add boolean context support to several ops

For some ops which return integer values and which have a reasonable
likelihood of being used in a boolean context, set the OPpTRUEBOOL
flag on the op as appropriate, and at runtime return &PL_sv_yes /
&PL_sv_zero rather than an integer value.

This is especially beneficial where the op doesn't have a targ, so has
to create a mortal SV to return the integer value.

Similarly, its a win where it may be expensive to calculate an integer
return value, such as pos() or length() converting between byte and char
offset.

Ops done:

    OP_SUBST
    OP_AASSIGN
    OP_POS
    OP_LENGTH
    OP_GREPWHILE

1 files changed, 18 insertions, 4 deletions

diff --git a/pp.c b/pp.c
index 24326b8d63..20fd474b2e 100644
--- a/pp.c
+++ b/pp.c
@@ -466,11 +466,15 @@ PP(pp_pos)
     else {
 	    const MAGIC * const mg = mg_find_mglob(sv);
 	    if (mg && mg->mg_len != -1) {
-		dTARGET;
 		STRLEN i = mg->mg_len;
-		if (mg->mg_flags & MGf_BYTES && DO_UTF8(sv))
-		    i = sv_pos_b2u_flags(sv, i, SV_GMAGIC|SV_CONST_RETURN);
-		SETu(i);
+                if (PL_op->op_private & OPpTRUEBOOL)
+                    SETs(i ? &PL_sv_yes : &PL_sv_zero);
+                else {
+                    dTARGET;
+                    if (mg->mg_flags & MGf_BYTES && DO_UTF8(sv))
+                        i = sv_pos_b2u_flags(sv, i, SV_GMAGIC|SV_CONST_RETURN);
+                    SETu(i);
+                }
 		return NORMAL;
 	    }
 	    SETs(&PL_sv_undef);
@@ -3258,6 +3262,11 @@ PP(pp_length)
 	if (!IN_BYTES) { /* reread to avoid using an C auto/register */
             if ((SvFLAGS(sv) & (SVf_POK|SVf_UTF8)) == SVf_POK)
                 goto simple_pv;
+            if ( SvPOK(sv) && (PL_op->op_private & OPpTRUEBOOL)) {
+                /* no need to convert from bytes to chars */
+                len = SvCUR(sv);
+                goto return_bool;
+            }
 	    len = sv_len_utf8_nomg(sv);
         }
 	else {
@@ -3265,6 +3274,11 @@ PP(pp_length)
             if (SvPOK_nog(sv)) {
               simple_pv:
                 len = SvCUR(sv);
+                if (PL_op->op_private & OPpTRUEBOOL) {
+                  return_bool:
+                    SETs(len ? &PL_sv_yes : &PL_sv_zero);
+                    return NORMAL;
+                }
             }
             else {
                 (void)sv_2pv_flags(sv, &len, 0|SV_CONST_RETURN);