gh-92228: disable the compiler's 'small exit block inlining' optimization for blocks that have a line number

Lib/test/test_dis.py

@@ -360,13 +360,13 @@ def bug42562():
     -->    BINARY_OP               11 (/)
            POP_TOP
 
-%3d        LOAD_FAST_CHECK          1 (tb)
+%3d     >> LOAD_FAST_CHECK          1 (tb)
            RETURN_VALUE
         >> PUSH_EXC_INFO
 
 %3d        LOAD_GLOBAL              0 (Exception)
            CHECK_EXC_MATCH
-           POP_JUMP_FORWARD_IF_FALSE    23 (to 82)
+           POP_JUMP_FORWARD_IF_FALSE    22 (to 80)
            STORE_FAST               0 (e)
 
 %3d        LOAD_FAST                0 (e)
@@ -376,9 +376,7 @@ def bug42562():
            LOAD_CONST               0 (None)
            STORE_FAST               0 (e)
            DELETE_FAST              0 (e)
-
-%3d        LOAD_FAST                1 (tb)
-           RETURN_VALUE
+           JUMP_BACKWARD           29 (to 14)
         >> LOAD_CONST               0 (None)
            STORE_FAST               0 (e)
            DELETE_FAST              0 (e)
@@ -396,7 +394,6 @@ def bug42562():
        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)
 
 def _fstring(a, b, c, d):
@@ -443,7 +440,7 @@ def _with(c):
            CALL                     2
            POP_TOP
 
-%3d        LOAD_CONST               2 (2)
+%3d     >> LOAD_CONST               2 (2)
            STORE_FAST               2 (y)
            LOAD_CONST               0 (None)
            RETURN_VALUE
@@ -456,11 +453,7 @@ def _with(c):
            POP_EXCEPT
            POP_TOP
            POP_TOP
-
-%3d        LOAD_CONST               2 (2)
-           STORE_FAST               2 (y)
-           LOAD_CONST               0 (None)
-           RETURN_VALUE
+           JUMP_BACKWARD           13 (to 30)
         >> COPY                     3
            POP_EXCEPT
            RERAISE                  1
@@ -472,7 +465,6 @@ def _with(c):
        _with.__code__.co_firstlineno + 1,
        _with.__code__.co_firstlineno + 3,
        _with.__code__.co_firstlineno + 1,
-       _with.__code__.co_firstlineno + 3,
        )
 
 async def _asyncwith(c):
@@ -510,7 +502,7 @@ async def _asyncwith(c):
            JUMP_BACKWARD_NO_INTERRUPT     4 (to 48)
         >> POP_TOP
 
-%3d        LOAD_CONST               2 (2)
+%3d     >> LOAD_CONST               2 (2)
            STORE_FAST               2 (y)
            LOAD_CONST               0 (None)
            RETURN_VALUE
@@ -529,11 +521,7 @@ async def _asyncwith(c):
            POP_EXCEPT
            POP_TOP
            POP_TOP
-
-%3d        LOAD_CONST               2 (2)
-           STORE_FAST               2 (y)
-           LOAD_CONST               0 (None)
-           RETURN_VALUE
+           JUMP_BACKWARD           19 (to 58)
         >> COPY                     3
            POP_EXCEPT
            RERAISE                  1
@@ -545,7 +533,6 @@ async def _asyncwith(c):
        _asyncwith.__code__.co_firstlineno + 1,
        _asyncwith.__code__.co_firstlineno + 3,
        _asyncwith.__code__.co_firstlineno + 1,
-       _asyncwith.__code__.co_firstlineno + 3,
        )
 
 

Lib/test/test_peepholer.py

@@ -345,6 +345,8 @@ 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 starts and the following code block begins.

Python/compile.c

@@ -9224,6 +9224,16 @@ optimize_basic_block(PyObject *const_cache, 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_loc.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.
  */
@@ -9240,6 +9250,10 @@ extend_block(basicblock *bb) {
     }
     if (basicblock_exits_scope(last->i_target) && 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 = basicblock_next_instr(bb);