def _int_arith_flags(rettype):
    """
    Return the modifier flags for integer arithmetic.
    """
    if rettype.signed:
        # Ignore the effects of signed overflow.  This is important for
        # optimization of some indexing operations.  For example
        # array[i+1] could see `i+1` trigger a signed overflow and
        # give a negative number.  With Python's indexing, a negative
        # index is treated differently: its resolution has a runtime cost.
        # Telling LLVM to ignore signed overflows allows it to optimize
        # away the check for a negative `i+1` if it knows `i` is positive.
        return ['nsw']
    else:
        return []

    Return the modifier flags for integer arithmetic.
    
numba/cpython/new_numbers.py