fix review comments

** nil check refine
** create a separate routine for From/To prefetch, avoid blocking the cirtical path

core/state/statedb.go

@@ -223,6 +223,9 @@ func (s *StateDB) StopPrefetcher() {
 }
 
 func (s *StateDB) TriePrefetchInAdvance(block *types.Block, signer types.Signer) {
+	if s.prefetcher == nil {
+		return
+	}
 	accounts := make(map[common.Address]struct{}, block.Transactions().Len()<<1)
 	for _, tx := range block.Transactions() {
 		from, err := types.Sender(signer, tx)
@@ -240,7 +243,7 @@ func (s *StateDB) TriePrefetchInAdvance(block *types.Block, signer types.Signer)
 		addressesToPrefetch = append(addressesToPrefetch, common.CopyBytes(addr[:])) // Copy needed for closure
 	}
 
-	if s.prefetcher != nil && len(addressesToPrefetch) > 0 {
+	if len(addressesToPrefetch) > 0 {
 		s.prefetcher.prefetch(s.originalRoot, addressesToPrefetch, emptyAddr)
 	}
 }

core/state_processor.go

@@ -406,7 +406,10 @@ func (p *StateProcessor) Process(block *types.Block, statedb *state.StateDB, cfg
 	statedb.MarkFullProcessed()
 
 	// do trie prefetch for the big state trie tree in advance based transaction's From/To address.
-	statedb.TriePrefetchInAdvance(block, signer)
+	go func() {
+		// trie prefetcher is thread safe now, ok now to prefetch in a separate routine
+		statedb.TriePrefetchInAdvance(block, signer)
+	}()
 
 	// usually do have two tx, one for validator set contract, another for system reward contract.
 	systemTxs := make([]*types.Transaction, 0, 2)