iter: new Invalid for range iterator (%d .. %d) error

we dont support reverse loops with constant ints (9..0).
now we exit earlier, already at compile-time.

Author: Reini Urban

op.c

@@ -7933,8 +7933,9 @@ Perl_newFOROP(pTHX_ I32 flags, OP *sv, OP *expr, OP *block, OP *cont)
             && SvIOK(leftsv = cSVOPx_sv(left))
             && SvIOK(rightsv = cSVOPx_sv(right)))
         {
-            if (SvIV(rightsv) - SvIV(leftsv) < 0)
-                DIE("Invalid for range (%"IVdf"..%"IVdf")", SvIV(leftsv), SvIV(rightsv));
+            if (UNLIKELY(SvIV(rightsv) < SvIV(leftsv)))
+                DIE(aTHX_ "Invalid for range iterator (%"IVdf" .. %"IVdf")",
+                    SvIV(leftsv), SvIV(rightsv));
             /* TODO: unroll loop for small constant ranges, if the body is not too big */
             if (SvIV(rightsv)-SvIV(leftsv) <= PERL_MAX_UNROLL_LOOP_COUNT) {
                 DEBUG_kv(Perl_deb(aTHX_ "TODO unroll loop (%"IVdf"..%"IVdf")\n",

pod/perlcdelta.pod

@@ -35,6 +35,18 @@ L</Selected Bug Fixes> section.
 
 =item *
 
+for loops got several enhancements:
+
+new special B<iter_ary> C<for (@ary)> and B<iter_lazyiv> C<for (0..9)>
+ops to avoid a run-time switch in the generic iter op.
+
+more aelem_u optimizations, less run-time out of bounds checks for
+shaped arrays in loops. E.g. in C<my @a[5]; $a[$_] for (0..4);> the
+compilers knows that the max index for C<@a> will be C<4>, which is
+within the allowed shape of C<@a>.
+
+=item *
+
 The C<multideref> OP has a new C<MDEREF_INDEX_uoob> flag.  This is
 used for unchecked out-of-bounds checks for arrays, to use the
 previous AvSHAPED array optimizations (aelem_u, aelemfast_lex_u) or
@@ -79,7 +91,34 @@ dual-life modules would have a F<Changes> file that could be cribbed.
 
 =item *
 
-L<XXX> has been upgraded from version A.xx to B.yy.
+L<Opcode> has been upgraded from version 1.34c to 1.35c, with the new
+iter_ary and iter_lazyiv ops.
+
+=back
+
+=head1 Diagnostics
+
+The following additions or changes have been made to diagnostic output,
+including warnings and fatal error messages.  For the complete list of
+diagnostic messages, see L<perldiag>.
+
+=head2 New Diagnostics
+
+=head3 New Errors
+
+=over 4
+
+=item Invalid for range iterator (%d .. %d)
+
+(F) A range with constant integers as a for loop cannot be reversed.
+
+We check now at compile-time that a range with C<for> loops with
+constant integers is incremental, the 2nd number must be higher. We
+don't support reverse loops with ranges, i.e. C<for (9..0)> is
+invalid, but C<for (0..$#ary)> with C<$#ary> being C<-1> is valid.
+
+Reverse constant strings ranges are still valid and lead to an empty
+loop.  i.e. C<for ('z'..'a')> is currently valid.
 
 =back
 

pod/perldiag.pod

@@ -2939,6 +2939,10 @@ instead, except within S<C<(?[   ])>>, where it is a fatal error.
 The S<<-- HERE> shows whereabouts in the regular expression the
 escape was discovered.
 
+=item Invalid for range iterator (%d .. %d)
+
+(F) A range with constant integers as a for loop cannot be reversed.
+
 =item Invalid function type %s
 
 (F) The declared type for the function must be a coretype or an existing

pp_ctl.c

@@ -2204,11 +2204,12 @@ PP(pp_enteriter)
 		cx->blk_loop.state_u.lazyiv.end = SvIV_nomg(right);
 	    }
 	    else {
+                SV *left =  newSVsv(sv);
 		cx->cx_type |= CXt_LOOP_LAZYSV;
-		cx->blk_loop.state_u.lazysv.cur = newSVsv(sv);
+		cx->blk_loop.state_u.lazysv.cur = left;
 		cx->blk_loop.state_u.lazysv.end = right;
 		SvREFCNT_inc_simple_void_NN(right);
-		(void) SvPV_force_nolen(cx->blk_loop.state_u.lazysv.cur);
+		(void) SvPV_force_nolen(left);
 		/* This will do the upgrade to SVt_PV, and warn if the value
 		   is uninitialised.  */
 		(void) SvPV_nolen_const(right);
@@ -2218,7 +2219,7 @@ PP(pp_enteriter)
 		    SvREFCNT_dec(right);
 		    cx->blk_loop.state_u.lazysv.end = &PL_sv_no;
 		}
-	    }
+            }
 	}
 	else /* SvTYPE(maybe_ary) == SVt_PVAV */ {
             /* for (@array) {} */

pp_hot.c

@@ -2747,6 +2747,7 @@ PP(pp_iter_lazyiv)
     itersvp = CxITERVAR(cx);
     assert(itersvp);
 
+    /* Note: no reverse support. (9..0) */
     cur = cx->blk_loop.state_u.lazyiv.cur;
     if (UNLIKELY(cur > cx->blk_loop.state_u.lazyiv.end)) {
         assert(PL_stack_sp < PL_stack_max);

t/porting/known_pod_issues.dat

@@ -336,8 +336,7 @@ pod/cperl.pod	Verbatim line length including indents exceeds 79 by	8
 pod/perl.pod	Verbatim line length including indents exceeds 79 by	8
 pod/perl5222cdelta.pod	? Should you be using F<...> or maybe L<...> instead of	1
 pod/perl5240cdelta.pod	Apparent broken link	1
-pod/perl5250cdelta.pod	Apparent internal link is missing its forward slash	4
-pod/perl5250cdelta.pod	unresolved internal link	1
+pod/perl5250cdelta.pod	Apparent internal link is missing its forward slash	3
 pod/perlandroid.pod	Verbatim line length including indents exceeds 79 by	3
 pod/perlapi.pod	Verbatim line length including indents exceeds 79 by	2
 pod/perlbook.pod	Verbatim line length including indents exceeds 79 by	1