Revert "Revert "8c34e50dc slowed down detruction with no DESTROY""

This reverts commit 95f9781bc2fad025553db0160ef9c2c5363312a1.

Now that the crash has been fixed by the preceding commit, we can
reinstate 7cc6787e9db.

2 files changed, 8 insertions, 6 deletions

diff --git a/mro.c b/mro.c
index 2d1d887fe8..be2038f4e7 100644
--- a/mro.c
+++ b/mro.c
@@ -578,6 +578,7 @@ Perl_mro_isa_changed_in(pTHX_ HV* stash)
                 revmeta->cache_gen++;
             if(revmeta->mro_nextmethod)
                 hv_clear(revmeta->mro_nextmethod);
+	    if (!SvOBJECT(revstash)) SvSTASH(revstash) = NULL;
 
 	    (void)
 	      hv_store(
@@ -1356,6 +1357,7 @@ Perl_mro_method_changed_in(pTHX_ HV *stash)
             mrometa->cache_gen++;
             if(mrometa->mro_nextmethod)
                 hv_clear(mrometa->mro_nextmethod);
+            if (!SvOBJECT(revstash)) SvSTASH(revstash) = NULL;
         }
     }
 
diff --git a/sv.c b/sv.c
index 9f5c157a87..a7914988e7 100644
--- a/sv.c
+++ b/sv.c
@@ -6338,12 +6338,12 @@ S_curse(pTHX_ SV * const sv, const bool check_refcnt) {
 	    if (!destructor) {
 		GV * const gv =
 		    gv_fetchmeth_autoload(stash, "DESTROY", 7, 0);
-		if (gv && (destructor = GvCV(gv))) {
-		    if (!SvOBJECT(stash))
-			SvSTASH(stash) = (HV *)destructor;
-		}
+		if (gv) destructor = GvCV(gv);
+		if (!SvOBJECT(stash))
+		    SvSTASH(stash) =
+			destructor ? (HV *)destructor : ((HV *)0)+1;
 	    }
-	    if (destructor
+	    if (destructor && destructor != ((CV *)0)+1
 		/* A constant subroutine can have no side effects, so
 		   don't bother calling it.  */
 		&& !CvCONST(destructor)
@@ -12000,7 +12000,7 @@ S_sv_dup_common(pTHX_ const SV *const sstr, CLONE_PARAMS *const param)
 		    SvOURSTASH_set(dstr, hv_dup_inc(SvOURSTASH(dstr), param));
 		} else if (SvMAGIC(dstr))
 		    SvMAGIC_set(dstr, mg_dup(SvMAGIC(dstr), param));
-		if (SvSTASH(dstr))
+		if (SvOBJECT(dstr) && SvSTASH(dstr))
 		    SvSTASH_set(dstr, hv_dup_inc(SvSTASH(dstr), param));
 		else SvSTASH_set(dstr, 0); /* don't copy DESTROY cache */
 	    }