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GeezerVison is showing github.com — gh-152192: Fix JUMP_BACKWARD passing a truncated oparg to the jit tracer by deadlovelll · Pull Request #152382 · python/cpython

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deadlovelll commented Jun 27, 2026 •

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The _JIT op used oparg as the counter in the while loop, truncating it before reaching _PyJit_TryInitializeTracing.
Fixed it by using a temporary value.

For more details see gh-152192

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          pythongh-152192: Fix JUMP_BACKWARD passing a truncated oparg to the j…

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…it tracer

deadlovelll requested a review from markshannon as a code owner

June 27, 2026 10:13

bedevere-app Bot added the awaiting review label

Jun 27, 2026

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Jun 27, 2026

Wrong oparg is passed to trace recorder when JUMP_BACKWARD_JIT is preceded by EXTENDED_ARG #152192

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deadlovelll commented

Jun 27, 2026

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Lib/test/test_capi/test_opt.py

		ext, jb = next((p, i) for p, i in zip(instrs, instrs[1:])
		if i.opname == "JUMP_BACKWARD" and p.opname == "EXTENDED_ARG")

		f(TIER2_THRESHOLD + 1)

deadlovelll Jun 27, 2026

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I want to clarify a little bit about this test.

When the jit counter is enabled we enter into _JIT op with original oparg = 299
Then oparg truncates with buggy loop

while (oparg > 255) {
       oparg >>= 8
       insert_exec_at--;
}

oparg becomes 1, and we pass it to _PyJit_TryInitializeTracing

In _PyJit_TryInitializeTracing we store this broken oparg

tracer->prev_state.instr_oparg = oparg;

And call ENTER_TRACING()

In next calls code uses TRACE_RECORD which in turn calls _PyJit_translate_single_bytecode_to_trace. This is the point where shift exists

    // Rewind EXTENDED_ARG so that we see the whole thing.
    // We must point to the first EXTENDED_ARG when deopting.
    int oparg = tracer->prev_state.instr_oparg;
    int opcode = this_instr->op.code;
    int rewind_oparg = oparg;
    while (rewind_oparg > 255) {
        rewind_oparg >>= 8;
        target--;
    }

We get the broken oparg from tracer->prev_state.instr_oparg and the loop does not executes because 1 < 255. target does not decrements and the instruction is not correct (should be EXTENDED_ARG but actually JUMP_BACKWARD)

P.S: I may not be fully correct in the internals but the test catches the shift on the buggy build

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markshannon requested changes

Jun 29, 2026

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This looks good. Thanks for fixing this.

Can you link to the issue in the test, then I'll merge.

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Lib/test/test_capi/test_opt.py

		self.assertTrue(any((opcode, oparg, operand) == ("_LOAD_FAST_BORROW", 259, 0)
		for opcode, oparg, _, operand in list(ex)))

		def test_jump_backward_extended_arg(self):

markshannon Jun 29, 2026

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Can you add a comment that this is testing for #152192

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deadlovelll Jun 29, 2026

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Hello! Thank you for the review. Just pushed the comment note on test

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bedevere-app Bot added awaiting changes and removed awaiting review labels

Jun 29, 2026

bedevere-app Bot commented Jun 29, 2026

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A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

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          added comment note on test

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deadlovelll requested a review from markshannon

June 29, 2026 11:18

markshannon approved these changes

Jun 30, 2026

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Looks good, thanks

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bedevere-app Bot added awaiting merge and removed awaiting changes labels

Jun 30, 2026

markshannon merged commit 9e10f14 into python:main

Jun 30, 2026

80 of 82 checks passed

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bedevere-app Bot removed the awaiting merge label

Jun 30, 2026

maurycy added a commit to maurycy/cpython that referenced this pull request

Jul 1, 2026


          Merge branch 'main' into maurycy/bye-abba

da7d768

* main: (266 commits)
  pythongh-151626: Fix tests that fail when PYTHONPYCACHEPREFIX is set (pythonGH-151952)
  pythongh-152728: IDLE - move 3 toplevel fix_xyz functions to idlelb.util (python#152729)
  pythongh-152711: Add pythoninfo-build command to Platforms/Android (python#152713)
  pythongh-152715: Add pythoninfo-build command to Platforms/Apple (python#152716)
  pythongh-152433: Windows: enable mmapmodule for UWP (python#152473)
  pythongh-152433: Windows: use GetFileSizeEx instead of GetFileSize for memory mapped files (python#152383)
  pythonGH-81881: Raise `SpecialFileError` for sockets and devices in `shutil.copyfile` (python#142693)
  pythongh-152502: Detect the curses mouse interface and is_* methods portably (pythonGH-152705)
  pythongh-145857: Replace `DELETE_GLOBAL` with `PUSH_NULL; STORE_GLOBAL` (pythonGH-146314)
  pythongh-145854: Replace `DELETE_NAME` with `PUSH_NULL; STORE_NAME` (pythonGH-146006)
  pythongh-152680: Detect container/VM in test.pythoninfo (python#152668)
  pythongh-152682: Fix NULL dereference on OOM in `symtable_visit_type_param_bound_or_default` (python#152684)
  pythongh-151881: Skip tk_inactive negativity check on Windows (pythonGH-152683)
  pythongh-152546: Refactor `mappingproxy.__new__` to use `PyDictProxy_New` (python#152547)
  pythongh-151126: Fix a possible crash during the startup with no memory under `Py_STACKREF_DEBUG` (python#152478)
  pythongh-152635: Raise MemoryError when the lock allocation fails in `_interpchannels.create()` (python#152642)
  pythongh-151029: Fix `test_remote_exec_deleted_static_executable` on static installed builds (pythonGH-152653)
  pythongh-121249: Deprecate using F/D type codes in the struct module (python#152309)
  pythongh-152192: Fix JUMP_BACKWARD passing a truncated oparg to the jit tracer (pythonGH-152382)
  Don't require the `_test{internal}capi` modules in `test_monitoring.py` (python#152311)
  ...

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deadlovelll commented Jun 27, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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deadlovelll commented Jun 27, 2026 •

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