make opslot_slab an offset in current slab

Each OPSLOT allocated within an OPSLAB contains a pointer, opslot_slab,
which points back to the first (head) slab of the slab chain (i.e. not
necessarily to the slab which the op is contained in).

This commit changes the pointer to be a 16-bit offset from the start of
the current slab, and adds a pointer at the start of each slab which
points back to the head slab.

The mapping from an op to the head slab is now a two-step process: use
the op's slot's opslot_offset field to find the start of the current
slab, then use that slab's new opslab_head pointer to find the head
slab.

The advantage of this is that it reduces the storage per op.  (It
probably doesn't make any practical difference yet, due to alignment
issues, but that will will be sorted shortly in this branch.)

1 files changed, 8 insertions, 3 deletions

diff --git a/op.h b/op.h
index ad6cf7fe49..9a6d6fb25a 100644
--- a/op.h
+++ b/op.h
@@ -691,15 +691,16 @@ least an C<UNOP>.
 struct opslot {
     /* keep opslot_next first */
     OPSLOT *	opslot_next;		/* next slot */
-    OPSLAB *	opslot_slab;		/* owner */
+    U16         opslot_offset;      /* offset from start of slab (in ptr units) */
     OP		opslot_op;		/* the op itself */
 };
 
 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 */
+    size_t	opslab_refcnt;		/* number of ops (head slab only) */
 # ifdef PERL_DEBUG_READONLY_OPS
     U16		opslab_size;		/* size of slab in pointers */
     bool	opslab_readonly;
@@ -711,7 +712,11 @@ struct opslab {
 # define OPSLOT_HEADER_P	(OPSLOT_HEADER/sizeof(I32 *))
 # define OpSLOT(o)		(assert_(o->op_slabbed) \
 				 (OPSLOT *)(((char *)o)-OPSLOT_HEADER))
-# define OpSLAB(o)		OpSLOT(o)->opslot_slab
+
+/* the first (head) opslab of the chain in which this op is allocated */
+# define OpSLAB(o) \
+    (((OPSLAB*)( (I32**)OpSLOT(o) - OpSLOT(o)->opslot_offset))->opslab_head)
+
 # define OpslabREFCNT_dec(slab)      \
 	(((slab)->opslab_refcnt == 1) \
 	 ? opslab_free_nopad(slab)     \