pp_subst PMOP (pp_hot.c)
Returns op_next or cPMOP->op_pmreplroot.

pp_substcont LOGOP (pp_ctl.c)
Gets PMOP pm from cLOGOP->op_other.
Returns pm->op_next or pm->op_pmreplstart.

pmreplroot is pp_substcont (I think).

sub pp_subst {
    ...
    my $sym = doop($op);
    runtime sprintf("if (op == ((PMOP*)(%s))->op_pmreplroot) goto %s;",
		    $sym, label($op->op_pmreplroot));
    push(@bblock_todo, $op->pmreplroot);
    ...
}

sub pp_substcont {
    ...
    doop($op);
    my $pmop = op->other;
    my $restartsym = objsym($pmop->pmreplstart);
    runtime sprintf("if (op == ((PMOP*)(%s))->op_pmreplstart) goto %s;",
		    $restartsym, label($pmop->pmreplstart));
    return $pmop->next;
}