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, 4 insertions, 1 deletions

diff --git a/hv.c b/hv.c
index 3fb3975cbc..6b662518f6 100644
--- a/hv.c
+++ b/hv.c
@@ -1863,7 +1863,10 @@ Perl_hv_undef_flags(pTHX_ HV *hv, U32 flags)
 	xhv->xhv_max   = 7;	/* HvMAX(hv) = 7 (it's a normal hash) */
 	HvARRAY(hv) = 0;
     }
-    HvPLACEHOLDERS_set(hv, 0);
+    /* if we're freeing the HV, the SvMAGIC field has been reused for
+     * other purposes, and so there can't be any placeholder magic */
+    if (SvREFCNT(hv))
+	HvPLACEHOLDERS_set(hv, 0);
 
     if (SvRMAGICAL(hv))
 	mg_clear(MUTABLE_SV(hv));