OP_EMPTYAVHV - optimized empty ANONLIST/ANONHASH

This commit introduces a new OP to replace cases of OP_ANONLIST and
OP_ANONHASH where there are zero elements, which is very common in
Perl code.

As an example, `my $x = {}` is currently implemented like this:
    ...
    6     <2> sassign vKS/2 ->7
    4        <@> anonhash sK* ->5
    3           <0> pushmark s ->4
    5        <0> padsv[$x:1,2] sRM*/LVINTRO ->6

The pushmark serves no meaningful purpose when there are zero
elements and the anonhash, besides undoing the pushmark,
performs work that is unnecessary for this special case.

The peephole optimizer, which also checks for applicability of a
related TARGMY optimization, transforms this example into:
    ...
    -     <1> ex-sassign vKS/2 ->4
    3        <@> emptyavhv[$x:1,2] vK*/LVINTRO,ANONHASH,TARGMY ->4
    -           <0> ex-pushmark s ->3
    -        <0> ex-padsv sRM*/LVINTRO ->-

Author: richardleach

ext/Opcode/Opcode.pm

@@ -336,6 +336,7 @@ invert_opset function.
     warn die lineseq nextstate scope enter leave
 
     rv2cv anoncode prototype coreargs avhvswitch anonconst
+    emptyavhv
 
     entersub leavesub leavesublv return method method_named
     method_super method_redir method_redir_super

lib/B/Deparse.pm

@@ -14,7 +14,7 @@ use B qw(class main_root main_start main_cv svref_2object opnumber perlstring
 	 OPf_WANT OPf_WANT_VOID OPf_WANT_SCALAR OPf_WANT_LIST
 	 OPf_KIDS OPf_REF OPf_STACKED OPf_SPECIAL OPf_MOD OPf_PARENS
 	 OPpLVAL_INTRO OPpOUR_INTRO OPpENTERSUB_AMPER OPpSLICE OPpKVSLICE
-         OPpCONST_BARE
+         OPpCONST_BARE OPpEMPTYAVHV_IS_HV
 	 OPpTRANS_SQUASH OPpTRANS_DELETE OPpTRANS_COMPLEMENT OPpTARGET_MY
 	 OPpEXISTS_SUB OPpSORT_NUMERIC OPpSORT_INTEGER OPpREPEAT_DOLIST
 	 OPpSORT_REVERSE OPpMULTIDEREF_EXISTS OPpMULTIDEREF_DELETE
@@ -2785,6 +2785,21 @@ sub pp_anonlist {
 
 *pp_anonhash = \&pp_anonlist;
 
+sub pp_emptyavhv {
+    my $self = shift;
+    my ($op, $cx, $forbid_parens) = @_;
+    my $val = ($op->private & OPpEMPTYAVHV_IS_HV) ? '{}' : '[]';
+    if ($op->private & OPpTARGET_MY) {
+        my $targ = $op->targ;
+        my $var = $self->maybe_my($op, $cx, $self->padname($targ),
+                           $self->padname_sv($targ),
+                           $forbid_parens);
+        return $self->maybe_parens("$var = $val", $cx, 7);
+    } else {
+        return $val;
+    }
+}
+
 sub pp_refgen {
     my $self = shift;	
     my($op, $cx) = @_;