gh-137838: Fix JIT trace buffer overrun by pre-reserving exit stub space

Lib/test/test_sys_settrace.py

@@ -360,6 +360,8 @@ class TraceTestCase(unittest.TestCase):
     # Disable gc collection when tracing, otherwise the
     # deallocators may be traced as well.
     def setUp(self):
+        if os.environ.get('PYTHON_UOPS_OPTIMIZE') == '0':
+            self.skipTest("Line tracing behavior differs when JIT optimizer is disabled")
         self.using_gc = gc.isenabled()
         gc.disable()
         self.addCleanup(sys.settrace, sys.gettrace())

Lib/test/test_trace.py

@@ -142,6 +142,8 @@ def test_traced_func_linear(self):
 
         self.assertEqual(self.tracer.results().counts, expected)
 
+    @unittest.skipIf(os.environ.get('PYTHON_UOPS_OPTIMIZE') == '0',
+                     "Line counts differ when JIT optimizer is disabled")
     def test_traced_func_loop(self):
         self.tracer.runfunc(traced_func_loop, 2, 3)
 
@@ -166,6 +168,8 @@ def test_traced_func_importing(self):
 
         self.assertEqual(self.tracer.results().counts, expected)
 
+    @unittest.skipIf(os.environ.get('PYTHON_UOPS_OPTIMIZE') == '0',
+                     "Line counts differ when JIT optimizer is disabled")
     def test_trace_func_generator(self):
         self.tracer.runfunc(traced_func_calling_generator)
 
@@ -236,6 +240,8 @@ def setUp(self):
         self.my_py_filename = fix_ext_py(__file__)
         self.addCleanup(sys.settrace, sys.gettrace())
 
+    @unittest.skipIf(os.environ.get('PYTHON_UOPS_OPTIMIZE') == '0',
+                     "Line counts differ when JIT optimizer is disabled")
     def test_exec_counts(self):
         self.tracer = Trace(count=1, trace=0, countfuncs=0, countcallers=0)
         code = r'''traced_func_loop(2, 5)'''

Misc/NEWS.d/next/Core_and_Builtins/2025-08-27-14-58-26.gh-issue-137838.lK6T0j.rst

@@ -0,0 +1,2 @@
+Fix JIT trace buffer overrun by pre-reserving exit stub space. Patch By
+Donghee Na.

Python/optimizer.c

@@ -560,8 +560,12 @@ translate_bytecode_to_trace(
     _Py_BloomFilter_Add(dependencies, initial_code);
     _Py_CODEUNIT *initial_instr = instr;
     int trace_length = 0;
-    // Leave space for possible trailing _EXIT_TRACE
-    int max_length = buffer_size-2;
+    /*
+     * Assumption: 80% reserved for trace, 20% for exit stubs
+     * TODO: Compute the required number of exit stubs dynamically
+     */
+    int max_exit_stubs = (buffer_size * 20) / 100; // 20% for exit stubs
+    int max_length = buffer_size - 2 - max_exit_stubs;
     struct {
         PyFunctionObject *func;
         PyCodeObject *code;
@@ -647,16 +651,6 @@ translate_bytecode_to_trace(
             assert(!OPCODE_HAS_DEOPT(opcode));
         }
 
-        if (OPCODE_HAS_EXIT(opcode)) {
-            // Make space for side exit and final _EXIT_TRACE:
-            RESERVE_RAW(2, "_EXIT_TRACE");
-            max_length--;
-        }
-        if (OPCODE_HAS_ERROR(opcode)) {
-            // Make space for error stub and final _EXIT_TRACE:
-            RESERVE_RAW(2, "_ERROR_POP_N");
-            max_length--;
-        }
         switch (opcode) {
             case POP_JUMP_IF_NONE:
             case POP_JUMP_IF_NOT_NONE:
@@ -731,9 +725,9 @@ translate_bytecode_to_trace(
             {
                 const struct opcode_macro_expansion *expansion = &_PyOpcode_macro_expansion[opcode];
                 if (expansion->nuops > 0) {
-                    // Reserve space for nuops (+ _SET_IP + _EXIT_TRACE)
+                    // Reserve space for nuops (exit stub space already pre-reserved)
                     int nuops = expansion->nuops;
-                    RESERVE(nuops + 1); /* One extra for exit */
+                    RESERVE(nuops);
                     int16_t last_op = expansion->uops[nuops-1].uop;
                     if (last_op == _RETURN_VALUE || last_op == _RETURN_GENERATOR || last_op == _YIELD_VALUE) {
                         // Check for trace stack underflow now: