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GeezerVison is showing github.com — gh-152240: Fix test_c_stack_unwind on Linux LoongArch builds by yzewei · Pull Request #152241 · python/cpython

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yzewei commented Jun 26, 2026 •

edited by bedevere-app Bot

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Fix test_c_stack_unwind on Linux LoongArch builds.

This change teaches _testinternalcapi's manual frame-pointer unwinder about
the LoongArch frame layout. On LoongArch, the frame pointer is the caller's
stack pointer; the previous frame pointer is at fp[-2], and the return
address is at fp[-1].

It also adds -funwind-tables for Linux LoongArch clang builds. clang/LoongArch
emits .debug_frame but not runtime .eh_frame unwind tables by default, so
glibc backtrace() cannot unwind through CPython frames unless unwind tables
are requested.

Validated on Linux LoongArch64:

GCC build:

../cpython/configure --with-pydebug CC=gcc
make -j8
PYTHON_JIT=0 ./python -m test -v test_c_stack_unwind

clang build with clang 20.1.8:

../cpython/configure --with-pydebug CC=clang
make -j8
PYTHON_JIT=0 ./python -m test -v test_c_stack_unwind

Also verified that -funwind-tables makes CPython C objects contain
.eh_frame, and the GNU backtrace test passes.

Issue: test_c_stack_unwind fails on Linux LoongArch builds #152240

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yzewei requested review from AA-Turner, corona10, emmatyping, erlend-aasland and itamaro as code owners

June 26, 2026 01:57

python-cla-bot Bot commented Jun 26, 2026 •

edited

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bedevere-app Bot mentioned this pull request

Jun 26, 2026

test_c_stack_unwind fails on Linux LoongArch builds #152240

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bedevere-app Bot added the awaiting review label

Jun 26, 2026

yzewei commented Jun 26, 2026

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Fix log:

Gcc:

PYTHON_JIT=0 ./python -m test -v test_c_stack_unwind
== CPython 3.16.0a0 (heads/main-dirty:a00464bc33) [GCC 15.2.0 20250808]
== Linux-6.18.16-main-16k-loongarch64-with-glibc2.42 little-endian
== Python build: release
== cwd: /mnt/build-cpython-main-gcc-release/build/test_python_worker_480813æ
== CPU count: 8
== encodings: locale=UTF-8 FS=utf-8
== resources: all test resources are disabled, use -u option to unskip tests

Using random seed: 4035734376
0:00:00 load avg: 0.08 mem: 27.8 MiB Run 1 test sequentially in a single process
0:00:00 load avg: 0.08 mem: 27.8 MiB [1/1] test_c_stack_unwind
test_gnu_backtrace_jit_frames_disappear_after_executor_free (test.test_c_stack_unwind.GnuBacktraceUnwindTests.test_gnu_backtrace_jit_frames_disappear_after_executor_free) ... skipped 'JIT is not available'
test_gnu_backtrace_unwinds_through_jit_frames (test.test_c_stack_unwind.GnuBacktraceUnwindTests.test_gnu_backtrace_unwinds_through_jit_frames) ... #00 0x7fffe3601470 -> other
#01 0x555558360384 -> python
#02 0x5555585569fc -> python
#03 0x55555855e82c -> python
#04 0x55555856d86c -> python
#05 0x555558360384 -> python
#06 0x5555585570a4 -> python
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{"length": 54, "python_frames": 51, "jit_frames": 0, "other_frames": 3, "jit_backend": null, "unwinder": "gnu_backtrace_unwind"}
ok
test_manual_unwind_finds_expected_frames (test.test_c_stack_unwind.ManualStackUnwindTests.test_manual_unwind_finds_expected_frames) ... #00 0x555558240384 -> python
#01 0x5555584369fc -> python
#02 0x55555843e82c -> python
#03 0x55555844d86c -> python
#04 0x555558240384 -> python
#05 0x5555584370a4 -> python
#06 0x555558443cbc -> python
#07 0x55555844d86c -> python
#08 0x555558240384 -> python
#09 0x5555584370a4 -> python
#10 0x555558443cbc -> python
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#12 0x555558240384 -> python
#13 0x5555584370a4 -> python
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{"length": 53, "python_frames": 51, "jit_frames": 0, "other_frames": 2, "jit_backend": null, "unwinder": "manual_frame_pointer_unwind"}
ok

----------------------------------------------------------------------
Ran 3 tests in 1.347s

OK (skipped=1)
0:00:01 load avg: 0.08 mem: 28.3 MiB [1/1] test_c_stack_unwind passed

== Tests result: SUCCESS ==

1 test OK.

Total duration: 1.4 sec
Total tests: run=3 skipped=1
Total test files: run=1/1
Result: SUCCESS

Clang

PYTHON_JIT=0 ./python -m test -v test_c_stack_unwind
== CPython 3.16.0a0 (heads/main-dirty:a00464bc33) [Clang 20.1.8 ]
== Linux-6.18.16-main-16k-loongarch64-with-glibc2.42 little-endian
== Python build: debug
== cwd: /mnt/build-cpython-main-clang-debug/build/test_python_worker_480864æ
== CPU count: 8
== encodings: locale=UTF-8 FS=utf-8
== resources: all test resources are disabled, use -u option to unskip tests

Using random seed: 721941152
0:00:00 load avg: 0.02 mem: 31.4 MiB Run 1 test sequentially in a single process
0:00:00 load avg: 0.02 mem: 31.4 MiB [1/1] test_c_stack_unwind
test_gnu_backtrace_jit_frames_disappear_after_executor_free (test.test_c_stack_unwind.GnuBacktraceUnwindTests.test_gnu_backtrace_jit_frames_disappear_after_executor_free) ... skipped 'JIT is not available'
test_gnu_backtrace_unwinds_through_jit_frames (test.test_c_stack_unwind.GnuBacktraceUnwindTests.test_gnu_backtrace_unwinds_through_jit_frames) ... #00 0x7ffff008dea4 -> other
#01 0x555556ec1308 -> python
#02 0x555556e341e8 -> python
#03 0x555556ff546c -> python
#04 0x555556ffc0c8 -> python
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{"length": 58, "python_frames": 55, "jit_frames": 0, "other_frames": 3, "jit_backend": null, "unwinder": "gnu_backtrace_unwind"}
ok
test_manual_unwind_finds_expected_frames (test.test_c_stack_unwind.ManualStackUnwindTests.test_manual_unwind_finds_expected_frames) ... #00 0x555556961308 -> python
#01 0x5555568d41e8 -> python
#02 0x555556a9546c -> python
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{"length": 57, "python_frames": 55, "jit_frames": 0, "other_frames": 2, "jit_backend": null, "unwinder": "manual_frame_pointer_unwind"}
ok

----------------------------------------------------------------------
Ran 3 tests in 2.709s

OK (skipped=1)
0:00:02 load avg: 0.10 mem: 31.9 MiB [1/1] test_c_stack_unwind passed

== Tests result: SUCCESS ==

1 test OK.

Total duration: 2.8 sec
Total tests: run=3 skipped=1
Total test files: run=1/1
Result: SUCCESS

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vstinner approved these changes

Jun 26, 2026

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The change LGTM, even if I don't know LoongArch architecture.

The -funwind-tables compiler flag is only needed by GnuBacktraceUnwindTests of test_c_stack_unwind? If yes, is it really worth it to add .eh_frame unwind tables just for a test?

@diegorusso @pablogsal: What do you think about the -funwind-tables flag?

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

Jun 26, 2026

yzewei commented Jun 29, 2026 •

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The change LGTM, even if I don't know LoongArch architecture.

The -funwind-tables compiler flag is only needed by GnuBacktraceUnwindTests of test_c_stack_unwind? If yes, is it really worth it to add .eh_frame unwind tables just for a test?

@diegorusso @pablogsal: What do you think about the -funwind-tables flag?

Indeed, adding -funwind-tables for testing purposes was not sufficiently necessary, and I removed this change. Thanks for the review.

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yzewei force-pushed the loongarch-clang-unwind branch from 0a7ad9f to 809c42d Compare

June 29, 2026 01:10


          pythongh-152240: Fix LoongArch frame pointer unwinding

50bb458

yzewei force-pushed the loongarch-clang-unwind branch from 809c42d to 50bb458 Compare

June 29, 2026 05:37

StanFromIreland mentioned this pull request

Jun 29, 2026

gh-151040: Fix test_c_stack_unwind on riscv64 #152370

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StanFromIreland reviewed

Jun 29, 2026

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Modules/_testinternalcapi.c Resolved

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vstinner added the needs backport to 3.15 bugs and security fixes label

Jun 29, 2026

vstinner merged commit 8458582 into python:main

Jun 29, 2026

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miss-islington-app Bot commented Jun 29, 2026

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Thanks @yzewei for the PR, and @vstinner for merging it 🌮🎉.. I'm working now to backport this PR to: 3.15.
🐍🍒⛏🤖

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bedevere-app Bot commented Jun 29, 2026

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GH-152578 is a backport of this pull request to the 3.15 branch.

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vstinner commented Jun 29, 2026

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I merged your change, thanks for the fix!

@yzewei:

Indeed, adding -funwind-tables for testing purposes was not sufficiently necessary, and I removed this change. Thanks for the review.

Hum, all test_c_stack_unwind tests still pass on LoongArch (without -funwind-tables)?

@StanFromIreland:

Please also expand this to all risc64, see the changes in #152370.

"Loongson moved to their own processor instruction set architecture (ISA) in 2021 (...) The ISA has been referred to as "a fork of MIPS64r6" due to a perceived lack of changes judging from instruction listings"

say https://en.wikipedia.org/wiki/Loongson#LoongArch

IMO LoongArch is different enough from RISC-V that it deserves to have its own code path.

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yzewei commented Jun 29, 2026

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I merged your change, thanks for the fix!

@yzewei:

Indeed, adding -funwind-tables for testing purposes was not sufficiently necessary, and I removed this change. Thanks for the review.

Hum, all test_c_stack_unwind tests still pass on LoongArch (without -funwind-tables)?

@StanFromIreland:

Please also expand this to all risc64, see the changes in #152370.

"Loongson moved to their own processor instruction set architecture (ISA) in 2021 (...) The ISA has been referred to as "a fork of MIPS64r6" due to a perceived lack of changes judging from instruction listings"

say https://en.wikipedia.org/wiki/Loongson#LoongArch

IMO LoongArch is different enough from RISC-V that it deserves to have its own code path.

Thanks for merging these changes!

Regarding the comparison between LoongArch and RISC-V: LoongArch is an independent Instruction Set Architecture (ISA), as clearly explained in the wiki.

Regarding the -funwind-tables flag: Without this flag, Clang/LoongArch builds do not generate the .eh_frame unwind tables required at runtime; although this PR fixes the tests, parts relying on GNU backtrace() would still fail.

In contrast, with GCC/LoongArch builds, GNU backtrace() works correctly even without this flag.

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vstinner commented Jul 10, 2026

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Ping @yzewei: Can you please sign the CLA for the backport? See #152578 (comment).

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xry111 commented Oct 11, 2026

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AFAIK unwinding via frame can work even without unwind tables given that all the frames have the same format that this PR has coded. Doing so is not related to unwind tables.

Unwinding via the tables will work when some frames are not canonical (for example, if a function does not call any other function the compiler can actually skip saving RA). But that will need an unwinder capable to parse the unwind table.

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yzewei commented Jun 26, 2026 • edited by bedevere-app Bot Loading Uh oh! There was an error while loading. Please reload this page.

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yzewei commented Jun 26, 2026

Fix log:

Gcc:

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yzewei commented Jun 26, 2026 •

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