`test_capi.test_opt.test_call_len_known_length` fails on JIT+debug builds

[ashm-dev]

Bug report

Bug description:

Bug Description

The test_capi test suite fails when CPython is built from the main branch with debug, optimizations (PGO), LTO, and the experimental JIT enabled.

Steps to Reproduce

1. Clone the cpython repository.
2. Configure the build with the following flags:

   ./configure --with-pydebug --enable-optimizations --with-lto --enable-experimental-jit

3. Build and run tests:

   sudo make -j"$(nproc)" test

Expected Behavior

All tests should pass.

Actual Behavior

The test run fails with one failed test: test_capi.
The specific failing test is test_call_len_known_length within test.test_capi.test_opt.TestUopsOptimization.

Test Output

sudo_make_j_nproc_test.txt

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-139393: fix _CALL_LEN JIT tests for tuples #139394

[picnixz]

Does it actually fail with just JIT + debug? or does it fail for JIT+LTO+PGO+debug (or maybe just LTO+PGO+debug) in particular? Stated otherwise, which feature is necessary for the test to fail?

[ashm-dev]

@picnixz I've completed testing with all requested configurations. Here is the summary of the results:

1. --with-pydebug --enable-optimizations --with-lto --enable-experimental-jit
   (debug + LTO + PGO + JIT)

   • Result: test_capi fails.

2. --with-pydebug --enable-experimental-jit
   (debug + JIT)

   • Result: test_capi passes, but test_gdb fails.

3. --with-pydebug --enable-optimizations --with-lto
   (debug + LTO + PGO)

   • Result: All tests pass successfully.

This confirms that the test_capi failure is specifically triggered by the combination of the experimental JIT and the optimizations (--enable-optimizations --with-lto).

Neither JIT alone nor the optimizations alone (in a debug build) cause test_capi to fail.

Logs:

pydebug_pgo_lto_sudo_make_j_nproc_test.txt

pydebug_jit_sudo_make_j_nproc_test.txt

[picnixz]

Thank you. I don't think LTO would be responsible for that, but can you also check debug+JIT+PGO?

[ashm-dev]

You were right! I've run the build with debug+JIT+PGO (by using ./configure --with-pydebug --enable-optimizations --enable-experimental-jit), and the test fails in the exact same way.

pydebug_jit_pgo_sudo_make_j_nproc_test.txt

[ashm-dev]

@picnixz I did additional testing with the debug+ LTO+JIT configuration (without PGO) to verify your assumption that LTO is not the cause.

Results for ./configure --with-pydebug --with-lto --enable-experimental-jit:

• test_capi also crashes with the same AssertionError error: '_CALL_LEN' not found.
• In addition, the test_gdb tests also crash in this build (test.test_gdb.test_backtrace, test.test_gdb.test_misc, test.test_gdb.test_pretty_print).

This clarifies the overall picture. It seems that your assumption that only PGO is the problem was not entirely correct. The error in test_capi is reproduced when combining JIT with any of the advanced optimizations (LTO or PGO).

pydebug_jit_lto_sudo_make_j_nproc_test.txt

[picnixz]

Disclaimer: I'm using clang-18 for the JIT instead of clang-19 so some attributes are not valid but it shouldn't change much I guess.

I think I know the cause. The test assumes that the length is >= _PY_NSMALLPOSINTS. But we recently updated _PY_NSMALLPOSINTS to be 1025 instead of 256 (in 7ce25ed). To be precise, I think we hit the following check:

PyObject *temp = PyLong_FromSsize_t(tuple_length);
if (temp == NULL) {
    goto error;
}
if (_Py_IsImmortal(temp)) {
    REPLACE_OP(this_instr, _POP_CALL_ONE_LOAD_CONST_INLINE_BORROW,
               0, (uintptr_t)temp);
}

For a small tuple length, we update the instruction. However, I can confirm that debug + JIT (no optimizations) also trigger the issue with clang-18. Can you verify once again that you have a clean build?