mapNextTx: use pointer as key, simplify value

Saves about 10% of application memory usage once the mempool warms up. Since the
mempool is DynamicUsage-regulated, this will translate to a larger mempool in
the same amount of space.

Map value type: eliminate the vin index; no users of the map need to know which
input of the transaction is spending the prevout.

Map key type: replace the COutPoint with a pointer to a COutPoint. A COutPoint
is 36 bytes, but each COutPoint is accessible from the same map entry's value.
A trivial DereferencingComparator functor allows indirect map keys, but the
resulting syntax is misleading: `map.find(&outpoint)`. Implement an indirectmap
that acts as a wrapper to a map that uses a DereferencingComparator, supporting
a syntax that accurately reflect the container's semantics: inserts and
iterators use pointers since they store pointers and need them to remain
constant and dereferenceable, but lookup functions take const references.

1 files changed, 22 insertions, 24 deletions

diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index 52c7793118..d7fb77661b 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -147,11 +147,11 @@ void CTxMemPool::UpdateTransactionsFromBlock(const std::vector<uint256> &vHashes
         if (it == mapTx.end()) {
             continue;
         }
-        std::map<COutPoint, CInPoint>::iterator iter = mapNextTx.lower_bound(COutPoint(hash, 0));
+        auto iter = mapNextTx.lower_bound(COutPoint(hash, 0));
         // First calculate the children, and update setMemPoolChildren to
         // include them, and update their setMemPoolParents to include this tx.
-        for (; iter != mapNextTx.end() && iter->first.hash == hash; ++iter) {
-            const uint256 &childHash = iter->second.ptx->GetHash();
+        for (; iter != mapNextTx.end() && iter->first->hash == hash; ++iter) {
+            const uint256 &childHash = iter->second->GetHash();
             txiter childIter = mapTx.find(childHash);
             assert(childIter != mapTx.end());
             // We can skip updating entries we've encountered before or that
@@ -365,11 +365,11 @@ void CTxMemPool::pruneSpent(const uint256 &hashTx, CCoins &coins)
 {
     LOCK(cs);
 
-    std::map<COutPoint, CInPoint>::iterator it = mapNextTx.lower_bound(COutPoint(hashTx, 0));
+    auto it = mapNextTx.lower_bound(COutPoint(hashTx, 0));
 
     // iterate over all COutPoints in mapNextTx whose hash equals the provided hashTx
-    while (it != mapNextTx.end() && it->first.hash == hashTx) {
-        coins.Spend(it->first.n); // and remove those outputs from coins
+    while (it != mapNextTx.end() && it->first->hash == hashTx) {
+        coins.Spend(it->first->n); // and remove those outputs from coins
         it++;
     }
 }
@@ -414,7 +414,7 @@ bool CTxMemPool::addUnchecked(const uint256& hash, const CTxMemPoolEntry &entry,
     const CTransaction& tx = newit->GetTx();
     std::set<uint256> setParentTransactions;
     for (unsigned int i = 0; i < tx.vin.size(); i++) {
-        mapNextTx[tx.vin[i].prevout] = CInPoint(&tx, i);
+        mapNextTx.insert(std::make_pair(&tx.vin[i].prevout, &tx));
         setParentTransactions.insert(tx.vin[i].prevout.hash);
     }
     // Don't bother worrying about child transactions of this one.
@@ -500,10 +500,10 @@ void CTxMemPool::removeRecursive(const CTransaction &origTx, std::list<CTransact
             // happen during chain re-orgs if origTx isn't re-accepted into
             // the mempool for any reason.
             for (unsigned int i = 0; i < origTx.vout.size(); i++) {
-                std::map<COutPoint, CInPoint>::iterator it = mapNextTx.find(COutPoint(origTx.GetHash(), i));
+                auto it = mapNextTx.find(COutPoint(origTx.GetHash(), i));
                 if (it == mapNextTx.end())
                     continue;
-                txiter nextit = mapTx.find(it->second.ptx->GetHash());
+                txiter nextit = mapTx.find(it->second->GetHash());
                 assert(nextit != mapTx.end());
                 txToRemove.insert(nextit);
             }
@@ -561,9 +561,9 @@ void CTxMemPool::removeConflicts(const CTransaction &tx, std::list<CTransaction>
     list<CTransaction> result;
     LOCK(cs);
     BOOST_FOREACH(const CTxIn &txin, tx.vin) {
-        std::map<COutPoint, CInPoint>::iterator it = mapNextTx.find(txin.prevout);
+        auto it = mapNextTx.find(txin.prevout);
         if (it != mapNextTx.end()) {
-            const CTransaction &txConflict = *it->second.ptx;
+            const CTransaction &txConflict = *it->second;
             if (txConflict != tx)
             {
                 removeRecursive(txConflict, removed);
@@ -671,10 +671,10 @@ void CTxMemPool::check(const CCoinsViewCache *pcoins) const
                 assert(coins && coins->IsAvailable(txin.prevout.n));
             }
             // Check whether its inputs are marked in mapNextTx.
-            std::map<COutPoint, CInPoint>::const_iterator it3 = mapNextTx.find(txin.prevout);
+            auto it3 = mapNextTx.find(txin.prevout);
             assert(it3 != mapNextTx.end());
-            assert(it3->second.ptx == &tx);
-            assert(it3->second.n == i);
+            assert(it3->first == &txin.prevout);
+            assert(it3->second == &tx);
             i++;
         }
         assert(setParentCheck == GetMemPoolParents(it));
@@ -701,10 +701,10 @@ void CTxMemPool::check(const CCoinsViewCache *pcoins) const
 
         // Check children against mapNextTx
         CTxMemPool::setEntries setChildrenCheck;
-        std::map<COutPoint, CInPoint>::const_iterator iter = mapNextTx.lower_bound(COutPoint(it->GetTx().GetHash(), 0));
+        auto iter = mapNextTx.lower_bound(COutPoint(it->GetTx().GetHash(), 0));
         int64_t childSizes = 0;
-        for (; iter != mapNextTx.end() && iter->first.hash == it->GetTx().GetHash(); ++iter) {
-            txiter childit = mapTx.find(iter->second.ptx->GetHash());
+        for (; iter != mapNextTx.end() && iter->first->hash == it->GetTx().GetHash(); ++iter) {
+            txiter childit = mapTx.find(iter->second->GetHash());
             assert(childit != mapTx.end()); // mapNextTx points to in-mempool transactions
             if (setChildrenCheck.insert(childit).second) {
                 childSizes += childit->GetTxSize();
@@ -738,14 +738,12 @@ void CTxMemPool::check(const CCoinsViewCache *pcoins) const
             stepsSinceLastRemove = 0;
         }
     }
-    for (std::map<COutPoint, CInPoint>::const_iterator it = mapNextTx.begin(); it != mapNextTx.end(); it++) {
-        uint256 hash = it->second.ptx->GetHash();
+    for (auto it = mapNextTx.cbegin(); it != mapNextTx.cend(); it++) {
+        uint256 hash = it->second->GetHash();
         indexed_transaction_set::const_iterator it2 = mapTx.find(hash);
         const CTransaction& tx = it2->GetTx();
         assert(it2 != mapTx.end());
-        assert(&tx == it->second.ptx);
-        assert(tx.vin.size() > it->second.n);
-        assert(it->first == it->second.ptx->vin[it->second.n].prevout);
+        assert(&tx == it->second);
     }
 
     assert(totalTxSize == checkTotal);
@@ -1044,8 +1042,8 @@ void CTxMemPool::TrimToSize(size_t sizelimit, std::vector<uint256>* pvNoSpendsRe
                 BOOST_FOREACH(const CTxIn& txin, tx.vin) {
                     if (exists(txin.prevout.hash))
                         continue;
-                    std::map<COutPoint, CInPoint>::iterator it = mapNextTx.lower_bound(COutPoint(txin.prevout.hash, 0));
-                    if (it == mapNextTx.end() || it->first.hash != txin.prevout.hash)
+                    auto it = mapNextTx.lower_bound(COutPoint(txin.prevout.hash, 0));
+                    if (it == mapNextTx.end() || it->first->hash != txin.prevout.hash)
                         pvNoSpendsRemaining->push_back(txin.prevout.hash);
                 }
             }