RESTORE_AVMA(oldvalue)

[Ilya Zakharevich <ilya@math.ohio-state.edu>]

As far as I understand, the only problem with interfacing PARI with
other math-related libraries (e.g., to allow Groebner bases supported)
is an absense of the type t_EXTERNAL_DATA.  The minimal support for
this type in the PARI core should be: cloning/uncloning,
create/destroy and a couple of others; these operations should go
through the dispatch table (the pointer to this table should be a part
of the data structure associated to this type).

However, the major obstacle for this to be done is the absense of
refcounts for PARI objects: whatever is on stack at the time of
"restoring the stack position" is just ignored (if it is not
gerepile()d).  This could lead to severe memory leaks.

The following patch + edit one-liner make it easy to change the policy
of "restoring the stack position" at run time: suppose that we have a
linked list LL of object on the stack which need to be free()ed; the
typical situation is LL == NULL.  Then a change like

#define RESTORE_AVMA(from)  (LL ? hard_way_restore_avma(from) : (avma = (from))

will:

  a) have a very little overhead in absense of LL;

  b) allow hard_way_restore_avma() to obj_free() the objects which are
     in front of the LL chain up to the position from.

Enjoy,
Ilya

P.S.  pfind is available from http://www.ilyaz.org/software/perl/scripts

--- ./src/headers/parigen.h-pre	Wed May 15 20:20:04 2002
+++ ./src/headers/parigen.h	Wed May 15 20:20:45 2002
@@ -156,3 +156,4 @@ typedef int (*QSCOMP)(const void *, cons
 #define setvarn(x,s)  (((GEN)(x))[1]=\
 		       (((GEN)(x))[1]&(~VARNBITS)) | evalvarn(s))
 
+#define RESTORE_AVMA(from)	(avma = (from))

=======================================================
After applying this patch run

  pfind src '=~ s/((?:^|[{;:,]|\belse)\s*)avma\s*=\s*((?:\(\w+\))?\w+)(\s*[;),])/$1RESTORE_AVMA($2)$3/g'

this replaces the bulk of assignments of the form "avma = from" to the
corresponding macro.  Around 70 assignments to avma remain unpatched,
but this is easy to fix by hand after the bulk work is done.

Enjoy,
Ilya