[D-runtime] Metadata for memory

[Steve Schveighoffer]

----- Original Message ----
> From: Fawzi Mohamed <fawzi at gmx.ch>
> 
> Actually I  don't see the need of more info about a block than size, and 
>character  (equivalent to the typeid of what was allocated).
> Anything else of interest  can be derived from that.

The "used" length of an array is not a static value.  It needs to be stored 
separate from the typeinfo.  The block size is identified by the pool or by 
doing a linear search on large blocks (this also needs to be addressed at some 
point).

> Given this one could think about hardcoding  that in some way in the block.
> The rest should be stored elsewhere imho (one  can expect that the table to 
>reach other info will be cached when used, as it  should be relatively small).
> 
> Still I would give more thought about  storing also those two pieces of 
>information outside the block.
> The pool  already knows the size, and one could add a (uint/size_t?) per 
>element to store  the  character.

Note that as I said above, the "used size" must be stored somewhere besides the 
"character", and also note that for arrays, a pad byte is inherent in the memory 
block to avoid pointers moving to the next block (which might possibly be 
unallocated).  So we already are using a byte inside the block for something, 
might as well make some use of it.

For non-arrays, I'm unsure.  An external pointer might be fine, but for small 
blocks, it can be up to 25% overhead if you are storing a size_t.

-Steve