optimise save/restore of PL_delaymagic.

A few places (pp_push, pp_unshift, pp_aassign) have to
set PL_delaymagic on entry, and restore it on exit. These are hot
pieces of code. Rather than using  ENTER/SAVEI16(PL_delaymagic)/LEAVE,
add an extra field to the jumpenv struct, and make the JUMPENV_PUSH / POP
macros automatically save and restore this var.

This means that pp_push etc only need to do a local save:

    U16 old_delaymagic = PL_delaymagic;
    PL_delaymagic = DM_DELAY;
    ....
    PL_delaymagic = old_delaymagic;

and in case of an exception being raised, PL_delaymagic still gets
restored.

This transfers the cost of saving PL_delaymagic from each call to
pp_aassign etc to each time a new run level is invoked. The latter should
be much less frequent.

Note that prior to this commit, pp_aassign wasn't actually saving and
restoring PL_delaymagic; it was just setting it to 0 at the end. So this
commit also makes pp_aassign safe against PL_delaymagic re-entrancy like
pp_push and pp_unshift already were.

1 files changed, 15 insertions, 0 deletions

diff --git a/intrpvar.h b/intrpvar.h
index 79bddeb8e2..7dc4be4e87 100644
--- a/intrpvar.h
+++ b/intrpvar.h
@@ -76,7 +76,22 @@ PERLVAR(I, curpm,	PMOP *)		/* what to do \ interps in REs from */
 
 PERLVAR(I, tainting,	bool)		/* doing taint checks */
 PERLVAR(I, tainted,	bool)		/* using variables controlled by $< */
+
+/* PL_delaymagic is currently used for two purposes: to assure simultaneous
+ * updates in ($<,$>) = ..., and to assure atomic update in push/unshift
+ * @ISA, It works like this: a few places such as pp_push set the DM_DELAY
+ * flag; then various places such as av_store() skip mg_set(ary) if this
+ * flag is set, and various magic vtable methods set flags like
+ * DM_ARRAY_ISA if they've seen something of that ilk. Finally when
+ * control returns to pp_push or whatever, it sees if any of those flags
+ * have been set, and if so finally calls mg_set().
+ *
+ * NB: PL_delaymagic is automatically saved and restored by JUMPENV_PUSH
+ * / POP. This removes the need to do ENTER/SAVEI16(PL_delaymagic)/LEAVE
+ * in hot code like pp_push.
+ */
 PERLVAR(I, delaymagic,	U16)		/* ($<,$>) = ... */
+
 PERLVAR(I, localizing,	U8)		/* are we processing a local() list? */
 PERLVAR(I, in_eval,	U8)		/* trap "fatal" errors? */
 PERLVAR(I, defgv,	GV *)           /* the *_ glob */