[Gc] memory alignment of small obiects allocation

[Xing Su]

hi! I am new user to libgc.
 From the documentation I found that libgc does well in managing small 
objects' allocation and collection.

In my application, I want to have some memory alignment constraints on 
my allocated objects.
For example, for each 12-byte objects I want them with a memory 
alignment of 8 bytes, and each 16-byte objects with alignment of 16 bytes.
  how can I achieve this?

Or if libgc can't do this, which data structure and which file should I 
focus on if I want modify the code myself?
Any help is greatly appreciated!