🚀 go-pugleaf

In article <1992Jul27.065551.3053@cs.kuleuven.ac.be> bimbart@cs.kuleuven.ac.be (Bart Demoen) writes:

   [...]

   Two more remarks: I want to make a link with BinProlog (by the way, the
   relation between BinProlog and BIMProlog as not the same as between Nu-Prolog
   and Mu-Prolog !) or a continuation based implementation of Prolog:
   continuations on the heap actually means also environments on the heap, so that
   it in fact puts uninitialized 'permanent' variables on the heap. Still, it trails as
   much as a WAM implementation with unsafe variables. And it is difficult to get rid
   of the extra dereference every time the variable is accessed.

I don't know how other WAM based Prologs do it, but in SICStus Prolog,
the compiler ensures that all permanent variables are initialized at
the first 'call' instruction of every clause.  The variables are initialized
by 'put_variable' if they are not otherwise initialized already.

This is done to prevent there from being random bit patterns in the
permanent variables in the event of a garbage collection.  Some other
changes to the instruction set are also necessary to prevent similar
trouble in the event of garbage collection after backtracking.

I don't have any figures, but I suspect that variables created by this
mechanism would significantly contribute to the heap growth if one
gets rid of unsafe variables.
--
Mats Carlsson
SICS, PO Box 1263, S-164 28  KISTA, Sweden    Internet: matsc@sics.se
Tel: +46 8 7521543     Ttx: 812 6154 7010 SICS     Fax: +46 8 7517230