Use Boost's hash_combine for 32 and 64-bit values

Issue #902

include/flexflow/utils/hash_utils.h

@@ -1,17 +1,117 @@
 #ifndef _FLEXFLOW_HASH_UTILS_H
 #define _FLEXFLOW_HASH_UTILS_H
 
+#include <cstdint>
+#include <climits>
 #include <functional>
 #include <tuple>
 #include <type_traits>
 #include <vector>
 
-// tuple hashing pulled from
-// https://www.variadic.xyz/2018/01/15/hashing-stdpair-and-stdtuple/
+// Copied directly from
+// https://github.com/boostorg/container_hash/blob/master/include/boost/container_hash/detail/hash_mix.hpp
+
+//
+// boost::hash_combine
+//
+namespace hash_detail {
+
+template <std::size_t Bits>
+struct hash_mix_impl;
+
+// hash_mix for 64 bit size_t
+//
+// The general "xmxmx" form of state of the art 64 bit mixers originates
+// from Murmur3 by Austin Appleby, which uses the following function as
+// its "final mix":
+//
+//	k ^= k >> 33;
+//	k *= 0xff51afd7ed558ccd;
+//	k ^= k >> 33;
+//	k *= 0xc4ceb9fe1a85ec53;
+//	k ^= k >> 33;
+//
+// (https://github.com/aappleby/smhasher/blob/master/src/MurmurHash3.cpp)
+//
+// It has subsequently been improved multiple times by different authors
+// by changing the constants. The most well known improvement is the
+// so-called "variant 13" function by David Stafford:
+//
+//	k ^= k >> 30;
+//	k *= 0xbf58476d1ce4e5b9;
+//	k ^= k >> 27;
+//	k *= 0x94d049bb133111eb;
+//	k ^= k >> 31;
+//
+// (https://zimbry.blogspot.com/2011/09/better-bit-mixing-improving-on.html)
+//
+// This mixing function is used in the splitmix64 RNG:
+// http://xorshift.di.unimi.it/splitmix64.c
+//
+// We use Jon Maiga's implementation from
+// http://jonkagstrom.com/mx3/mx3_rev2.html
+//
+// 	x ^= x >> 32;
+//	x *= 0xe9846af9b1a615d;
+//	x ^= x >> 32;
+//	x *= 0xe9846af9b1a615d;
+//	x ^= x >> 28;
+//
+// An equally good alternative is Pelle Evensen's Moremur:
+//
+//	x ^= x >> 27;
+//	x *= 0x3C79AC492BA7B653;
+//	x ^= x >> 33;
+//	x *= 0x1C69B3F74AC4AE35;
+//	x ^= x >> 27;
+//
+// (https://mostlymangling.blogspot.com/2019/12/stronger-better-morer-moremur-better.html)
+
+template <>
+struct hash_mix_impl<64> {
+  inline static std::uint64_t fn(std::uint64_t x) {
+    std::uint64_t const m = (std::uint64_t(0xe9846af) << 32) + 0x9b1a615d;
+
+    x ^= x >> 32;
+    x *= m;
+    x ^= x >> 32;
+    x *= m;
+    x ^= x >> 28;
+
+    return x;
+  }
+};
+
+// hash_mix for 32 bit size_t
+//
+// We use the "best xmxmx" implementation from
+// https://github.com/skeeto/hash-prospector/issues/19
+
+template <>
+struct hash_mix_impl<32> {
+  inline static std::uint32_t fn(std::uint32_t x) {
+    std::uint32_t const m1 = 0x21f0aaad;
+    std::uint32_t const m2 = 0x735a2d97;
+
+    x ^= x >> 16;
+    x *= m1;
+    x ^= x >> 15;
+    x *= m2;
+    x ^= x >> 15;
+
+    return x;
+  }
+};
+
+inline std::size_t hash_mix(std::size_t v) {
+  return hash_mix_impl<sizeof(std::size_t) * CHAR_BIT>::fn(v);
+}
+
+} // namespace hash_detail
+
 template <class T>
 inline void hash_combine(std::size_t &seed, T const &v) {
-  std::hash<T> hasher;
-  seed ^= hasher(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
+  seed = hash_detail::hash_mix(seed + 0x9e3779b9 + std::hash<T>()(v));
 }
 
 namespace std {