opslabs: change opslab_first to opslab_free_space

Currently a OPSLAB maintains a pointer to the lowest allocated OPSLOT
within the slab (slots are allocated downwards). Replace this pointer
with a U16 indicating how many pointer-sized words are free below the
lowest allocated slot.

1 files changed, 3 insertions, 1 deletions

diff --git a/op.h b/op.h
index e97683efae..5a61b6a331 100644
--- a/op.h
+++ b/op.h
@@ -696,13 +696,15 @@ struct opslot {
 };
 
 struct opslab {
-    OPSLOT *	opslab_first;		/* first op in this slab */
     OPSLAB *	opslab_next;		/* next slab */
     OPSLAB *	opslab_head;		/* first slab in chain */
     OP *	opslab_freed;		/* chain of freed ops */
     size_t	opslab_refcnt;		/* number of ops (head slab only) */
     U16		opslab_size;		/* size of slab in pointers,
                                            including header */
+    U16         opslab_free_space;	/* space available in this slab
+                                           for allocating new ops (in ptr
+                                           units) */
 # ifdef PERL_DEBUG_READONLY_OPS
     bool	opslab_readonly;
 # endif