accounts/abi faster unpacking of int256 (ethereum#20850)

accounts/abi/unpack.go

@@ -27,13 +27,9 @@ import (
 
 var (
 	// MaxUint256 is the maximum value that can be represented by a uint256
-	MaxUint256 = big.NewInt(0).Add(
-		big.NewInt(0).Exp(big.NewInt(2), big.NewInt(256), nil),
-		big.NewInt(-1))
+	MaxUint256 = new(big.Int).Sub(new(big.Int).Lsh(common.Big1, 256), common.Big1)
 	// MaxInt256 is the maximum value that can be represented by a int256
-	MaxInt256 = big.NewInt(0).Add(
-		big.NewInt(0).Exp(big.NewInt(2), big.NewInt(255), nil),
-		big.NewInt(-1))
+	MaxInt256 = new(big.Int).Sub(new(big.Int).Lsh(common.Big1, 255), common.Big1)
 )
 
 // ReadInteger reads the integer based on its kind and returns the appropriate value
@@ -56,17 +52,17 @@ func ReadInteger(typ byte, kind reflect.Kind, b []byte) interface{} {
 	case reflect.Int64:
 		return int64(binary.BigEndian.Uint64(b[len(b)-8:]))
 	default:
-		// the only case lefts for integer is int256/uint256.
-		// big.SetBytes can't tell if a number is negative, positive on itself.
-		// On EVM, if the returned number > max int256, it is negative.
+		// the only case left for integer is int256/uint256.
 		ret := new(big.Int).SetBytes(b)
 		if typ == UintTy {
 			return ret
 		}
-
-		if ret.Cmp(MaxInt256) > 0 {
-			ret.Add(MaxUint256, big.NewInt(0).Neg(ret))
-			ret.Add(ret, big.NewInt(1))
+		// big.SetBytes can't tell if a number is negative or positive in itself.
+		// On EVM, if the returned number > max int256, it is negative.
+		// A number is > max int256 if the bit at position 255 is set.
+		if ret.Bit(255) == 1 {
+			ret.Add(MaxUint256, new(big.Int).Neg(ret))
+			ret.Add(ret, common.Big1)
 			ret.Neg(ret)
 		}
 		return ret

accounts/abi/unpack_test.go

@@ -1009,7 +1009,7 @@ func TestUnpackTuple(t *testing.T) {
 			t.Errorf("unexpected value unpacked: want %x, got %x", 1, v.A)
 		}
 		if v.B.Cmp(big.NewInt(-1)) != 0 {
-			t.Errorf("unexpected value unpacked: want %x, got %x", v.B, -1)
+			t.Errorf("unexpected value unpacked: want %x, got %x", -1, v.B)
 		}
 	}