From 735fd68dacdf25e51691d2e804d5e9db643e7833 Mon Sep 17 00:00:00 2001 From: Lars Knoll Date: Sun, 9 Feb 2020 15:11:25 +0100 Subject: [PATCH] Implement better hash functions for integer types MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The hash function provides rather good mixing of the input bits. It spreads numbers out evenly through the uint range, a change of one bit in the input changes around half the output bits, and it is pretty fast. Using this as a hash function over the simple hash(int) == int has the advantage that it reduces the amount of collisions for badly distributed keys. In addition, it allows us to always use power of two sizes for the hash table, leading to better performance for inserts and lookups. the 32 and 64 bit hash functions where chosen from https://nullprogram.com/blog/2018/07/31/. I selected the ones that give a very good distribution of the hash values while using the integer for both multiplication steps. This should be slighty faster than using two different numbers. While the result is still being cast to a uint, the method is prepared so it can handle 64 bit keys and seeds. Fixes: QTBUG-29009 Change-Id: Id7a1b97b3c0d219e65de2e6e1fe6faf092f8ce16 Reviewed-by: Fabian Kosmale Reviewed-by: Mårten Nordheim Reviewed-by: Thiago Macieira --- src/corelib/tools/qhashfunctions.h | 58 ++++++++++++++++++++++-------- 1 file changed, 44 insertions(+), 14 deletions(-) diff --git a/src/corelib/tools/qhashfunctions.h b/src/corelib/tools/qhashfunctions.h index 2ff3464912..2e62e93ac4 100644 --- a/src/corelib/tools/qhashfunctions.h +++ b/src/corelib/tools/qhashfunctions.h @@ -68,25 +68,55 @@ class QLatin1String; Q_CORE_EXPORT int qGlobalQHashSeed(); Q_CORE_EXPORT void qSetGlobalQHashSeed(int newSeed); +namespace QHashPrivate { + +Q_DECL_CONST_FUNCTION constexpr uint hash(size_t key, uint seed) noexcept +{ + key ^= seed; + if constexpr (sizeof(size_t) == 4) { + key ^= key >> 16; + key *= UINT32_C(0x45d9f3b); + key ^= key >> 16; + key *= UINT32_C(0x45d9f3b); + key ^= key >> 16; + return key; + } else { + key ^= key >> 32; + key *= UINT64_C(0xd6e8feb86659fd93); + key ^= key >> 32; + key *= UINT64_C(0xd6e8feb86659fd93); + key ^= key >> 32; + return uint(key); + } +} + +} + Q_CORE_EXPORT Q_DECL_PURE_FUNCTION uint qHashBits(const void *p, size_t size, uint seed = 0) noexcept; -Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(char key, uint seed = 0) noexcept { return uint(key) ^ seed; } -Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(uchar key, uint seed = 0) noexcept { return uint(key) ^ seed; } -Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(signed char key, uint seed = 0) noexcept { return uint(key) ^ seed; } -Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(ushort key, uint seed = 0) noexcept { return uint(key) ^ seed; } -Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(short key, uint seed = 0) noexcept { return uint(key) ^ seed; } -Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(uint key, uint seed = 0) noexcept { return key ^ seed; } -Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(int key, uint seed = 0) noexcept { return uint(key) ^ seed; } +Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(char key, uint seed = 0) noexcept +{ return QHashPrivate::hash(size_t(key), seed); } +Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(uchar key, uint seed = 0) noexcept +{ return QHashPrivate::hash(size_t(key), seed); } +Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(signed char key, uint seed = 0) noexcept +{ return QHashPrivate::hash(size_t(key), seed); } +Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(ushort key, uint seed = 0) noexcept +{ return QHashPrivate::hash(size_t(key), seed); } +Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(short key, uint seed = 0) noexcept +{ return QHashPrivate::hash(size_t(key), seed); } +Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(uint key, uint seed = 0) noexcept +{ return QHashPrivate::hash(size_t(key), seed); } +Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(int key, uint seed = 0) noexcept +{ return QHashPrivate::hash(size_t(key), seed); } Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(ulong key, uint seed = 0) noexcept -{ - return (sizeof(ulong) > sizeof(uint)) - ? (uint(((key >> (8 * sizeof(uint) - 1)) ^ key) & (~0U)) ^ seed) - : (uint(key & (~0U)) ^ seed); -} -Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(long key, uint seed = 0) noexcept { return qHash(ulong(key), seed); } +{ return QHashPrivate::hash(size_t(key), seed); } +Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(long key, uint seed = 0) noexcept +{ return QHashPrivate::hash(size_t(key), seed); } Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(quint64 key, uint seed = 0) noexcept { - return uint(((key >> (8 * sizeof(uint) - 1)) ^ key) & (~0U)) ^ seed; + if (sizeof(quint64) > sizeof(size_t)) + key ^= (key >> 32); + return QHashPrivate::hash(size_t(key), seed); } Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(qint64 key, uint seed = 0) noexcept { return qHash(quint64(key), seed); } Q_CORE_EXPORT Q_DECL_CONST_FUNCTION uint qHash(float key, uint seed = 0) noexcept;