consensus/ethash: less allocation during mining (ethereum#23199)

Author: fxfactorial

consensus/ethash/sealer.go

@@ -140,8 +140,9 @@ func (ethash *Ethash) mine(block *types.Block, id int, seed uint64, abort chan s
 	)
 	// Start generating random nonces until we abort or find a good one
 	var (
-		attempts = int64(0)
-		nonce    = seed
+		attempts  = int64(0)
+		nonce     = seed
+		powBuffer = new(big.Int)
 	)
 	logger := ethash.config.Log.New("miner", id)
 	logger.Trace("Started ethash search for new nonces", "seed", seed)
@@ -163,7 +164,7 @@ search:
 			}
 			// Compute the PoW value of this nonce
 			digest, result := hashimotoFull(dataset.dataset, hash, nonce)
-			if new(big.Int).SetBytes(result).Cmp(target) <= 0 {
+			if powBuffer.SetBytes(result).Cmp(target) <= 0 {
 				// Correct nonce found, create a new header with it
 				header = types.CopyHeader(header)
 				header.Nonce = types.EncodeNonce(nonce)