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

3 files changed, 3 insertions, 3 deletions

diff --git a/core/types/transaction.go b/core/types/transaction.go
index 9382acb70..ab0bba4dc 100644
--- a/core/types/transaction.go
+++ b/core/types/transaction.go
@@ -42,7 +42,7 @@ var (
 
 // deriveSigner makes a *best* guess about which signer to use.
 func deriveSigner(V *big.Int) Signer {
-	if V.BitLen() > 0 && isProtectedV(V) {
+	if V.Sign() != 0 && isProtectedV(V) {
 		return EIP155Signer{chainId: deriveChainId(V)}
 	} else {
 		return HomesteadSigner{}
diff --git a/core/types/transaction_signing.go b/core/types/transaction_signing.go
index 7d7b63e9f..e7eb7c5f0 100644
--- a/core/types/transaction_signing.go
+++ b/core/types/transaction_signing.go
@@ -167,7 +167,7 @@ func (s EIP155Signer) WithSignature(tx *Transaction, sig []byte) (*Transaction,
 	cpy.data.R = new(big.Int).SetBytes(sig[:32])
 	cpy.data.S = new(big.Int).SetBytes(sig[32:64])
 	cpy.data.V = new(big.Int).SetBytes([]byte{sig[64]})
-	if s.chainId.BitLen() > 0 {
+	if s.chainId.Sign() != 0 {
 		cpy.data.V = big.NewInt(int64(sig[64] + 35))
 		cpy.data.V.Add(cpy.data.V, s.chainIdMul)
 	}
diff --git a/core/types/transaction_signing_test.go b/core/types/transaction_signing_test.go
index 3216fcfad..7f799fb10 100644
--- a/core/types/transaction_signing_test.go
+++ b/core/types/transaction_signing_test.go
@@ -71,7 +71,7 @@ func TestEIP155ChainId(t *testing.T) {
 		t.Error("didn't expect tx to be protected")
 	}
 
-	if tx.ChainId().BitLen() > 0 {
+	if tx.ChainId().Sign() != 0 {
 		t.Error("expected chain id to be 0 got", tx.ChainId())
 	}
 }