[Sablevm-developer] Generational GC and Hashcode

[Carl L. <le...@ia...>]

I am finishing up an implementation for Generational garbage collection 
for SableVM and have a question regarding the hashcode.  I have based the 
GC on the original copying GC but am not sure I understand how the 
hashcode is handled.

From what I have read, the hashcode is used by various hashing functions 
when storing objects, and thus the hashcode should be constant for the 
life of the object.  It is also suggested that the hashcode be distributed 
across the range of integers, and although does not need to be unique for 
each object, uniqueness will provide better performance.  It is suggested 
to use the heap address of the object for the hashcode, but in a copying 
collector, such as in SableVM, the location of the object will not be 
static.

Could someone explain to me how the lockword bits denoting SVM_HASH_NONE, 
SVM_HASH_NOT_MOVED, SVM_HASH_MOVED work in conjunction with the garbage 
collector, specifically with env->vm->heap.hashcode_base?  I feel I 
understand the intention, but not the details.  I think the major 
difference between the current collector and a generational collector is 
that only a small subset of the objects are relocated during a collection 
of the nursery.

Carl Lebsack
PhD Student
Iowa State University