bpo-46724: Use JUMP_ABSOLUTE for all backward jumps. (GH-31326)

* Make sure all backward jumps use JUMP_ABSOLUTE.

* Add news.

* Fix up news item.

* Make test use consistent style.

Author: markshannon

Lib/test/test_compile.py

@@ -1008,6 +1008,16 @@ def if_else_break():
             elif instr.opname in HANDLED_JUMPS:
                 self.assertNotEqual(instr.arg, (line + 1)*INSTR_SIZE)
 
+    def test_no_wraparound_jump(self):
+        # See https://bugs.python.org/issue46724
+
+        def while_not_chained(a, b, c):
+            while not (a < b < c):
+                pass
+
+        for instr in dis.Bytecode(while_not_chained):
+            self.assertNotEqual(instr.opname, "EXTENDED_ARG")
+
 @requires_debug_ranges()
 class TestSourcePositions(unittest.TestCase):
     # Ensure that compiled code snippets have correct line and column numbers

Misc/NEWS.d/next/Core and Builtins/2022-02-14-14-44-06.bpo-46724.jym_K6.rst

@@ -0,0 +1,2 @@
+Make sure that all backwards jumps use the ``JUMP_ABSOLUTE`` instruction, rather
+than ``JUMP_FORWARD`` with an argument of ``(2**32)+offset``.

Python/compile.c

@@ -7534,6 +7534,11 @@ normalize_jumps(struct assembler *a)
                 last->i_opcode = JUMP_FORWARD;
             }
         }
+        if (last->i_opcode == JUMP_FORWARD) {
+            if (last->i_target->b_visited == 1) {
+                last->i_opcode = JUMP_ABSOLUTE;
+            }
+        }
     }
 }