[OT] About GC: The Future of Rust : GC integration

[deadalnix via Digitalmars-d]

On Wednesday, 8 June 2016 at 04:15:17 UTC, H. S. Teoh wrote:
> I thought the usual implementation is to store the metadata 
> immediately before the pointer, so it's O(1) to look it up.
>

No, it gets you terrible fragmentation and it makes it easy to 
corrupt allocator data with just a buffer overflow/underflow, 
which is a big nono.

But the general idea remain: the metadata address can indeed be 
found in O(1) from the address of the data, usually with some 
cheap bit twiddling, for most allocations (for instance, jemalloc 
does it in O(1) fro all allocation < 256k and in log(n) for 
larger ones).