diff options
| -rw-r--r-- | src/common/bloom.cpp | 13 | ||||
| -rw-r--r-- | src/cuckoocache.h | 27 | ||||
| -rw-r--r-- | src/util/fastrange.h | 20 |
3 files changed, 30 insertions, 30 deletions
diff --git a/src/common/bloom.cpp b/src/common/bloom.cpp index c3603b5d2a..0bb72dbcbb 100644 --- a/src/common/bloom.cpp +++ b/src/common/bloom.cpp @@ -11,6 +11,7 @@ #include <script/standard.h> #include <span.h> #include <streams.h> +#include <util/fastrange.h> #include <algorithm> #include <cmath> @@ -191,14 +192,6 @@ static inline uint32_t RollingBloomHash(unsigned int nHashNum, uint32_t nTweak, return MurmurHash3(nHashNum * 0xFBA4C795 + nTweak, vDataToHash); } - -// A replacement for x % n. This assumes that x and n are 32bit integers, and x is a uniformly random distributed 32bit value -// which should be the case for a good hash. -// See https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/ -static inline uint32_t FastMod(uint32_t x, size_t n) { - return ((uint64_t)x * (uint64_t)n) >> 32; -} - void CRollingBloomFilter::insert(Span<const unsigned char> vKey) { if (nEntriesThisGeneration == nEntriesPerGeneration) { @@ -223,7 +216,7 @@ void CRollingBloomFilter::insert(Span<const unsigned char> vKey) uint32_t h = RollingBloomHash(n, nTweak, vKey); int bit = h & 0x3F; /* FastMod works with the upper bits of h, so it is safe to ignore that the lower bits of h are already used for bit. */ - uint32_t pos = FastMod(h, data.size()); + uint32_t pos = FastRange32(h, data.size()); /* The lowest bit of pos is ignored, and set to zero for the first bit, and to one for the second. */ data[pos & ~1] = (data[pos & ~1] & ~(((uint64_t)1) << bit)) | ((uint64_t)(nGeneration & 1)) << bit; data[pos | 1] = (data[pos | 1] & ~(((uint64_t)1) << bit)) | ((uint64_t)(nGeneration >> 1)) << bit; @@ -235,7 +228,7 @@ bool CRollingBloomFilter::contains(Span<const unsigned char> vKey) const for (int n = 0; n < nHashFuncs; n++) { uint32_t h = RollingBloomHash(n, nTweak, vKey); int bit = h & 0x3F; - uint32_t pos = FastMod(h, data.size()); + uint32_t pos = FastRange32(h, data.size()); /* If the relevant bit is not set in either data[pos & ~1] or data[pos | 1], the filter does not contain vKey */ if (!(((data[pos & ~1] | data[pos | 1]) >> bit) & 1)) { return false; diff --git a/src/cuckoocache.h b/src/cuckoocache.h index 15cb55c3ce..d0dc61c7e6 100644 --- a/src/cuckoocache.h +++ b/src/cuckoocache.h @@ -5,6 +5,8 @@ #ifndef BITCOIN_CUCKOOCACHE_H #define BITCOIN_CUCKOOCACHE_H +#include <util/fastrange.h> + #include <algorithm> // std::find #include <array> #include <atomic> @@ -219,13 +221,8 @@ private: * One option would be to implement the same trick the compiler uses and compute the * constants for exact division based on the size, as described in "{N}-bit Unsigned * Division via {N}-bit Multiply-Add" by Arch D. Robison in 2005. But that code is - * somewhat complicated and the result is still slower than other options: - * - * Instead we treat the 32-bit random number as a Q32 fixed-point number in the range - * [0, 1) and simply multiply it by the size. Then we just shift the result down by - * 32-bits to get our bucket number. The result has non-uniformity the same as a - * mod, but it is much faster to compute. More about this technique can be found at - * https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/ . + * somewhat complicated and the result is still slower than an even simpler option: + * see the FastRange32 function in util/fastrange.h. * * The resulting non-uniformity is also more equally distributed which would be * advantageous for something like linear probing, though it shouldn't matter @@ -241,14 +238,14 @@ private: */ inline std::array<uint32_t, 8> compute_hashes(const Element& e) const { - return {{(uint32_t)(((uint64_t)hash_function.template operator()<0>(e) * (uint64_t)size) >> 32), - (uint32_t)(((uint64_t)hash_function.template operator()<1>(e) * (uint64_t)size) >> 32), - (uint32_t)(((uint64_t)hash_function.template operator()<2>(e) * (uint64_t)size) >> 32), - (uint32_t)(((uint64_t)hash_function.template operator()<3>(e) * (uint64_t)size) >> 32), - (uint32_t)(((uint64_t)hash_function.template operator()<4>(e) * (uint64_t)size) >> 32), - (uint32_t)(((uint64_t)hash_function.template operator()<5>(e) * (uint64_t)size) >> 32), - (uint32_t)(((uint64_t)hash_function.template operator()<6>(e) * (uint64_t)size) >> 32), - (uint32_t)(((uint64_t)hash_function.template operator()<7>(e) * (uint64_t)size) >> 32)}}; + return {{FastRange32(hash_function.template operator()<0>(e), size), + FastRange32(hash_function.template operator()<1>(e), size), + FastRange32(hash_function.template operator()<2>(e), size), + FastRange32(hash_function.template operator()<3>(e), size), + FastRange32(hash_function.template operator()<4>(e), size), + FastRange32(hash_function.template operator()<5>(e), size), + FastRange32(hash_function.template operator()<6>(e), size), + FastRange32(hash_function.template operator()<7>(e), size)}}; } /** invalid returns a special index that can never be inserted to diff --git a/src/util/fastrange.h b/src/util/fastrange.h index 963d21c03a..77cb883ce0 100644 --- a/src/util/fastrange.h +++ b/src/util/fastrange.h @@ -7,11 +7,21 @@ #include <cstdint> -// Map a value x that is uniformly distributed in the range [0, 2^64) to a -// value uniformly distributed in [0, n) by returning the upper 64 bits of -// x * n. -// -// See: https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/ +/* This file offers implementations of the fast range reduction technique described + * in https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/ + * + * In short, they take an integer x and a range n, and return the upper bits of + * (x * n). If x is uniformly distributed over its domain, the result is as close to + * uniformly distributed over [0, n) as (x mod n) would be, but significantly faster. + */ + +/** Fast range reduction with 32-bit input and 32-bit range. */ +static inline uint32_t FastRange32(uint32_t x, uint32_t n) +{ + return (uint64_t{x} * n) >> 32; +} + +/** Fast range reduction with 64-bit input and 64-bit range. */ static inline uint64_t FastRange64(uint64_t x, uint64_t n) { #ifdef __SIZEOF_INT128__ |