Perl_op_convert_list: Avoid unnecessary processing of CONST OPs

[richardleach]

When Perl_op_convert_list is called to stringify a CONST OP, a fair amount
of activity occurs in order to return either an unchanged (if called without the
OPf_FOLDED flag) or only slightly changed (with OPf_FOLDED) OP.

In these commits, any changes are done directly and the CONST OP returned
without wrapping it in a new list OP and constant-folding it back again.

Please note: This almost certainly should be a defer-next-dev PR!

[tonycoz]

The change looks reasonable, but the added folding code doesn't execute much, from coverage testing against make test_harness:

 13511958: 5535:    if (type == OP_STRINGIFY && OP_TYPE_IS(o, OP_CONST) ) {
        -: 5536:        /* Don't wrap a single CONST in a list, process that list,
        -: 5537:         * then constant fold the list back to the starting OP. */
        -: 5538:        assert(!OpSIBLING(o));
        -: 5539:
  2208678: 5540:        if (flags & OPf_FOLDED) {
      167: 5541:            o->op_folded = 1;
      167: 5542:            SV *sv = cSVOPx_sv(o);
        -: 5543:
      167: 5544:            if (SvPOK(sv))
      165: 5545:                SvFLAGS(sv) |= SVs_PADTMP;
        -: 5546:            else {
        2: 5547:                SV *newsv = newSV_type(SVt_PV);
        2: 5548:                SvFLAGS(newsv) |= SVs_PADTMP;
        2: 5549:                sv_copypv_nomg(newsv, sv);
        2: 5550:                SvFLAGS(newsv) |= (SVf_READONLY|SVf_PROTECT);
        2: 5551:                SvREFCNT_dec_NN(sv);
        -: 5552:#ifdef USE_ITHREADS
        -: 5553:                if (cSVOPx(o)->op_sv) {
        -: 5554:                    cSVOPx(o)->op_sv = newsv;
        -: 5555:                } else {
        -: 5556:                    PAD_SVl((o)->op_targ) = newsv;
        -: 5557:                }
        -: 5558:#else
        2: 5559:                cSVOPx(o)->op_sv = newsv;
        -: 5560:#endif
        -: 5561:            }
        -: 5562:        }
  2208678: 5563:        return o;
        -: 5564:
        -: 5565:    }
 11303280: 5566:    if (!o || o->op_type != OP_LIST)

[richardleach]

Thanks for looking at this, @tonycoz.

but the added folding code doesn't execute much

In that case, I'll remove the folding code and add in an extra comment about its absence.

How did you generate that coverage report please?

[richardleach]

I'll remove the folding code and add in an extra comment about its absence.

Now done.

[tonycoz]

How did you generate that coverage report please?

I built perl following the instructions under "GCC gcov profiling" in perlhacktips, then:

• make test_harness
• gcov op.c