[3.11] gh-92228: disable the compiler's 'small exit block inlining' optimization for blocks that have a line number (GH-94592) (GH-94643)

Inlining of code that corresponds to source code lines, can make it hard to distinguish later between code which is only reachable from except handlers, and that which is reachable in normal control flow. This caused problems with the debugger's jump feature.

This PR turns off the inlining optimisation for code which has line numbers. We still inline things like the implicit "return None"..
(cherry picked from commit bde06e1b83)

Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com>

Lib/test/test_dis.py

@@ -366,14 +366,12 @@ def bug42562():
            LOAD_CONST               2 (0)
     -->    BINARY_OP               11 (/)
            POP_TOP
-
-%3d        LOAD_FAST                1 (tb)
-           RETURN_VALUE
+           JUMP_FORWARD            30 (to 76)
         >> PUSH_EXC_INFO
 
 %3d        LOAD_GLOBAL              0 (Exception)
            CHECK_EXC_MATCH
-           POP_JUMP_FORWARD_IF_FALSE    18 (to 72)
+           POP_JUMP_FORWARD_IF_FALSE    17 (to 68)
            STORE_FAST               0 (e)
 
 %3d        LOAD_FAST                0 (e)
@@ -383,9 +381,7 @@ def bug42562():
            LOAD_CONST               0 (None)
            STORE_FAST               0 (e)
            DELETE_FAST              0 (e)
-
-%3d        LOAD_FAST                1 (tb)
-           RETURN_VALUE
+           JUMP_FORWARD             8 (to 76)
         >> LOAD_CONST               0 (None)
            STORE_FAST               0 (e)
            DELETE_FAST              0 (e)
@@ -395,15 +391,17 @@ def bug42562():
         >> COPY                     3
            POP_EXCEPT
            RERAISE                  1
+
+%3d     >> LOAD_FAST                1 (tb)
+           RETURN_VALUE
 ExceptionTable:
 """ % (TRACEBACK_CODE.co_firstlineno,
        TRACEBACK_CODE.co_firstlineno + 1,
        TRACEBACK_CODE.co_firstlineno + 2,
-       TRACEBACK_CODE.co_firstlineno + 5,
        TRACEBACK_CODE.co_firstlineno + 3,
        TRACEBACK_CODE.co_firstlineno + 4,
-       TRACEBACK_CODE.co_firstlineno + 5,
-       TRACEBACK_CODE.co_firstlineno + 3)
+       TRACEBACK_CODE.co_firstlineno + 3,
+       TRACEBACK_CODE.co_firstlineno + 5)
 
 def _fstring(a, b, c, d):
     return f'{a} {b:4} {c!r} {d!r:4}'

Lib/test/test_peepholer.py

@@ -344,6 +344,8 @@ def f(x):
         self.assertEqual(len(returns), 1)
         self.check_lnotab(f)
 
+    @unittest.skip("Following gh-92228 the return has two predecessors "
+                   "and that prevents jump elimination.")
     def test_elim_jump_to_return(self):
         # JUMP_FORWARD to RETURN -->  RETURN
         def f(cond, true_value, false_value):

Lib/test/test_sys_settrace.py

@@ -2042,6 +2042,15 @@ def test_no_jump_within_except_block(output):
             output.append(6)
         output.append(7)
 
+    @jump_test(6, 1, [1, 5, 1, 5])
+    def test_jump_over_try_except(output):
+        output.append(1)
+        try:
+            1 / 0
+        except ZeroDivisionError as e:
+            output.append(5)
+        x = 42  # has to be a two-instruction block
+
     @jump_test(2, 4, [1, 4, 5, -4])
     def test_jump_across_with(output):
         output.append(1)

Misc/NEWS.d/next/Core and Builtins/2022-07-06-14-02-26.gh-issue-92228.44Cbly.rst

@@ -0,0 +1 @@
+Disable the compiler's inline-small-exit-blocks optimization for exit blocks that are associated with source code lines. This fixes a bug where the debugger cannot tell where an exception handler ends and the following code block begins.

Python/compile.c

@@ -8976,6 +8976,16 @@ optimize_basic_block(struct compiler *c, basicblock *bb, PyObject *consts)
     return -1;
 }
 
+static bool
+basicblock_has_lineno(const basicblock *bb) {
+    for (int i = 0; i < bb->b_iused; i++) {
+        if (bb->b_instr[i].i_lineno > 0) {
+            return true;
+        }
+    }
+    return false;
+}
+
 /* If this block ends with an unconditional jump to an exit block,
  * then remove the jump and extend this block with the target.
  */
@@ -8992,6 +9002,10 @@ extend_block(basicblock *bb) {
     }
     if (last->i_target->b_exit && last->i_target->b_iused <= MAX_COPY_SIZE) {
         basicblock *to_copy = last->i_target;
+        if (basicblock_has_lineno(to_copy)) {
+            /* copy only blocks without line number (like implicit 'return None's) */
+            return 0;
+        }
         last->i_opcode = NOP;
         for (int i = 0; i < to_copy->b_iused; i++) {
             int index = compiler_next_instr(bb);