Add pool based memory resource & allocator

A memory resource similar to std::pmr::unsynchronized_pool_resource, but
optimized for node-based containers.

Co-Authored-By: Pieter Wuille <pieter@wuille.net>

src/bench/pool.cpp

@@ -0,0 +1,50 @@
+// Copyright (c) 2022 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <bench/bench.h>
+#include <support/allocators/pool.h>
+
+#include <unordered_map>
+
+template <typename Map>
+void BenchFillClearMap(benchmark::Bench& bench, Map& map)
+{
+    size_t batch_size = 5000;
+
+    // make sure each iteration of the benchmark contains exactly 5000 inserts and one clear.
+    // do this at least 10 times so we get reasonable accurate results
+
+    bench.batch(batch_size).minEpochIterations(10).run([&] {
+        auto rng = ankerl::nanobench::Rng(1234);
+        for (size_t i = 0; i < batch_size; ++i) {
+            map[rng()];
+        }
+        map.clear();
+    });
+}
+
+static void PoolAllocator_StdUnorderedMap(benchmark::Bench& bench)
+{
+    auto map = std::unordered_map<uint64_t, uint64_t>();
+    BenchFillClearMap(bench, map);
+}
+
+static void PoolAllocator_StdUnorderedMapWithPoolResource(benchmark::Bench& bench)
+{
+    using Map = std::unordered_map<uint64_t,
+                                   uint64_t,
+                                   std::hash<uint64_t>,
+                                   std::equal_to<uint64_t>,
+                                   PoolAllocator<std::pair<const uint64_t, uint64_t>,
+                                                 sizeof(std::pair<const uint64_t, uint64_t>) + 4 * sizeof(void*),
+                                                 alignof(void*)>>;
+
+    // make sure the resource supports large enough pools to hold the node. We do this by adding the size of a few pointers to it.
+    auto pool_resource = Map::allocator_type::ResourceType();
+    auto map = Map{0, std::hash<uint64_t>{}, std::equal_to<uint64_t>{}, &pool_resource};
+    BenchFillClearMap(bench, map);
+}
+
+BENCHMARK(PoolAllocator_StdUnorderedMap, benchmark::PriorityLevel::HIGH);
+BENCHMARK(PoolAllocator_StdUnorderedMapWithPoolResource, benchmark::PriorityLevel::HIGH);