process xhv_backreferences early in S_hfreeentries

When deleting a stash, make the algorithm
    GvSTASH($_) = NULL for (@xhv_backreferences);
    delete xhv_backreferences;
    free each stash entry;

Previously the algorithm was

    hide xhv_backreferences as ordinary backref magic;
    free each stash entry:
	this may trigger a sv_del_backref() for each GV being freed
    delete @xhv_backreferences

The new method is:

* more efficient: one scan through @xhv_backreferences rather than lots of
calls to sv_del_backref(), removing elements one by one;

* makes the code simpler; the 'hide xhv_backreferences as backref magic'
hack no longer needs to be done

* removes a bug whereby GVs that had a refcnt > 1 (the usual case) were
left with a GvSTASH pointing to the freed stash; it's now NULL instead. I
couldn't think of a test for this.

There are two drawbacks:

* If the GV gets freed at the same time as the stash, the freeing code
sees the GV with a GVSTASH of NULL rather than still pointing to the
stash.

* As far as I can see, the only difference this currently makes is that
mro_method_changed_in() is no longer called by sv_clear(), but since we're
blowing away the whole stash anyway, method resolution doesn't really
bother us any more.

At some point in the future I might set GvSTASH to %__ANON__ rather than
NULL.

1 files changed, 0 insertions, 7 deletions

diff --git a/proto.h b/proto.h
index 1824377d3b..688a2f03a9 100644
--- a/proto.h
+++ b/proto.h
@@ -6459,13 +6459,6 @@ PERL_CALLCONV AV**	Perl_hv_backreferences_p(pTHX_ HV *hv)
 #define PERL_ARGS_ASSERT_HV_BACKREFERENCES_P	\
 	assert(hv)
 
-#if defined(PERL_IN_DUMP_C) || defined(PERL_IN_HV_C) || defined(PERL_IN_SV_C)
-PERL_CALLCONV void	Perl_hv_kill_backrefs(pTHX_ HV *hv)
-			__attribute__nonnull__(pTHX_1);
-#define PERL_ARGS_ASSERT_HV_KILL_BACKREFS	\
-	assert(hv)
-
-#endif
 PERL_CALLCONV void	Perl_hv_clear_placeholders(pTHX_ HV *hv)
 			__attribute__nonnull__(pTHX_1);
 #define PERL_ARGS_ASSERT_HV_CLEAR_PLACEHOLDERS	\