Add microbenchmarks to profile more code paths.

The new benchmarks exercise script validation, CCoinsDBView caching,
mempool eviction, and wallet coin selection code.

All of the benchmarks added here are extremely simple and don't
necessarily mirror common real world conditions or interesting
performance edge cases. Details about how specific benchmarks can be
improved are noted in comments.

Github-Issue: #7883

src/bench/coin_selection.cpp

@@ -0,0 +1,62 @@
+// Copyright (c) 2012-2015 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.h"
+#include "wallet/wallet.h"
+
+#include <boost/foreach.hpp>
+#include <set>
+
+using namespace std;
+
+static void addCoin(const CAmount& nValue, const CWallet& wallet, vector<COutput>& vCoins)
+{
+    int nInput = 0;
+
+    static int nextLockTime = 0;
+    CMutableTransaction tx;
+    tx.nLockTime = nextLockTime++; // so all transactions get different hashes
+    tx.vout.resize(nInput + 1);
+    tx.vout[nInput].nValue = nValue;
+    CWalletTx* wtx = new CWalletTx(&wallet, tx);
+
+    int nAge = 6 * 24;
+    COutput output(wtx, nInput, nAge, true, true);
+    vCoins.push_back(output);
+}
+
+// Simple benchmark for wallet coin selection. Note that it maybe be necessary
+// to build up more complicated scenarios in order to get meaningful
+// measurements of performance. From laanwj, "Wallet coin selection is probably
+// the hardest, as you need a wider selection of scenarios, just testing the
+// same one over and over isn't too useful. Generating random isn't useful
+// either for measurements."
+// (https://github.com/bitcoin/bitcoin/issues/7883#issuecomment-224807484)
+static void CoinSelection(benchmark::State& state)
+{
+    const CWallet wallet;
+    vector<COutput> vCoins;
+    LOCK(wallet.cs_wallet);
+
+    while (state.KeepRunning()) {
+        // Empty wallet.
+        BOOST_FOREACH (COutput output, vCoins)
+            delete output.tx;
+        vCoins.clear();
+
+        // Add coins.
+        for (int i = 0; i < 1000; i++)
+            addCoin(1000 * COIN, wallet, vCoins);
+        addCoin(3 * COIN, wallet, vCoins);
+
+        set<pair<const CWalletTx*, unsigned int> > setCoinsRet;
+        CAmount nValueRet;
+        bool success = wallet.SelectCoinsMinConf(1003 * COIN, 1, 6, vCoins, setCoinsRet, nValueRet);
+        assert(success);
+        assert(nValueRet == 1003 * COIN);
+        assert(setCoinsRet.size() == 2);
+    }
+}
+
+BENCHMARK(CoinSelection);