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

1 files changed, 3 insertions, 3 deletions

diff --git a/core/state/state_object.go b/core/state/state_object.go
index ebb103806..17726dd0c 100644
--- a/core/state/state_object.go
+++ b/core/state/state_object.go
@@ -93,7 +93,7 @@ type stateObject struct {
 
 // empty returns whether the account is considered empty.
 func (s *stateObject) empty() bool {
-	return s.data.Nonce == 0 && s.data.Balance.BitLen() == 0 && bytes.Equal(s.data.CodeHash, emptyCodeHash)
+	return s.data.Nonce == 0 && s.data.Balance.Sign() == 0 && bytes.Equal(s.data.CodeHash, emptyCodeHash)
 }
 
 // Account is the Ethereum consensus representation of accounts.
@@ -243,7 +243,7 @@ func (self *stateObject) CommitTrie(db trie.Database, dbw trie.DatabaseWriter) e
 func (c *stateObject) AddBalance(amount *big.Int) {
 	// EIP158: We must check emptiness for the objects such that the account
 	// clearing (0,0,0 objects) can take effect.
-	if amount.Cmp(common.Big0) == 0 {
+	if amount.Sign() == 0 {
 		if c.empty() {
 			c.touch()
 		}
@@ -260,7 +260,7 @@ func (c *stateObject) AddBalance(amount *big.Int) {
 // SubBalance removes amount from c's balance.
 // It is used to remove funds from the origin account of a transfer.
 func (c *stateObject) SubBalance(amount *big.Int) {
-	if amount.Cmp(common.Big0) == 0 {
+	if amount.Sign() == 0 {
 		return
 	}
 	c.SetBalance(new(big.Int).Sub(c.Balance(), amount))