fix sort optimizer to not hang inside loops

p4raw-id: //depot/perl@5815

Author: Gurusamy Sarathy

op.c

@@ -5999,6 +5999,12 @@ Perl_ck_sort(pTHX_ OP *o)
 		    for (k = kLISTOP->op_first->op_next; k; k = k->op_next) {
 			if (k->op_next == kid)
 			    k->op_next = 0;
+			/* don't descend into loops */
+			else if (k->op_type == OP_ENTERLOOP
+				 || k->op_type == OP_ENTERITER)
+			{
+			    k = cLOOPx(k)->op_lastop;
+			}
 		    }
 		}
 		else

t/op/sort.t

@@ -13,6 +13,15 @@ print "1..49\n";
   $ENV{PERL_DESTRUCT_LEVEL} = 0 unless $ENV{PERL_DESTRUCT_LEVEL} > 3;
 }
 
+# these shouldn't hang
+{
+    no warnings;
+    sort { for ($_ = 0;; $_++) {} } @a;
+    sort { while(1) {}            } @a;
+    sort { while(1) { last; }     } @a;
+    sort { while(0) { last; }     } @a;
+}
+
 sub Backwards { $a lt $b ? 1 : $a gt $b ? -1 : 0 }
 sub Backwards_stacked($$) { my($a,$b) = @_; $a lt $b ? 1 : $a gt $b ? -1 : 0 }