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gh-149202: Fix frame pointer unwinding on ppc64le and armv7/clang #149409

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gh-149202: Fix frame pointer unwinding on ppc64le and armv7/clang #149409

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c68855d
gh-149202: Fix frame pointer unwinding on s390x and ARM
pablogsal May 4, 2026
ad2c814
Advertise frame pointer support via _Py_WITH_FRAME_POINTERS
encukou May 5, 2026
242a58e
Use -mbackchain only on s390x
encukou May 5, 2026
9b74182
Adjust comment
encukou May 5, 2026
efb829e
Merge in the main branch
encukou May 5, 2026
a612c88
Regen configure
encukou May 5, 2026
955261c
Merge in the main branch
encukou May 5, 2026
92ee35e
Merge in the main branch
encukou May 6, 2026
02617af
Adjust docs & comment
encukou May 6, 2026
cfc9b62
Account for ppc64le
encukou May 6, 2026
4ad01d6
Fix comment
encukou May 6, 2026
1db3418
Also use -mno-thumb for 32-bit ARM
encukou May 6, 2026
b7cb2b3
Merge in the main branch
encukou May 6, 2026
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5 changes: 2 additions & 3 deletions Doc/using/configure.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -789,14 +789,13 @@ also be used to improve performance.

- ``-fno-omit-frame-pointer`` and/or ``-mno-omit-leaf-frame-pointer``
are added when the compiler supports them.
- ``-marm`` is added on 32-bit ARM when supported,
- ``-marm`` and/or ``-mno-thumb`` is added on 32-bit ARM when supported,
- on s390x platforms, when supported, ``-mbackchain`` is added *instead*.
of the above frame pointer flags.

Frame pointers enable profilers, debuggers, and system tracing tools
(``perf``, ``eBPF``, ``dtrace``, ``gdb``) to walk the C call stack
without DWARF metadata.
The flags propagate to third-party C
without DWARF metadata. The flags propagate to third-party C
extensions through :mod:`sysconfig`. On compilers that do not
understand them, the build silently skips them.

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6 changes: 6 additions & 0 deletions Modules/_testinternalcapi.c
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Expand Up @@ -92,6 +92,12 @@ static const uintptr_t min_frame_pointer_addr = 0x1000;
# define FRAME_POINTER_NEXT_OFFSET 0
# define FRAME_POINTER_RETURN_OFFSET \
(S390X_FRAME_RETURN_ADDRESS_OFFSET / (Py_ssize_t)sizeof(uintptr_t))
#elif defined(__powerpc64__) || defined(__ppc64__)
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@encukou encukou May 6, 2026

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@vstinner as the powerpc expert: Are these the right macros to detect ppc64le?

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@vstinner vstinner May 6, 2026

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#if defined(__powerpc64__) || defined(__ppc64__) looks to good to me according to existing CPython code. __powerpc64__ macro is defined on Linux. The __ppc64__ macro is maybe needed on macOS or Windows, I'm not sure, but it's good to check for the two macros :-)

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@encukou encukou May 6, 2026

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Well, the Linux stack layout might not apply to Mac... but then again, the last ppc Mac came out 20 years ago.

// ppc64le puts the return address at fp[2]; it saves the Condition Register
// in fp[1]. See:
// https://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html#STACK
# define FRAME_POINTER_NEXT_OFFSET 0
# define FRAME_POINTER_RETURN_OFFSET 2
#else
# define FRAME_POINTER_NEXT_OFFSET 0
# define FRAME_POINTER_RETURN_OFFSET 1
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45 changes: 44 additions & 1 deletion configure
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4 changes: 4 additions & 0 deletions configure.ac
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Expand Up @@ -2549,9 +2549,13 @@ AS_VAR_IF([ac_cv_gcc_compat], [yes], [
frame_pointer_cflags="$frame_pointer_cflags -mno-omit-leaf-frame-pointer"
], [], [-Werror])
AS_CASE([$host_cpu], [arm|armv*], [
dnl GCC uses "-marm"; clang uses "-mno-thumb"
AX_CHECK_COMPILE_FLAG([-marm], [
frame_pointer_cflags="$frame_pointer_cflags -marm"
], [], [-Werror])
AX_CHECK_COMPILE_FLAG([-mno-thumb], [
frame_pointer_cflags="$frame_pointer_cflags -mno-thumb"
], [], [-Werror])
])
AS_CASE([$host_cpu], [s390*], [
AX_CHECK_COMPILE_FLAG([-mbackchain], [
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