rocksdb/util/math.h
Peter Dillinger c4d8838a2b New bit manipulation functions and 128-bit value library (#7338)
Summary:
These new functions and 128-bit value bit operations are
expected to be used in a forthcoming Bloom filter alternative.

No functional changes to production code, just new code only called by
unit tests, cosmetic changes to existing headers, and fix an existing
function for a yet-unused template instantiation (BitsSetToOne on
something signed and smaller than 32 bits).

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7338

Test Plan:
Unit tests included. Works with and without
TEST_UINT128_COMPAT=1 to check compatibility with and without
__uint128_t. Also added that parameter to the CircleCI build
build-linux-shared_lib-alt_namespace-status_checked.

Reviewed By: jay-zhuang

Differential Revision: D23494945

Pulled By: pdillinger

fbshipit-source-id: 5c0dc419100d9df5d4d9abb153b2855d5aea39e8
2020-09-03 09:32:59 -07:00

127 lines
4.6 KiB
C++

// Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
#pragma once
#include <assert.h>
#include <stdint.h>
#ifdef _MSC_VER
#include <intrin.h>
#endif
namespace ROCKSDB_NAMESPACE {
// Fast implementation of floor(log2(v)). Undefined for 0 or negative
// numbers (in case of signed type).
template <typename T>
inline int FloorLog2(T v) {
static_assert(std::is_integral<T>::value, "non-integral type");
assert(v > 0);
#ifdef _MSC_VER
static_assert(sizeof(T) <= sizeof(uint64_t), "type too big");
unsigned long lz = 0;
if (sizeof(T) <= sizeof(uint32_t)) {
_BitScanReverse(&lz, static_cast<uint32_t>(v));
} else {
_BitScanReverse64(&lz, static_cast<uint64_t>(v));
}
return 63 - static_cast<int>(lz);
#else
static_assert(sizeof(T) <= sizeof(unsigned long long), "type too big");
if (sizeof(T) <= sizeof(unsigned int)) {
int lz = __builtin_clz(static_cast<unsigned int>(v));
return int{sizeof(unsigned int)} * 8 - 1 - lz;
} else if (sizeof(T) <= sizeof(unsigned long)) {
int lz = __builtin_clzl(static_cast<unsigned long>(v));
return int{sizeof(unsigned long)} * 8 - 1 - lz;
} else {
int lz = __builtin_clzll(static_cast<unsigned long long>(v));
return int{sizeof(unsigned long long)} * 8 - 1 - lz;
}
#endif
}
// Number of low-order zero bits before the first 1 bit. Undefined for 0.
template <typename T>
inline int CountTrailingZeroBits(T v) {
static_assert(std::is_integral<T>::value, "non-integral type");
assert(v != 0);
#ifdef _MSC_VER
static_assert(sizeof(T) <= sizeof(uint64_t), "type too big");
unsigned long tz = 0;
if (sizeof(T) <= sizeof(uint32_t)) {
_BitScanForward(&tz, static_cast<uint32_t>(v));
} else {
_BitScanForward64(&tz, static_cast<uint64_t>(v));
}
return static_cast<int>(tz);
#else
static_assert(sizeof(T) <= sizeof(unsigned long long), "type too big");
if (sizeof(T) <= sizeof(unsigned int)) {
return __builtin_ctz(static_cast<unsigned int>(v));
} else if (sizeof(T) <= sizeof(unsigned long)) {
return __builtin_ctzl(static_cast<unsigned long>(v));
} else {
return __builtin_ctzll(static_cast<unsigned long long>(v));
}
#endif
}
// Number of bits set to 1. Also known as "population count".
template <typename T>
inline int BitsSetToOne(T v) {
static_assert(std::is_integral<T>::value, "non-integral type");
#ifdef _MSC_VER
static_assert(sizeof(T) <= sizeof(uint64_t), "type too big");
if (sizeof(T) < sizeof(uint32_t)) {
// This bit mask is to avoid a compiler warning on unused path
constexpr auto mm = 8 * sizeof(uint32_t) - 1;
// The bit mask is to neutralize sign extension on small signed types
constexpr uint32_t m = (uint32_t{1} << ((8 * sizeof(T)) & mm)) - 1;
return static_cast<int>(__popcnt(static_cast<uint32_t>(v) & m));
} else if (sizeof(T) == sizeof(uint32_t)) {
return static_cast<int>(__popcnt(static_cast<uint32_t>(v)));
} else {
return static_cast<int>(__popcnt64(static_cast<uint64_t>(v)));
}
#else
static_assert(sizeof(T) <= sizeof(unsigned long long), "type too big");
if (sizeof(T) < sizeof(unsigned int)) {
// This bit mask is to avoid a compiler warning on unused path
constexpr auto mm = 8 * sizeof(unsigned int) - 1;
// This bit mask is to neutralize sign extension on small signed types
constexpr unsigned int m = (1U << ((8 * sizeof(T)) & mm)) - 1;
return __builtin_popcount(static_cast<unsigned int>(v) & m);
} else if (sizeof(T) == sizeof(unsigned int)) {
return __builtin_popcount(static_cast<unsigned int>(v));
} else if (sizeof(T) <= sizeof(unsigned long)) {
return __builtin_popcountl(static_cast<unsigned long>(v));
} else {
return __builtin_popcountll(static_cast<unsigned long long>(v));
}
#endif
}
template <typename T>
inline int BitParity(T v) {
static_assert(std::is_integral<T>::value, "non-integral type");
#ifdef _MSC_VER
// bit parity == oddness of popcount
return BitsSetToOne(v) & 1;
#else
static_assert(sizeof(T) <= sizeof(unsigned long long), "type too big");
if (sizeof(T) <= sizeof(unsigned int)) {
// On any sane systen, potential sign extension here won't change parity
return __builtin_parity(static_cast<unsigned int>(v));
} else if (sizeof(T) <= sizeof(unsigned long)) {
return __builtin_parityl(static_cast<unsigned long>(v));
} else {
return __builtin_parityll(static_cast<unsigned long long>(v));
}
#endif
}
} // namespace ROCKSDB_NAMESPACE