There is a less costly way to avoid the tag bit in integer: 
"conservative GC": any int which happens to point in an alloccated block 
(or only at the beginning if you do not consider C but ML) is considered 
as a pointer. You will have very few wrong pointers (especially in the 
second case). Moreover, array of int or float, or block of memory can be 
tagged with a flag saying they do not old pointer.

The Boehm GC for C and C++ is very succefull to do that and very often 
allow you to share data-structure in C as you would in ML (not caring 
about who will release first the data) and gain both speed and memory.

Does anyone have  a comparison between two identical GC except one 
should have a tag bit and the other be conservative ?

The cost of conservative GC is the test to know if an int is pointing in 
(or at the beginning) of an allocated block which require for instance a 
hashtbl of allocated blocks by adress ranges. I don't know if the gain 
for arithmetic + easier C interface would compensate the lost in the GC 
for a real GC like Caml's.


--
Christophe Raffalli
Université de Savoie
Batiment Le Chablais, bureau 21
73376 Le Bourget-du-Lac Cedex

tél: (33) 4 79 75 81 03
fax: (33) 4 79 75 87 42
mail: Christophe.Raffalli@univ-savoie.fr
www: http://www.lama.univ-savoie.fr/~RAFFALLI
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