gpgscm: Avoid cell allocation overhead.

* tests/gpgscm/scheme-private.h (struct scheme): New fields
'inhibit_gc', 'reserved_cells', and 'reserved_lineno'.
* tests/gpgscm/scheme.c (GC_ENABLED): New macro.
(USE_GC_LOCKING): Likewise.
(gc_reservations): Likewise.
(gc_reservation_failure): New function.
(_gc_disable): Likewise.
(gc_disable): New macro.
(gc_enable): Likewise.
(gc_enabled): Likewise.
(gc_consume): Likewise.
(get_cell_x): Consume reserved cell if garbage collection is disabled.
(_get_cell): Assert that gc is enabled.
(get_cell): Only record cell in the list of recently allocated cells
if gc is enabled.
(get_vector_object): Likewise.
(gc): Assert that gc is enabled.
(s_return): Add comment, adjust call to '_s_return'.
(s_return_enable_gc): New macro.
(_s_return): Add flag 'enable_gc' and re-enable gc if set.
(oblist_add_by_name): Use the new facilities to protect the
allocations.
(new_frame_in_env): Likewise.
(new_slot_spec_in_env): Likewise.
(s_save): Likewise.
(opexe_0): Likewise.
(opexe_1): Likewise.
(opexe_2): Likewise.
(opexe_5): Likewise.
(opexe_6): Likewise.
(scheme_init_custom_alloc): Initialize the new fields.
--

Every time a cell is allocated, the interpreter may run out of free
cells and do a garbage collection.  This is problematic because it
might garbage collect objects that have been allocated, but are not
yet made available to the interpreter.

Previously, we would plug such newly allocated cells into the list of
newly allocated objects rooted at car(sc->sink), but that requires
allocating yet another cell increasing pressure on the memory
management system.

A faster alternative is to preallocate the cells needed for an
operation and make sure the garbage collection is not run until all
allocated objects are plugged in.  This can be done with gc_disable
and gc_enable.

This optimization can be applied incrementally.  This commit picks all
low-hanging fruits.

Signed-off-by: Justus Winter <justus@g10code.com>

tests/gpgscm/scheme-private.h

@@ -121,6 +121,11 @@ pointer COMPILE_HOOK;  /* *compile-hook* */
 
 pointer free_cell;       /* pointer to top of free cells */
 long    fcells;          /* # of free cells */
+size_t  inhibit_gc;      /* nesting of gc_disable */
+size_t  reserved_cells;  /* # of reserved cells */
+#ifndef NDEBUG
+int     reserved_lineno;   /* location of last reservation */
+#endif
 
 pointer inport;
 pointer outport;