gh-152240: Fix LoongArch frame pointer unwinding

Author: yzewei

Misc/NEWS.d/next/Build/2026-06-26-16-30-00.gh-issue-152240.loongarch.rst

@@ -0,0 +1,2 @@
+Fix C stack unwinding tests on Linux LoongArch builds by teaching the manual
+frame pointer unwinder to recognize the LoongArch frame layout.

Modules/_testinternalcapi.c

@@ -98,6 +98,12 @@ static const uintptr_t min_frame_pointer_addr = 0x1000;
 // https://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html#STACK
 #  define FRAME_POINTER_NEXT_OFFSET 0
 #  define FRAME_POINTER_RETURN_OFFSET 2
+#elif defined(__loongarch__)
+// On LoongArch, the frame pointer is the caller's stack pointer.
+// The saved frame pointer is stored at fp[-2], and the return
+// address is stored at fp[-1].
+#  define FRAME_POINTER_NEXT_OFFSET -2
+#  define FRAME_POINTER_RETURN_OFFSET -1
 #else
 #  define FRAME_POINTER_NEXT_OFFSET 0
 #  define FRAME_POINTER_RETURN_OFFSET 1