Martin Sebor <mse...@gmail.com> writes:
> On 11/08/2017 02:32 AM, Richard Sandiford wrote:
>> Martin Sebor <mse...@gmail.com> writes:
>>> I haven't done nearly a thorough review but the dtor followed by
>>> the placement new in the POLY_SET_COEFF() macro caught my eye so
>>> I thought I'd ask sooner rather than later. Given the macro
>>> definition:
>>>
>>> + The dummy comparison against a null C * is just a way of checking
>>> + that C gives the right type. */
>>> +#define POLY_SET_COEFF(C, RES, I, VALUE) \
>>> + ((void) (&(RES).coeffs[0] == (C *) 0), \
>>> + wi::int_traits<C>::precision_type == wi::FLEXIBLE_PRECISION \
>>> + ? (void) ((RES).coeffs[I] = VALUE) \
>>> + : (void) ((RES).coeffs[I].~C (), new (&(RES).coeffs[I]) C (VALUE)))
>>>
>>> is the following use well-defined?
>>>
>>> +template<unsigned int N, typename C>
>>> +inline poly_int_pod<N, C>&
>>> +poly_int_pod<N, C>::operator <<= (unsigned int a)
>>> +{
>>> + POLY_SET_COEFF (C, *this, 0, this->coeffs[0] << a);
>>>
>>> It looks to me as though the VALUE argument in the ctor invoked
>>> by the placement new expression is evaluated after the dtor has
>>> destroyed the very array element the VALUE argument expands to.
>>
>> Good catch! It should simply have been doing <<= on each coefficient --
>> I must have got carried away when converting to POLY_SET_COEFF.
>>
>> I double-checked the other uses and think that's the only one.
>>
>>> Whether or not is, in fact, a problem, it seems to me that using
>>> a function template rather than a macro would be a clearer and
>>> safer way to do the same thing. (Safer in that the macro also
>>> evaluates its arguments multiple times, which is often a source
>>> of subtle bugs.)
>>
>> That would slow down -O0 builds though, by introducing an extra
>> function call and set of temporaries even when the coefficients
>> are primitive integers.
>
> Would decorating the function template with attribute always_inline
> help?
It would remove the call itself, but we'd still have the extra temporary
objects that were the function argument and return value.
Thanks,
Richard
