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114 lines
4.3 KiB
114 lines
4.3 KiB
// Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
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// This source code is licensed under both the GPLv2 (found in the
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// COPYING file in the root directory) and Apache 2.0 License
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// (found in the LICENSE.Apache file in the root directory).
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// fastrange/FastRange: A faster alternative to % for mapping a hash value
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// to an arbitrary range. See https://github.com/lemire/fastrange
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//
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// Generally recommended are FastRange32 for mapping results of 32-bit
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// hash functions and FastRange64 for mapping results of 64-bit hash
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// functions. FastRange is less forgiving than % if the input hashes are
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// not well distributed over the full range of the type (32 or 64 bits).
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//
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// Also included is a templated implementation FastRangeGeneric for use
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// in generic algorithms, but not otherwise recommended because of
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// potential ambiguity. Unlike with %, it is critical to use the right
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// FastRange variant for the output size of your hash function.
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#pragma once
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#include <cstddef>
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#include <cstdint>
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#include <type_traits>
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#include "rocksdb/rocksdb_namespace.h"
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#ifdef TEST_UINT128_COMPAT
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#undef HAVE_UINT128_EXTENSION
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#endif
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namespace ROCKSDB_NAMESPACE {
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namespace detail {
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// Using a class template to support partial specialization
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template <typename Hash, typename Range>
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struct FastRangeGenericImpl {
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// only reach this on no supported specialization
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};
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template <typename Range>
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struct FastRangeGenericImpl<uint32_t, Range> {
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static inline Range Fn(uint32_t hash, Range range) {
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static_assert(std::is_unsigned<Range>::value, "must be unsigned");
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static_assert(sizeof(Range) <= sizeof(uint32_t),
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"cannot be larger than hash (32 bits)");
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uint64_t product = uint64_t{range} * hash;
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return static_cast<Range>(product >> 32);
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}
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};
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template <typename Range>
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struct FastRangeGenericImpl<uint64_t, Range> {
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static inline Range Fn(uint64_t hash, Range range) {
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static_assert(std::is_unsigned<Range>::value, "must be unsigned");
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static_assert(sizeof(Range) <= sizeof(uint64_t),
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"cannot be larger than hash (64 bits)");
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#ifdef HAVE_UINT128_EXTENSION
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// Can use compiler's 128-bit type. Trust it to do the right thing.
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__uint128_t wide = __uint128_t{range} * hash;
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return static_cast<Range>(wide >> 64);
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#else
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// Fall back: full decomposition.
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// NOTE: GCC seems to fully understand this code as 64-bit x 64-bit
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// -> 128-bit multiplication and optimize it appropriately
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uint64_t range64 = range; // ok to shift by 32, even if Range is 32-bit
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uint64_t tmp = uint64_t{range64 & 0xffffFFFF} * uint64_t{hash & 0xffffFFFF};
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tmp >>= 32;
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tmp += uint64_t{range64 & 0xffffFFFF} * uint64_t{hash >> 32};
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// Avoid overflow: first add lower 32 of tmp2, and later upper 32
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uint64_t tmp2 = uint64_t{range64 >> 32} * uint64_t{hash & 0xffffFFFF};
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tmp += static_cast<uint32_t>(tmp2);
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tmp >>= 32;
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tmp += (tmp2 >> 32);
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tmp += uint64_t{range64 >> 32} * uint64_t{hash >> 32};
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return static_cast<Range>(tmp);
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#endif
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}
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};
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} // namespace detail
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// Now an omnibus templated function (yay parameter inference).
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//
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// NOTICE:
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// This templated version is not recommended for typical use because
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// of the potential to mix a 64-bit FastRange with a 32-bit bit hash,
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// most likely because you put your 32-bit hash in an "unsigned long"
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// which is 64 bits on some platforms. That doesn't really matter for
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// an operation like %, but 64-bit FastRange gives extremely bad results,
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// mostly zero, on 32-bit hash values. And because good hashing is not
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// generally required for correctness, this kind of mistake could go
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// unnoticed with just unit tests. Plus it could vary by platform.
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template <typename Hash, typename Range>
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inline Range FastRangeGeneric(Hash hash, Range range) {
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return detail::FastRangeGenericImpl<Hash, Range>::Fn(hash, range);
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}
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// The most popular / convenient / recommended variants:
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// Map a quality 64-bit hash value down to an arbitrary size_t range.
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// (size_t is standard for mapping to things in memory.)
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inline size_t FastRange64(uint64_t hash, size_t range) {
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return FastRangeGeneric(hash, range);
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}
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// Map a quality 32-bit hash value down to an arbitrary uint32_t range.
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inline uint32_t FastRange32(uint32_t hash, uint32_t range) {
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return FastRangeGeneric(hash, range);
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}
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} // namespace ROCKSDB_NAMESPACE
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