Re: [07/23] Add a class that multiplexes two pointer types

[Nathan Sidwell <nathan@acm.org>]

On 12/17/20 10:38 AM, Richard Sandiford via Gcc-patches wrote:
> Tom Tromey <tom@tromey.com> writes:
>>>>>>> "Richard" == Richard Sandiford via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>>
>> Richard> +// A class that stores a choice "A or B", where A has type T1 * and B has
>> Richard> +// type T2 *.  Both T1 and T2 must have an alignment greater than 1, since
>> Richard> +// the low bit is used to identify B over A.  T1 and T2 can be the same.
>>
>> It seems like the alignment requirement could be static_assert'd, which
>> would make using this class a bit safer.
> 
> Yeah, I wondered about doing that, but in principle there's nothing
> to stop people using the class for something like a char*, provided that
> the start is suitably aligned.  So having gcc_checking_assert is a
> compromise: it only provides run-time checking rather than compile-time
> checking, but it doesn't place any artificial limits on how the class
> can be used.

I don;t know whether our optimizers are smart enough (credulous enough?) 
to know that the low bits of any pointer to an aligned type must be 
zero, and therefore optimize the assert away in those cases?

If they're not something like

  assert (align (T) > 1 || !(reinterpret_cast<intptr_t> (ptr) & 1))

?

-- 
Nathan Sidwell