common/math: optimize PaddedBigBytes, use it more

name              old time/op    new time/op    delta
PaddedBigBytes-8    71.1ns ± 5%    46.1ns ± 1%  -35.15%  (p=0.000 n=20+19)

name              old alloc/op   new alloc/op   delta
PaddedBigBytes-8     48.0B ± 0%     32.0B ± 0%  -33.33%  (p=0.000 n=20+20)

accounts/abi/numbers.go

@@ -59,7 +59,7 @@ var (
 
 // U256 converts a big Int into a 256bit EVM number.
 func U256(n *big.Int) []byte {
-	return common.LeftPadBytes(math.U256(n).Bytes(), 32)
+	return math.PaddedBigBytes(math.U256(n), 32)
 }
 
 // packNum packs the given number (using the reflect value) and will cast it to appropriate number representation

accounts/abi/packing.go

@@ -20,6 +20,7 @@ import (
 	"reflect"
 
 	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/math"
 )
 
 // packBytesSlice packs the given bytes as [L, V] as the canonical representation
@@ -45,9 +46,9 @@ func packElement(t Type, reflectValue reflect.Value) []byte {
 		return common.LeftPadBytes(reflectValue.Bytes(), 32)
 	case BoolTy:
 		if reflectValue.Bool() {
-			return common.LeftPadBytes(common.Big1.Bytes(), 32)
+			return math.PaddedBigBytes(common.Big1, 32)
 		} else {
-			return common.LeftPadBytes(common.Big0.Bytes(), 32)
+			return math.PaddedBigBytes(common.Big0, 32)
 		}
 	case BytesTy:
 		if reflectValue.Kind() == reflect.Array {

accounts/keystore/keystore_passphrase.go

@@ -36,6 +36,7 @@ import (
 	"path/filepath"
 
 	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/math"
 	"github.com/ethereum/go-ethereum/crypto"
 	"github.com/ethereum/go-ethereum/crypto/randentropy"
 	"github.com/pborman/uuid"
@@ -115,8 +116,7 @@ func EncryptKey(key *Key, auth string, scryptN, scryptP int) ([]byte, error) {
 		return nil, err
 	}
 	encryptKey := derivedKey[:16]
-	keyBytes0 := crypto.FromECDSA(key.PrivateKey)
-	keyBytes := common.LeftPadBytes(keyBytes0, 32)
+	keyBytes := math.PaddedBigBytes(key.PrivateKey.D, 32)
 
 	iv := randentropy.GetEntropyCSPRNG(aes.BlockSize) // 16
 	cipherText, err := aesCTRXOR(encryptKey, keyBytes, iv)

common/math/big.go

@@ -28,6 +28,13 @@ var (
 	MaxBig256 = new(big.Int).Set(tt256m1)
 )
 
+const (
+	// number of bits in a big.Word
+	wordBits = 32 << (uint64(^big.Word(0)) >> 63)
+	// number of bytes in a big.Word
+	wordBytes = wordBits / 8
+)
+
 // ParseBig256 parses s as a 256 bit integer in decimal or hexadecimal syntax.
 // Leading zeros are accepted. The empty string parses as zero.
 func ParseBig256(s string) (*big.Int, bool) {
@@ -91,12 +98,24 @@ func FirstBitSet(v *big.Int) int {
 // PaddedBigBytes encodes a big integer as a big-endian byte slice. The length
 // of the slice is at least n bytes.
 func PaddedBigBytes(bigint *big.Int, n int) []byte {
-	bytes := bigint.Bytes()
-	if len(bytes) >= n {
-		return bytes
+	if bigint.BitLen()/8 >= n {
+		return bigint.Bytes()
 	}
 	ret := make([]byte, n)
-	return append(ret[:len(ret)-len(bytes)], bytes...)
+	readBits(ret, bigint.Bits())
+	return ret
+}
+
+// reads little-endian words as big-endian bytes into buf.
+func readBits(buf []byte, words []big.Word) {
+	i := len(buf)
+	for _, d := range words {
+		for j := 0; j < wordBytes && i > 0; j++ {
+			i--
+			buf[i] = byte(d)
+			d >>= 8
+		}
+	}
 }
 
 // U256 encodes as a 256 bit two's complement number. This operation is destructive.
@@ -119,9 +138,6 @@ func S256(x *big.Int) *big.Int {
 	}
 }
 
-// wordSize is the size number of bits in a big.Word.
-const wordSize = 32 << (uint64(^big.Word(0)) >> 63)
-
 // Exp implements exponentiation by squaring.
 // Exp returns a newly-allocated big integer and does not change
 // base or exponent. The result is truncated to 256 bits.
@@ -131,7 +147,7 @@ func Exp(base, exponent *big.Int) *big.Int {
 	result := big.NewInt(1)
 
 	for _, word := range exponent.Bits() {
-		for i := 0; i < wordSize; i++ {
+		for i := 0; i < wordBits; i++ {
 			if word&1 == 1 {
 				U256(result.Mul(result, base))
 			}

common/math/big_test.go

@@ -131,6 +131,13 @@ func TestPaddedBigBytes(t *testing.T) {
 	}
 }
 
+func BenchmarkPaddedBigBytes(b *testing.B) {
+	bigint := MustParseBig256("123456789123456789123456789123456789")
+	for i := 0; i < b.N; i++ {
+		PaddedBigBytes(bigint, 32)
+	}
+}
+
 func TestU256(t *testing.T) {
 	tests := []struct{ x, y *big.Int }{
 		{x: big.NewInt(0), y: big.NewInt(0)},

core/vm/instructions.go

@@ -252,10 +252,11 @@ func opXor(pc *uint64, evm *EVM, contract *Contract, memory *Memory, stack *Stac
 	evm.interpreter.intPool.put(y)
 	return nil, nil
 }
+
 func opByte(pc *uint64, evm *EVM, contract *Contract, memory *Memory, stack *Stack) ([]byte, error) {
 	th, val := stack.pop(), stack.pop()
 	if th.Cmp(big.NewInt(32)) < 0 {
-		byte := evm.interpreter.intPool.get().SetInt64(int64(common.LeftPadBytes(val.Bytes(), 32)[th.Int64()]))
+		byte := evm.interpreter.intPool.get().SetInt64(int64(math.PaddedBigBytes(val, 32)[th.Int64()]))
 		stack.push(byte)
 	} else {
 		stack.push(new(big.Int))

core/vm/logger.go

@@ -23,6 +23,7 @@ import (
 	"unicode"
 
 	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/math"
 )
 
 type Storage map[common.Hash]common.Hash
@@ -180,7 +181,7 @@ func StdErrFormat(logs []StructLog) {
 		fmt.Fprintln(os.Stderr, "STACK =", len(log.Stack))
 
 		for i := len(log.Stack) - 1; i >= 0; i-- {
-			fmt.Fprintf(os.Stderr, "%04d: %x\n", len(log.Stack)-i-1, common.LeftPadBytes(log.Stack[i].Bytes(), 32))
+			fmt.Fprintf(os.Stderr, "%04d: %x\n", len(log.Stack)-i-1, math.PaddedBigBytes(log.Stack[i], 32))
 		}
 
 		const maxMem = 10

crypto/signature_cgo.go

@@ -23,7 +23,7 @@ import (
 	"crypto/elliptic"
 	"fmt"
 
-	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/math"
 	"github.com/ethereum/go-ethereum/crypto/secp256k1"
 )
 
@@ -53,7 +53,7 @@ func Sign(hash []byte, prv *ecdsa.PrivateKey) (sig []byte, err error) {
 	if len(hash) != 32 {
 		return nil, fmt.Errorf("hash is required to be exactly 32 bytes (%d)", len(hash))
 	}
-	seckey := common.LeftPadBytes(prv.D.Bytes(), prv.Params().BitSize/8)
+	seckey := math.PaddedBigBytes(prv.D, prv.Params().BitSize/8)
 	defer zeroBytes(seckey)
 	return secp256k1.Sign(hash, seckey)
 }

internal/ethapi/api.go

@@ -720,7 +720,7 @@ func FormatLogs(structLogs []vm.StructLog) []StructLogRes {
 		}
 
 		for i, stackValue := range trace.Stack {
-			formattedStructLogs[index].Stack[i] = fmt.Sprintf("%x", common.LeftPadBytes(stackValue.Bytes(), 32))
+			formattedStructLogs[index].Stack[i] = fmt.Sprintf("%x", math.PaddedBigBytes(stackValue, 32))
 		}
 
 		for i := 0; i+32 <= len(trace.Memory); i += 32 {

tests/util.go

@@ -71,7 +71,7 @@ func checkLogs(tlog []Log, logs []*types.Log) error {
 					}
 				}
 			}
-			genBloom := common.LeftPadBytes(types.LogsBloom([]*types.Log{logs[i]}).Bytes(), 256)
+			genBloom := math.PaddedBigBytes(types.LogsBloom([]*types.Log{logs[i]}), 256)
 
 			if !bytes.Equal(genBloom, common.Hex2Bytes(log.BloomF)) {
 				return fmt.Errorf("bloom mismatch")