diff --git a/benchmark/uint64.cpp b/benchmark/uint64.cpp
new file mode 100644
index 00000000..b040c62e
--- /dev/null
+++ b/benchmark/uint64.cpp
@@ -0,0 +1,233 @@
+// Copyright 2021 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#define _SILENCE_CXX17_OLD_ALLOCATOR_MEMBERS_DEPRECATION_WARNING
+
+#include <boost/unordered_map.hpp>
+#include <boost/multi_index_container.hpp>
+#include <boost/multi_index/hashed_index.hpp>
+#include <boost/multi_index/member.hpp>
+#include <boost/core/detail/splitmix64.hpp>
+#ifdef HAVE_ABSEIL
+# include "absl/container/node_hash_map.h"
+# include "absl/container/flat_hash_map.h"
+#endif
+#include <unordered_map>
+#include <map>
+#include <cstdint>
+#include <iostream>
+#include <chrono>
+
+using namespace std::chrono_literals;
+
+static void print_time( std::chrono::steady_clock::time_point & t1, char const* label, std::uint64_t s, std::size_t size )
+{
+    auto t2 = std::chrono::steady_clock::now();
+
+    std::cout << label << ": " << ( t2 - t1 ) / 1ms << " ms (s=" << s << ", size=" << size << ")\n";
+
+    t1 = t2;
+}
+
+constexpr unsigned N = 2'000'000;
+constexpr int K = 10;
+
+template<class Map> void test_insert( Map& map, std::chrono::steady_clock::time_point & t1 )
+{
+    for( unsigned i = 1; i <= N; ++i )
+    {
+        map.insert( { i, i } );
+    }
+
+    print_time( t1, "Consecutive insert",  0, map.size() );
+
+    {
+        boost::detail::splitmix64 rng;
+
+        for( unsigned i = 1; i <= N; ++i )
+        {
+            map.insert( { rng(), i } );
+        }
+    }
+
+    print_time( t1, "Random insert",  0, map.size() );
+
+    for( unsigned i = 1; i <= N; ++i )
+    {
+        map.insert( { (std::uint64_t)i << 32, i } );
+    }
+
+    print_time( t1, "Consecutive shifted insert",  0, map.size() );
+
+    std::cout << std::endl;
+}
+
+template<class Map> void test_lookup( Map& map, std::chrono::steady_clock::time_point & t1 )
+{
+    std::uint64_t s;
+    
+    s = 0;
+
+    for( int j = 0; j < K; ++j )
+    {
+        for( unsigned i = 1; i <= N * 2; ++i )
+        {
+            auto it = map.find( i );
+            if( it != map.end() ) s += it->second;
+        }
+    }
+
+    print_time( t1, "Consecutive lookup",  s, map.size() );
+
+    s = 0;
+
+    for( int j = 0; j < K; ++j )
+    {
+        boost::detail::splitmix64 rng;
+
+        for( unsigned i = 1; i <= N * 2; ++i )
+        {
+            auto it = map.find( rng() );
+            if( it != map.end() ) s += it->second;
+        }
+    }
+
+    print_time( t1, "Random lookup",  s, map.size() );
+
+    s = 0;
+
+    for( int j = 0; j < K; ++j )
+    {
+        for( unsigned i = 1; i <= N * 2; ++i )
+        {
+            auto it = map.find( (std::uint64_t)i << 32 );
+            if( it != map.end() ) s += it->second;
+        }
+    }
+
+    print_time( t1, "Consecutive shifted lookup",  s, map.size() );
+
+    std::cout << std::endl;
+}
+
+template<class Map> void test_iteration( Map& map, std::chrono::steady_clock::time_point & t1 )
+{
+    auto it = map.begin();
+
+    while( it != map.end() )
+    {
+        if( it->second & 1 )
+        {
+            map.erase( it++ );
+        }
+        else
+        {
+            ++it;
+        }
+    }
+
+    print_time( t1, "Iterate and erase odd elements",  0, map.size() );
+
+    std::cout << std::endl;
+}
+
+template<class Map> void test_erase( Map& map, std::chrono::steady_clock::time_point & t1 )
+{
+    for( unsigned i = 1; i <= N; ++i )
+    {
+        map.erase( i );
+    }
+
+    print_time( t1, "Consecutive erase",  0, map.size() );
+
+    {
+        boost::detail::splitmix64 rng;
+
+        for( unsigned i = 1; i <= N; ++i )
+        {
+            map.erase( rng() );
+        }
+    }
+
+    print_time( t1, "Random erase",  0, map.size() );
+
+    for( unsigned i = 1; i <= N; ++i )
+    {
+        map.erase( (std::uint64_t)i << 32 );
+    }
+
+    print_time( t1, "Consecutive shifted erase",  0, map.size() );
+
+    std::cout << std::endl;
+}
+
+std::vector< std::pair<std::string, long long> > times;
+
+template<template<class...> class Map> void test( char const* label )
+{
+    std::cout << label << ":\n\n";
+
+    Map<std::uint64_t, std::uint64_t> map;
+
+    auto t0 = std::chrono::steady_clock::now();
+    auto t1 = t0;
+
+    test_insert( map, t1 );
+    test_lookup( map, t1 );
+    test_iteration( map, t1 );
+    test_lookup( map, t1 );
+    test_erase( map, t1 );
+
+    auto tN = std::chrono::steady_clock::now();
+    std::cout << "Total: " << ( tN - t0 ) / 1ms << " ms\n\n";
+
+    times.push_back( { label, ( tN - t0 ) / 1ms } );
+}
+
+// multi_index emulation of unordered_map
+
+template<class K, class V> struct pair
+{
+    K first;
+    mutable V second;
+};
+
+using namespace boost::multi_index;
+
+template<class K, class V> using multi_index_map = multi_index_container<
+  pair<K, V>,
+  indexed_by<
+    hashed_unique< member<pair<K, V>, K, &pair<K, V>::first> >
+  >
+>;
+
+int main()
+{
+    test<std::unordered_map>( "std::unordered_map" );
+    test<boost::unordered_map>( "boost::unordered_map" );
+    test<multi_index_map>( "multi_index_map" );
+
+    // test<std::map>( "std::map" );
+
+#ifdef HAVE_ABSEIL
+
+    test<absl::node_hash_map>( "absl::node_hash_map" );
+    test<absl::flat_hash_map>( "absl::flat_hash_map" );
+
+#endif
+
+    std::cout << "---\n\n";
+
+    for( auto const& x: times )
+    {
+        std::cout << x.first << ": " << x.second << " ms\n";
+    }
+}
+
+#ifdef HAVE_ABSEIL
+# include "absl/container/internal/raw_hash_set.cc"
+# include "absl/hash/internal/hash.cc"
+# include "absl/hash/internal/low_level_hash.cc"
+# include "absl/hash/internal/city.cc"
+#endif