From d054cdb0a9fbeeddfa67b2ef90ca115307f28fb0 Mon Sep 17 00:00:00 2001
From: Father Chrysostomos <sprout@cpan.org>
Date: Thu, 12 Sep 2013 18:26:30 -0700
Subject: [PATCH] Preserve undef identity in const ops

This fixes the one remaining issue in ticket #105906.

Under ithreads, the constants are transferred to the pad at compile
time.  Since the pads are AVs, and since AVs used &PL_sv_undef to rep-
resent unused slots, &PL_sv_undef could not be stored in the pad, and
so was copied.

That meant the same constant would produce a different scalar at each
call site:

$  ./perl -Ilib -le 'BEGIN { $::{z} = \undef }  for(z,z) { print \$_ }'
SCALAR(0x7fecb38062e0)
SCALAR(0x7fecb3806100)

Commit ce0d59f changed the way AVs work, so now we *can* store
&PL_sv_undef in an AV meaningfully, and there is no longer any need
for this copying hack.
---
 op.c         | 22 ++++------------------
 t/op/undef.t |  9 ++++++++-
 2 files changed, 12 insertions(+), 19 deletions(-)

diff --git a/op.c b/op.c
index 9be7bbc..ae3739e 100644
--- a/op.c
+++ b/op.c
@@ -1754,24 +1754,10 @@ S_finalize_op(pTHX_ OP* o)
 	 * for reference counts, sv_upgrade() etc. */
 	if (cSVOPo->op_sv) {
 	    const PADOFFSET ix = pad_alloc(OP_CONST, SVf_READONLY);
-	    if (o->op_type != OP_METHOD_NAMED
-		&& cSVOPo->op_sv == &PL_sv_undef) {
-		/* PL_sv_undef is hack - it's unsafe to store it in the
-		   AV that is the pad, because av_fetch treats values of
-		   PL_sv_undef as a "free" AV entry and will merrily
-		   replace them with a new SV, causing pad_alloc to think
-		   that this pad slot is free. (When, clearly, it is not)
-		*/
-		SvOK_off(PAD_SVl(ix));
-		SvPADTMP_on(PAD_SVl(ix));
-		SvREADONLY_on(PAD_SVl(ix));
-	    }
-	    else {
-		SvREFCNT_dec(PAD_SVl(ix));
-		PAD_SETSV(ix, cSVOPo->op_sv);
-		/* XXX I don't know how this isn't readonly already. */
-		if (!SvIsCOW(PAD_SVl(ix))) SvREADONLY_on(PAD_SVl(ix));
-	    }
+	    SvREFCNT_dec(PAD_SVl(ix));
+	    PAD_SETSV(ix, cSVOPo->op_sv);
+	    /* XXX I don't know how this isn't readonly already. */
+	    if (!SvIsCOW(PAD_SVl(ix))) SvREADONLY_on(PAD_SVl(ix));
 	    cSVOPo->op_sv = NULL;
 	    o->op_targ = ix;
 	}
diff --git a/t/op/undef.t b/t/op/undef.t
index eafa6db..d7d78c0 100644
--- a/t/op/undef.t
+++ b/t/op/undef.t
@@ -10,7 +10,7 @@ use strict;
 
 use vars qw(@ary %ary %hash);
 
-plan 85;
+plan 86;
 
 ok !defined($a);
 
@@ -168,6 +168,13 @@ like $@, qr/^Modification of a read/;
     is $k, undef, 'each undef at end';
 }
 
+# part of #105906: inlined undef constant getting copied
+BEGIN { $::{z} = \undef }
+for (z,z) {
+    push @_, \$_;
+}
+is $_[0], $_[1], 'undef constants preserve identity';
+
 # this will segfault if it fails
 
 sub PVBM () { 'foo' }
-- 
2.7.4