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

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".

5 files changed, 33 insertions, 20 deletions

diff --git a/Lib/test/test_dis.py b/Lib/test/test_dis.py
index 2560048..440e4ed 100644
--- a/Lib/test/test_dis.py
+++ b/Lib/test/test_dis.py
@@ -360,13 +360,13 @@ dis_traceback = """\
     -->    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 @@ dis_traceback = """\
            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 @@ ExceptionTable:
        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 @@ dis_with = """\
            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 @@ dis_with = """\
            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 @@ ExceptionTable:
        _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 @@ dis_asyncwith = """\
            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 @@ dis_asyncwith = """\
            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 @@ ExceptionTable:
        _asyncwith.__code__.co_firstlineno + 1,
        _asyncwith.__code__.co_firstlineno + 3,
        _asyncwith.__code__.co_firstlineno + 1,
-       _asyncwith.__code__.co_firstlineno + 3,
        )
 
 
diff --git a/Lib/test/test_peepholer.py b/Lib/test/test_peepholer.py
index 2c3b1ab..e03c42c 100644
--- a/Lib/test/test_peepholer.py
+++ b/Lib/test/test_peepholer.py
@@ -345,6 +345,8 @@ class TestTranforms(BytecodeTestCase):
         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):
diff --git a/Lib/test/test_sys_settrace.py b/Lib/test/test_sys_settrace.py
index 7ec290d..f03b03e 100644
--- a/Lib/test/test_sys_settrace.py
+++ b/Lib/test/test_sys_settrace.py
@@ -2042,6 +2042,15 @@ class JumpTestCase(unittest.TestCase):
             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)
diff --git a/Misc/NEWS.d/next/Core and Builtins/2022-07-06-14-02-26.gh-issue-92228.44Cbly.rst b/Misc/NEWS.d/next/Core and Builtins/2022-07-06-14-02-26.gh-issue-92228.44Cbly.rst
new file mode 100644
index 0000000..458ad89
--- /dev/null
+++ b/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.
diff --git a/Python/compile.c b/Python/compile.c
index 7717689..f36a6e8 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -9224,6 +9224,16 @@ error:
     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);