block_connected: don't serialize block hash twice

In the validation:block_connected tracepoint, we call block->GetHash(),
which ends up calling CBlockHeader::GetHash(), executing around 8000
serialization instructions. We don't need to do this extra work, because
block->GetHash() is already called further up in the function. Let's
save that value as a local variable and re-use it in our tracepoint so
there is no unnecessary tracepoint overhead.

Signed-off-by: William Casarin <jb55@jb55.com>

src/validation.cpp

@@ -1866,7 +1866,10 @@ bool CChainState::ConnectBlock(const CBlock& block, BlockValidationState& state,
 {
     AssertLockHeld(cs_main);
     assert(pindex);
-    assert(*pindex->phashBlock == block.GetHash());
+
+    uint256 block_hash{block.GetHash()};
+    assert(*pindex->phashBlock == block_hash);
+
     int64_t nTimeStart = GetTimeMicros();
 
     // Check it again in case a previous version let a bad block in
@@ -1900,7 +1903,7 @@ bool CChainState::ConnectBlock(const CBlock& block, BlockValidationState& state,
 
     // Special case for the genesis block, skipping connection of its transactions
     // (its coinbase is unspendable)
-    if (block.GetHash() == m_params.GetConsensus().hashGenesisBlock) {
+    if (block_hash == m_params.GetConsensus().hashGenesisBlock) {
         if (!fJustCheck)
             view.SetBestBlock(pindex->GetBlockHash());
         return true;
@@ -2158,7 +2161,7 @@ bool CChainState::ConnectBlock(const CBlock& block, BlockValidationState& state,
     LogPrint(BCLog::BENCH, "    - Index writing: %.2fms [%.2fs (%.2fms/blk)]\n", MILLI * (nTime5 - nTime4), nTimeIndex * MICRO, nTimeIndex * MILLI / nBlocksTotal);
 
     TRACE6(validation, block_connected,
-        block.GetHash().data(),
+        block_hash.data(),
         pindex->nHeight,
         block.vtx.size(),
         nInputs,