Keeping a list of instances and garbage-collection

[grauzone]

Actually, scratch that. Any 4 byte pattern can look like a pointer. 
Unless you manage to encode it in a way the 4 byte cells look like 
they're pointing into an address range not managed by the GC. For 
example, most OSes reserve the last 1 or 2 GBs for the kernel. If your 
byte quadruple looks like a pointer into the kernel, it's always safe 
not to be scanned.

The only simple and reliable way is to store the pointer in a malloc'ed 
or NO_SCAN'ed memory area.