Pointer top 16 bits use

[deadalnix via Digitalmars-d]

On Saturday, 7 May 2016 at 12:32:53 UTC, Marco Leise wrote:
> Am Sat, 07 May 2016 12:02:07 +0000
> schrieb Nicholas Wilson <iamthewilsonator at hotmail.com>:
>
>> On Saturday, 7 May 2016 at 09:01:11 UTC, Ali Çehreli wrote:
>> > On 05/06/2016 11:08 PM, Nicholas Wilson wrote:
>> >> In Dicebot's DConf talk
>> >
>> > That was deadalnix. :)
>> >
>> > Ali
>> 
>> Touché.
>
> Too many d's for the day. :) I agree with Benjamin, that we 
> can't rely on the 16-bits being reserved forever.
>
> I have a question about the lower 3 bits OTOH. The minimal GC 
> allocation size is >= 8 bytes, right? I.e. if we set the lower 
> 3 bits to 0b111, and the pointer points to a 4 byte structure, 
> it will still be recognized as an internal pointer due to the 
> overallocation.

You could have allocated an array, so that's a problem, you can 
only use 2 bits. taggedPointer will check for this at compile 
time.

But, yes, LSB are safe to use is you wrap it properly because it 
will be seen as an interior pointer.