Overflow checking for multiplication

[yzhang90]

What's your issue about?

NOT in EVM is bitwise NOT, not logical NOT. So the overflow checking for multiplication (['assert', ['or', ['eq', ['sdiv', 'ans', 'l'], 'r'], ['not', 'l']]] ) is wrong. Consider the following the test cases:

def test_mul_overflow(t, assert_tx_failed, get_contract_with_gas_estimation, chain):
    mul_code = """

@public
def _num_mul(x: decimal, y: int128) -> decimal:
    return x * y

    """

    c = get_contract_with_gas_estimation(mul_code)

    t.s = chain
    NUM_1 = 85070591730234615865843651857942052864.0
    NUM_2 = 136112946768375385385349842973

    assert_tx_failed(lambda: c._num_mul(NUM_1, NUM_2))

NUM_1 * NUM_2 > 2**127, but no exception is thrown.

How can it be fixed?

Use ISZERO to test whether l is zero.

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