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

encukou · web-flow · commit 612140e7ddc3 · 2026-05-07T10:37:00.000+02:00
- ppc64's backchain format is also different from x86
- On 32-bit ARM, clang needs `-mno-thumb`, not `-marm` like GCC
diff --git a/Doc/using/configure.rst b/Doc/using/configure.rst
@@ -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.
 
diff --git a/Modules/_testinternalcapi.c b/Modules/_testinternalcapi.c
@@ -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__)
+// 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
diff --git a/configure b/configure
diff --git a/configure.ac b/configure.ac
@@ -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], [
