optimise away OP_KEYS op in scalar/void context

In something like

    if (keys %h) { ... }

the 'keys %h' is implemented as the op sequences

    gv[*h] s
    rv2hv lKRM/1
    keys[t2] sK/1
or
    padhv[%h:1,6] lRM
    keys[t2] sK/1

It turns out that (%h) in scalar and void context now behaves very
similarly to (keys %h) (except that it reset the iterator), so in these
cases, convert the two ops

    rv2hv/padhv, keys

into the single op

    rv2hv/padhv

with a private flag indicating that the op is handling the 'keys' action
by itself.

As well as one less op to execute, this brings the boolean-context
optimisation already present in padhv/rv2sv to keys. So

    if (keys %h) { ... }

is no longer slower than

    if (%h) { ... }

1 files changed, 28 insertions, 5 deletions

diff --git a/pp_hot.c b/pp_hot.c
index 62a51f8ce1..0e5a541c11 100644
--- a/pp_hot.c
+++ b/pp_hot.c
@@ -1035,20 +1035,43 @@ PP(pp_rv2av)
 	    SETi(maxarg);
 	}
     } else {
+        bool tied;
 	/* The guts of pp_rv2hv  */
 	if (gimme == G_ARRAY) { /* array wanted */
 	    *PL_stack_sp = sv;
 	    return Perl_do_kv(aTHX);
 	}
-	else if ((PL_op->op_private & OPpTRUEBOOL
+
+        if (PL_op->op_private & OPpRV2HV_ISKEYS)
+            /* 'keys %h' masquerading as '%h': reset iterator */
+            (void)hv_iterinit(MUTABLE_HV(sv));
+
+        tied = SvRMAGICAL(sv) && mg_find(sv, PERL_MAGIC_tied);
+
+	if ( (   PL_op->op_private & OPpTRUEBOOL
 	      || (  PL_op->op_private & OPpMAYBE_TRUEBOOL
-		 && block_gimme() == G_VOID  ))
-	      && (!SvRMAGICAL(sv) || !mg_find(sv, PERL_MAGIC_tied)))
+		 && block_gimme() == G_VOID)
+             )
+	     && !tied)
 	    SETs(HvUSEDKEYS(MUTABLE_HV(sv)) ? &PL_sv_yes : &PL_sv_zero);
 	else if (gimme == G_SCALAR) {
 	    dTARG;
-	    TARG = Perl_hv_scalar(aTHX_ MUTABLE_HV(sv));
-	    SETTARG;
+            if (PL_op->op_private & OPpRV2HV_ISKEYS) {
+                IV i;
+                if (tied) {
+                    i = 0;
+                    while (hv_iternext(MUTABLE_HV(sv)))
+                        i++;
+                }
+                else
+                    i = HvUSEDKEYS(MUTABLE_HV(sv));
+                (void)POPs;
+                mPUSHi(i);
+            }
+            else {
+                TARG = Perl_hv_scalar(aTHX_ MUTABLE_HV(sv));
+                SETTARG;
+            }
 	}
     }
     RETURN;