fix slowdown in nested hash freeing

Commit 104d7b69 made sv_clear free hashes iteratively rather than recursively;
however, my code didn't record the current hash index when freeing a
nested hash, which made the code go quadratic when freeing a large hash
with inner hashes, e.g.:

    my $r; $r->{$_} = { a => 1 } for 1..10_0000;

This was noticeable on such things as CPAN.pm being very slow to exit.

This commit fixes this by squirrelling away the old hash index in the
now-unused SvMAGIC field of the hash being freed.

1 files changed, 1 insertions, 0 deletions

diff --git a/sv.h b/sv.h
index 935f4fff15..60ff740501 100644
--- a/sv.h
+++ b/sv.h
@@ -440,6 +440,7 @@ union _xivu {
 union _xmgu {
     MAGIC*  xmg_magic;		/* linked list of magicalness */
     HV*	    xmg_ourstash;	/* Stash for our (when SvPAD_OUR is true) */
+    STRLEN  xmg_hash_index;	/* used while freeing hash entries */
 };
 
 struct xpv {