench> 7.78ns ± 1% 4.32ns ± 1% -44.45% (p=0.000 n=18+20) BM_LatencyHash<RandValues<std::pair<int*, int*>>, CarbonHashBench> 7.78ns ± 2% 4.33ns ± 1% -44.42% (p=0.000 n=20+20) BM_LatencyHash<RandValues<std::pair<uint64_t, int*>>, CarbonHashBench> 7.62ns ± 1% 4.32ns ± 1% -43.24% (p=0.000 n=20+20) BM_LatencyHash<RandValues<std::pair<int*, uint64_t>>, CarbonHashBench> 7.77ns ± 1% 4.33ns ± 1% -44.34% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 4>, CarbonHashBench> 8.15ns ± 3% 1.94ns ± 5% -76.16% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 8>, CarbonHashBench> 7.02ns ± 3% 1.69ns ± 4% -75.94% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 16>, CarbonHashBench> 7.83ns ± 2% 3.50ns ± 3% -55.34% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 32>, CarbonHashBench> 9.17ns ± 1% 4.43ns ± 2% -51.65% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 64>, CarbonHashBench> 11.3ns ± 1% 7.6ns ± 1% -32.04% (p=0.000 n=20+19) BM_LatencyHash<RandStrings< true, 256>, CarbonHashBench> 23.0ns ± 1% 14.7ns ± 1% -36.14% (p=0.000 n=20+19) BM_LatencyHash<RandStrings< true, 512>, CarbonHashBench> 32.9ns ± 0% 21.4ns ± 1% -34.96% (p=0.000 n=17+19) BM_LatencyHash<RandStrings< true, 1024>, CarbonHashBench> 52.2ns ± 1% 34.4ns ± 1% -34.01% (p=0.000 n=19+18) BM_LatencyHash<RandStrings< true, 2048>, CarbonHashBench> 92.1ns ± 1% 62.8ns ± 1% -31.82% (p=0.000 n=19+19) BM_LatencyHash<RandStrings< true, 4096>, CarbonHashBench> 169ns ± 1% 117ns ± 1% -30.53% (p=0.000 n=20+19) BM_LatencyHash<RandStrings< true, 8192>, CarbonHashBench> 319ns ± 1% 224ns ± 1% -29.78% (p=0.000 n=20+18) ``` ... and on an ARM server: ``` BM_LatencyHash<RandValues<uint8_t>, CarbonHashBench> 8.38ns ± 0% 5.27ns ± 0% -37.04% (p=0.000 n=20+20) BM_LatencyHash<RandValues<uint16_t>, CarbonHashBench> 8.39ns ± 1% 5.28ns ± 0% -37.01% (p=0.000 n=19+19) BM_LatencyHash<RandValues<std::pair<uint8_t, uint8_t>>, CarbonHashBench> 8.07ns ± 0% 7.02ns ± 0% -13.10% (p=0.000 n=19+20) BM_LatencyHash<RandValues<uint32_t>, CarbonHashBench> 8.48ns ± 1% 5.32ns ± 1% -37.25% (p=0.000 n=20+20) BM_LatencyHash<RandValues<std::pair<uint16_t, uint16_t>>, CarbonHashBench> 9.34ns ± 2% 7.09ns ± 2% -24.14% (p=0.000 n=19+20) BM_LatencyHash<RandValues<uint64_t>, CarbonHashBench> 9.76ns ± 3% 5.37ns ± 2% -44.98% (p=0.000 n=20+20) BM_LatencyHash<RandValues<int*>, CarbonHashBench> 9.76ns ± 3% 5.37ns ± 2% -44.98% (p=0.000 n=20+20) BM_LatencyHash<RandValues<std::pair<uint32_t, uint32_t>>, CarbonHashBench> 10.1ns ± 2% 7.2ns ± 3% -29.36% (p=0.000 n=19+20) BM_LatencyHash<RandValues<std::pair<uint64_t, uint32_t>>, CarbonHashBench> 11.9ns ± 2% 7.3ns ± 4% -38.68% (p=0.000 n=19+20) BM_LatencyHash<RandValues<std::pair<uint32_t, uint64_t>>, CarbonHashBench> 11.3ns ± 2% 7.3ns ± 4% -35.16% (p=0.000 n=19+19) BM_LatencyHash<RandValues<std::pair<int*, uint32_t>>, CarbonHashBench> 11.9ns ± 2% 7.3ns ± 4% -38.68% (p=0.000 n=19+20) BM_LatencyHash<RandValues<std::pair<uint32_t, int*>>, CarbonHashBench> 11.3ns ± 2% 7.3ns ± 4% -35.16% (p=0.000 n=19+19) BM_LatencyHash<RandValues<__uint128_t>, CarbonHashBench> 10.3ns ± 2% 8.7ns ± 3% -15.81% (p=0.000 n=19+20) BM_LatencyHash<RandValues<std::pair<uint64_t, uint64_t>>, CarbonHashBench> 11.6ns ± 3% 8.7ns ± 3% -25.44% (p=0.000 n=20+20) BM_LatencyHash<RandValues<std::pair<int*, int*>>, CarbonHashBench> 11.6ns ± 3% 8.7ns ± 3% -25.44% (p=0.000 n=20+20) BM_LatencyHash<RandValues<std::pair<uint64_t, int*>>, CarbonHashBench> 11.6ns ± 3% 8.7ns ± 3% -25.44% (p=0.000 n=20+20) BM_LatencyHash<RandValues<std::pair<int*, uint64_t>>, CarbonHashBench> 11.6ns ± 3% 8.7ns ± 3% -25.44% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 4>, CarbonHashBench> 9.39ns ± 2% 2.66ns ± 3% -71.66% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 8>, CarbonHashBench> 10.7ns ± 3% 2.9ns ± 3% -72.97% (p=0.000 n=19+18) BM_LatencyHash<RandStrings< true, 16>, CarbonHashBench> 11.8ns ± 1% 6.1ns ± 2% -47.75% (p=0.000 n=19+20) BM_LatencyHash<RandStrings< true, 32>, CarbonHashBench> 13.9ns ± 1% 8.3ns ± 1% -40.71% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 64>, CarbonHashBench> 16.8ns ± 1% 13.8ns ± 0% -17.83% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 256>, CarbonHashBench> 31.7ns ± 1% 31.2ns ± 0% -1.76% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 512>, CarbonHashBench> 43.5ns ± 0% 49.8ns ± 0% +14.56% (p=0.000 n=18+20) BM_LatencyHash<RandStrings< true, 1024>, CarbonHashBench> 66.2ns ± 0% 86.9ns ± 0% +31.39% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 2048>, CarbonHashBench> 112ns ± 0% 163ns ± 0% +46.09% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 4096>, CarbonHashBench> 201ns ± 0% 312ns ± 0% +55.49% (p=0.000 n=20+20) BM_LatencyHash<RandStrings< true, 8192>, CarbonHashBench> 379ns ± 0% 610ns ± 0% +61.08% (p=0.000 n=20+20) ``` Note that there is a significant regression on long strings compared to LLVM's hash function on the ARM server I have access to. This doesn't show up on the M1 at all, and is likely specific to inadequate throughput for the 64-bit multiply operations. This seems fine as a) our priority is for short strings, and b) the M1 and other ARM CPUs are likely to improve here over time given the prevalent use of this core technique. For example, Abseil's current hash algorithm has the same long-string behavior (and performance bottleneck) on this server. --------- Co-authored-by: josh11b <josh11b@users.noreply.github.com> Co-authored-by: Geoff Romer <gromer@google.com> |