Add OPpTARGET_MY optimization to OP_UNDEF

This allows the existing `undef` OP to act on a pad SV. The following
two cases are optimized:

`undef my $x`, currently implemented as:
    4     <1> undef vK/1 ->5
    3        <0> padsv[$x:1,2] sRM/LVINTRO ->4

`my $a = undef`, currently implemented as:

    5     <2> sassign vKS/2 ->6
    3        <0> undef s ->4
    4        <0> padsv[$x:1,2] sRM*/LVINTRO ->5

These are now just represented as:

   3     <1> undef[$x:1,2] vK/SOMEFLAGS ->4

Note: The two cases are not quite functionally identical, as `$x = undef`
clears the SV flags but preserves any PV allocation for later reuse,
whereas `undef $x` does free any PV allocation. This behaviour difference
is preserved through use of the OPpUNDEF_KEEP_PV flag.

1 files changed, 18 insertions, 6 deletions

diff --git a/pp.c b/pp.c
index 185d7b7b34..6605d38279 100644
--- a/pp.c
+++ b/pp.c
@@ -879,11 +879,21 @@ PP(pp_undef)
         RETPUSHUNDEF;
     }
 
-    sv = TOPs;
-    if (!sv)
-    {
-        SETs(&PL_sv_undef);
-        return NORMAL;
+    if (PL_op->op_private & OPpTARGET_MY) {
+        SV** const padentry = &PAD_SVl(PL_op->op_targ);
+        sv = *padentry;
+        EXTEND(SP,1);sp++;PUTBACK;
+        if ((PL_op->op_private & (OPpLVAL_INTRO|OPpPAD_STATE)) == OPpLVAL_INTRO) {
+            save_clearsv(padentry);
+        }
+    } else {
+        sv = TOPs;
+
+        if (!sv)
+        {
+            SETs(&PL_sv_undef);
+            return NORMAL;
+        }
     }
 
     if (SvTHINKFIRST(sv))
@@ -960,7 +970,9 @@ PP(pp_undef)
             break;
         }
     default:
-        if (SvTYPE(sv) >= SVt_PV && SvPVX_const(sv) && SvLEN(sv)) {
+        if (SvTYPE(sv) >= SVt_PV && SvPVX_const(sv) && SvLEN(sv)
+            && !(PL_op->op_private & OPpUNDEF_KEEP_PV)
+        ) {
             SvPV_free(sv);
             SvPV_set(sv, NULL);
             SvLEN_set(sv, 0);