all: unify big.Int zero checks, use common/math in more places (#3716)

* 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)

* all: unify big.Int zero checks

Various checks were in use. This commit replaces them all with Int.Sign,
which is cheaper and less code.

eg templates:

    func before(x *big.Int) bool { return x.BitLen() == 0 }
    func after(x *big.Int) bool  { return x.Sign() == 0 }

    func before(x *big.Int) bool { return x.BitLen() > 0 }
    func after(x *big.Int) bool  { return x.Sign() != 0 }

    func before(x *big.Int) int { return x.Cmp(common.Big0) }
    func after(x *big.Int) int  { return x.Sign() }

* common/math, crypto/secp256k1: make ReadBits public in package math

5 files changed, 8 insertions, 7 deletions

diff --git a/accounts/abi/bind/backends/simulated.go b/accounts/abi/bind/backends/simulated.go
index 5e2fcbae7..e21ffe1f8 100644
--- a/accounts/abi/bind/backends/simulated.go
+++ b/accounts/abi/bind/backends/simulated.go
@@ -237,7 +237,7 @@ func (b *SimulatedBackend) callContract(ctx context.Context, call ethereum.CallM
 	if call.GasPrice == nil {
 		call.GasPrice = big.NewInt(1)
 	}
-	if call.Gas == nil || call.Gas.BitLen() == 0 {
+	if call.Gas == nil || call.Gas.Sign() == 0 {
 		call.Gas = big.NewInt(50000000)
 	}
 	if call.Value == nil {
diff --git a/accounts/abi/numbers.go b/accounts/abi/numbers.go
index 36fb6705e..10afa6511 100644
--- a/accounts/abi/numbers.go
+++ b/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
diff --git a/accounts/abi/packing.go b/accounts/abi/packing.go
index 5054dcf13..1d7f85e2b 100644
--- a/accounts/abi/packing.go
+++ b/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 {
diff --git a/accounts/keystore/keystore_passphrase.go b/accounts/keystore/keystore_passphrase.go
index 8ef510fcf..2eae25841 100644
--- a/accounts/keystore/keystore_passphrase.go
+++ b/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)
diff --git a/accounts/usbwallet/ledger_wallet.go b/accounts/usbwallet/ledger_wallet.go
index 37e04e598..99d0b21d1 100644
--- a/accounts/usbwallet/ledger_wallet.go
+++ b/accounts/usbwallet/ledger_wallet.go
@@ -416,7 +416,7 @@ func (w *ledgerWallet) selfDerive() {
 				break
 			}
 			// If the next account is empty, stop self-derivation, but add it nonetheless
-			if balance.BitLen() == 0 && nonce == 0 {
+			if balance.Sign() == 0 && nonce == 0 {
 				empty = true
 			}
 			// We've just self-derived a new account, start tracking it locally