libcxxabi: declare __gnu_unwind_frame in cxa_personality

[kraj]

ARM EHABI builds of libcxxabi fail with clang-22+ because cxa_personality.cpp calls __gnu_unwind_frame without a visible declaration, triggering:

error: use of undeclared identifier '__gnu_unwind_frame'

Add an extern "C" forward declaration before the EHABI unwind helper so the source compiles correctly.

[llvmbot]

@llvm/pr-subscribers-libcxxabi

Author: Khem Raj (kraj)

Changes

ARM EHABI builds of libcxxabi fail with clang-22+ because cxa_personality.cpp calls __gnu_unwind_frame without a visible declaration, triggering:

error: use of undeclared identifier '__gnu_unwind_frame'

Add an extern "C" forward declaration before the EHABI unwind helper so the source compiles correctly.

---

Full diff: https://github.com/llvm/llvm-project/pull/189787.diff

1 Files Affected:

• (modified) libcxxabi/src/cxa_personality.cpp (+2)

diff --git a/libcxxabi/src/cxa_personality.cpp b/libcxxabi/src/cxa_personality.cpp
index 35f4e38077248..ddff277cc7b81 100644
--- a/libcxxabi/src/cxa_personality.cpp
+++ b/libcxxabi/src/cxa_personality.cpp
@@ -1116,6 +1116,8 @@ __gxx_personality_seh0(PEXCEPTION_RECORD ms_exc, void *this_frame,
 
 #else
 
+extern "C" _Unwind_Reason_Code __gnu_unwind_frame(_Unwind_Exception*,
+                                                  _Unwind_Context*);
 // Helper function to unwind one frame.
 // ARM EHABI 7.3 and 7.4: If the personality function returns _URC_CONTINUE_UNWIND, the
 // personality routine should update the virtual register set (VRS) according to the

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[MaskRay]

Seems better than #189280 as there is no unrelated formatting.
But ideally __gnu_unwind_frame should only be declared with ARM EABI is used, and with a comment.

[kraj]

Seems better than #189280 as there is no unrelated formatting. But ideally __gnu_unwind_frame should only be declared with ARM EABI is used, and with a comment.

Good point. This is already in the _LIBCXXABI_ARM_EHABI branch, but I can make that explicit in the #else comment and add a short note explaining that __gnu_unwind_frame is the ARM EHABI unwinder entry point used by continue_unwind(). but i feel that might become noisy

[planetmarshall]

#116261 moved this declaration to libunwind, so there should probably be a preprocessor guard in case it's already been declared (by building libcxxabi with libunwind).

[planetmarshall]

Seems better than #189280 as there is no unrelated formatting.

The formatting changes were required by the CI checks. In the end I'm not bothered which PR gets merged, so long as it's fixed!

[alexrp]

This is a fairly significant regression in 22.x and the fix is basically zero-risk, so let's just get this in.