bigint: don't allocate in ToPreallocatedBytes

Turns out, it's almost always allocating because we're mostly dealing with
small integers while the buffer size is calculated in 8-byte chunks here, so
preallocated buffer is always insufficient.

name                   old time/op    new time/op    delta
ToPreallocatedBytes-8    28.5ns ± 7%    19.7ns ± 5%   -30.72%  (p=0.000 n=10+10)

name                   old alloc/op   new alloc/op   delta
ToPreallocatedBytes-8     16.0B ± 0%      0.0B       -100.00%  (p=0.000 n=10+10)

name                   old allocs/op  new allocs/op  delta
ToPreallocatedBytes-8      1.00 ± 0%      0.00       -100.00%  (p=0.000 n=10+10)

Fix StorageItem reuse at the same time. We don't copy when getting values from
the storage, but we don when we're putting them, so buffer reuse could corrupt
old values.

pkg/core/dao/dao.go

@@ -7,12 +7,14 @@ import (
 	"errors"
 	"fmt"
 	iocore "io"
+	"math/big"
 	"sync"
 
 	"github.com/nspcc-dev/neo-go/pkg/core/block"
 	"github.com/nspcc-dev/neo-go/pkg/core/state"
 	"github.com/nspcc-dev/neo-go/pkg/core/storage"
 	"github.com/nspcc-dev/neo-go/pkg/core/transaction"
+	"github.com/nspcc-dev/neo-go/pkg/encoding/bigint"
 	"github.com/nspcc-dev/neo-go/pkg/io"
 	"github.com/nspcc-dev/neo-go/pkg/smartcontract/trigger"
 	"github.com/nspcc-dev/neo-go/pkg/util"
@@ -387,6 +389,14 @@ func (dao *Simple) PutStorageItem(id int32, key []byte, si state.StorageItem) {
 	dao.Store.Put(stKey, si)
 }
 
+// PutBigInt serializaed and puts the given integer for the given id with the given
+// key into the given store.
+func (dao *Simple) PutBigInt(id int32, key []byte, n *big.Int) {
+	var buf [bigint.MaxBytesLen]byte
+	stData := bigint.ToPreallocatedBytes(n, buf[:])
+	dao.PutStorageItem(id, key, stData)
+}
+
 // DeleteStorageItem drops a storage item for the given id with the
 // given key from the store.
 func (dao *Simple) DeleteStorageItem(id int32, key []byte) {

pkg/core/native/management.go

@@ -601,8 +601,7 @@ func (m *Management) getNextContractID(d *dao.Simple) (int32, error) {
 	id := bigint.FromBytes(si)
 	ret := int32(id.Int64())
 	id.Add(id, intOne)
-	si = bigint.ToPreallocatedBytes(id, si)
-	d.PutStorageItem(m.ID, keyNextAvailableID, si)
+	d.PutBigInt(m.ID, keyNextAvailableID, id)
 	return ret, nil
 }
 

pkg/core/native/native_neo.go

@@ -440,7 +440,7 @@ func (n *NEO) PostPersist(ic *interop.Context) error {
 				}
 				cache.gasPerVoteCache[cs[i].Key] = *tmp
 
-				ic.DAO.PutStorageItem(n.ID, key, bigint.ToBytes(tmp))
+				ic.DAO.PutBigInt(n.ID, key, tmp)
 			}
 		}
 	}
@@ -1088,8 +1088,7 @@ func (n *NEO) modifyVoterTurnout(d *dao.Simple, amount *big.Int) error {
 	}
 	votersCount := bigint.FromBytes(si)
 	votersCount.Add(votersCount, amount)
-	si = bigint.ToPreallocatedBytes(votersCount, si)
-	d.PutStorageItem(n.ID, key, si)
+	d.PutBigInt(n.ID, key, votersCount)
 	return nil
 }
 
@@ -1218,5 +1217,5 @@ func (n *NEO) putGASRecord(dao *dao.Simple, index uint32, value *big.Int) {
 	key := make([]byte, 5)
 	key[0] = prefixGASPerBlock
 	binary.BigEndian.PutUint32(key[1:], index)
-	dao.PutStorageItem(n.ID, key, bigint.ToBytes(value))
+	dao.PutBigInt(n.ID, key, value)
 }

pkg/core/native/native_nep17.go

@@ -108,8 +108,7 @@ func (c *nep17TokenNative) getTotalSupply(d *dao.Simple) (state.StorageItem, *bi
 }
 
 func (c *nep17TokenNative) saveTotalSupply(d *dao.Simple, si state.StorageItem, supply *big.Int) {
-	si = state.StorageItem(bigint.ToPreallocatedBytes(supply, si))
-	d.PutStorageItem(c.ID, totalSupplyKey, si)
+	d.PutBigInt(c.ID, totalSupplyKey, supply)
 }
 
 func (c *nep17TokenNative) Transfer(ic *interop.Context, args []stackitem.Item) stackitem.Item {

pkg/core/native/oracle.go

@@ -355,8 +355,7 @@ func (o *Oracle) RequestInternal(ic *interop.Context, url string, filter *string
 	itemID := bigint.FromBytes(si)
 	id := itemID.Uint64()
 	itemID.Add(itemID, intOne)
-	si = bigint.ToPreallocatedBytes(itemID, si)
-	ic.DAO.PutStorageItem(o.ID, prefixRequestID, si)
+	ic.DAO.PutBigInt(o.ID, prefixRequestID, itemID)
 
 	// Should be executed from the contract.
 	_, err := ic.GetContract(ic.VM.GetCallingScriptHash())

pkg/core/native/util.go

@@ -37,7 +37,7 @@ func putConvertibleToDAO(id int32, d *dao.Simple, key []byte, conv stackitem.Con
 }
 
 func setIntWithKey(id int32, dao *dao.Simple, key []byte, value int64) {
-	dao.PutStorageItem(id, key, bigint.ToBytes(big.NewInt(value)))
+	dao.PutBigInt(id, key, big.NewInt(value))
 }
 
 func getIntWithKey(id int32, dao *dao.Simple, key []byte) int64 {

pkg/encoding/bigint/bench_test.go

@@ -0,0 +1,15 @@
+package bigint
+
+import (
+	"math/big"
+	"testing"
+)
+
+func BenchmarkToPreallocatedBytes(b *testing.B) {
+	v := big.NewInt(100500)
+	buf := make([]byte, 4)
+
+	for i := 0; i < b.N; i++ {
+		_ = ToPreallocatedBytes(v, buf[:0])
+	}
+}

pkg/encoding/bigint/bigint.go

@@ -1,7 +1,6 @@
 package bigint
 
 import (
-	"encoding/binary"
 	"math/big"
 	"math/bits"
 
@@ -109,36 +108,26 @@ func ToBytes(n *big.Int) []byte {
 func ToPreallocatedBytes(n *big.Int, data []byte) []byte {
 	sign := n.Sign()
 	if sign == 0 {
-		return data
+		return data[:0]
 	}
 
 	if sign < 0 {
 		n.Add(n, bigOne)
 		defer func() { n.Sub(n, bigOne) }()
 		if n.Sign() == 0 { // n == -1
-			return append(data, 0xFF)
+			return append(data[:0], 0xFF)
 		}
 	}
 
-	var ws = n.Bits()
+	lb := n.BitLen()/8 + 1
 
-	lb := len(ws) * wordSizeBytes
 	if c := cap(data); c < lb {
-		data = make([]byte, lb, lb+1)
+		data = make([]byte, lb)
 	} else {
 		data = data[:lb]
 	}
-	data = wordsToBytes(ws, data)
-
-	size := len(data)
-	for ; data[size-1] == 0; size-- {
-	}
-
-	data = data[:size]
-
-	if data[size-1]&0x80 != 0 {
-		data = append(data, 0)
-	}
+	_ = n.FillBytes(data)
+	slice.Reverse(data)
 
 	if sign == -1 {
 		for i := range data {
@@ -148,17 +137,3 @@ func ToPreallocatedBytes(n *big.Int, data []byte) []byte {
 
 	return data
 }
-
-func wordsToBytes(ws []big.Word, bs []byte) []byte {
-	if wordSizeBytes == 8 {
-		for i := range ws {
-			binary.LittleEndian.PutUint64(bs[i*wordSizeBytes:], uint64(ws[i]))
-		}
-	} else {
-		for i := range ws {
-			binary.LittleEndian.PutUint32(bs[i*wordSizeBytes:], uint32(ws[i]))
-		}
-	}
-
-	return bs
-}