Added some comments

Some clarifications after a code review by Mike Hearn.

src/main.cpp

@@ -698,6 +698,8 @@ bool CTxMemPool::accept(CTransaction &tx, bool fCheckInputs,
             return false;
 
         // do all inputs exist?
+        // Note that this does not check for the presence of actual outputs (see the next check for that),
+        // only helps filling in pfMissingInputs (to determine missing vs spent).
         BOOST_FOREACH(const CTxIn txin, tx.vin) {
             if (!view.HaveCoins(txin.prevout.hash)) {
                 if (pfMissingInputs)
@@ -706,6 +708,7 @@ bool CTxMemPool::accept(CTransaction &tx, bool fCheckInputs,
             }
         }
 
+        // are the actual inputs available?
         if (!tx.HaveInputs(view))
             return error("CTxMemPool::accept() : inputs already spent");
 
@@ -1956,9 +1959,13 @@ bool CBlock::CheckBlock(bool fCheckPOW, bool fCheckMerkleRoot) const
         if (!tx.CheckTransaction())
             return DoS(tx.nDoS, error("CheckBlock() : CheckTransaction failed"));
 
+    // Build the merkle tree already. We need it anyway later, and it makes the
+    // block cache the transaction hashes, which means they don't need to be
+    // recalculated many times during this block's validation.
+    BuildMerkleTree();
+
     // Check for duplicate txids. This is caught by ConnectInputs(),
     // but catching it earlier avoids a potential DoS attack:
-    BuildMerkleTree();
     set<uint256> uniqueTx;
     for (unsigned int i=0; i<vtx.size(); i++) {
         uniqueTx.insert(GetTxHash(i));