[libunwind] Add a _LIBUNWIND_VERSION macro

[ldionne]

Summary

This allows us to detect whether we're being compiled with LLVM's libunwind
more easily, without CMake having to set explicit variables.

As discussed in https://llvm.org/D119538.

[emaste]

LGTM (modulo the lint complaints)

[ldionne]

In D121015#3360735, @emaste wrote:

LGTM (modulo the lint complaints)

I think the code would be inconsistent with surrounding code if I listened to them, so I'm tempted to ignore them instead. LMK if you disagree.

@mgorny Is this patch OK with you? Can you test it locally? I suspect that, in its current form, it's not going to quite work for you unless you have basically ToT libunwind headers installed on your system, right?

[MaskRay]

Look good to ignore clang-format diagnostics.

[ldionne]

Gentle ping @mgorny.

[mgorny]

I'm sorry, my replacement PSU just arrived today and I'm slowly catching up with everything. I presume we don't need to worry about backwards compatibility, so LGTM.

[ldionne]

In D121015#3379587, @mgorny wrote:

I'm sorry, my replacement PSU just arrived today and I'm slowly catching up with everything. I presume we don't need to worry about backwards compatibility, so LGTM.

No worries. If you think this works for your use case, then I think this should be good to go. I verified and we do take that code path when we build the runtimes with LIBCXXABI_USE_LLVM_UNWINDER.

However, upon thinking about it again, I think it's safer to add a temporary workaround to detect LLVM's libunwind in libc++abi for a couple of releases, until we can assume that libunwind headers from LLVM 15 (or newer) are being used to build libc++abi. That should take care of folks who might be building libc++abi against an out-of-sync LLVM libunwind (e.g. as part of a SDK or a system-provided libunwind). I'll add a workaround and you folks can tell me what you think -- sorry for the churn.

[ldionne]

Add workaround for building libc++abi against older libunwind. What do people think about this?

[ldionne]

I'll ship this since it should be strictly safer than the previous version of the patch.

libcxx/docs/Contributing.rst

 =======================
 
 After branching for an LLVM release:
 
 1. Update ``_LIBCPP_VERSION`` in ``libcxx/include/__config``
 2. Update the ``libcxx/include/__libcpp_version`` file
 3. Update the version number in ``libcxx/docs/conf.py``
 4. Update ``_LIBCPPABI_VERSION`` in ``libcxxabi/include/cxxabi.h``
+5. Update ``_LIBUNWIND_VERSION`` in ``libunwind/include/__libunwind_config.h``
 
 Exporting new symbols from the library
 ======================================
 
 When exporting new symbols from libc++, you must update the ABI lists located in ``lib/abi``.
 To test whether the lists are up-to-date, please run the target ``check-cxx-abilist``.
 To regenerate the lists, use the target ``generate-cxx-abilist``.
 The ABI lists must be updated for all supported platforms; currently Linux and

libcxxabi/CMakeLists.txt

 
 # Prevent libc++abi from having library dependencies on libc++
 add_definitions(-D_LIBCPP_DISABLE_EXTERN_TEMPLATE)
 
 if (MSVC)
   add_definitions(-D_CRT_SECURE_NO_WARNINGS)
 endif()
 
-# Define LIBCXXABI_USE_LLVM_UNWINDER for conditional compilation.
-if (LIBCXXABI_USE_LLVM_UNWINDER)
-  add_definitions(-DLIBCXXABI_USE_LLVM_UNWINDER)
-endif()
-
 if (LIBCXXABI_SILENT_TERMINATE)
   add_definitions(-DLIBCXXABI_SILENT_TERMINATE)
 endif()
 
 if (LIBCXXABI_NON_DEMANGLING_TERMINATE)
   add_definitions(-DLIBCXXABI_NON_DEMANGLING_TERMINATE)
 endif()
 

libcxxabi/src/cxa_personality.cpp

 #include <typeinfo>
 
 #include "__cxxabi_config.h"
 #include "cxa_exception.h"
 #include "cxa_handlers.h"
 #include "private_typeinfo.h"
 #include "unwind.h"
 
+// TODO: This is a temporary workaround for libc++abi to recognize that it's being
+// built against LLVM's libunwind. LLVM's libunwind started reporting _LIBUNWIND_VERSION
+// in LLVM 15 -- we can remove this workaround after shipping LLVM 17. Once we remove
+// this workaround, it won't be possible to build libc++abi against libunwind headers
+// from LLVM 14 and before anymore.
+#if defined(____LIBUNWIND_CONFIG_H__) && !defined(_LIBUNWIND_VERSION)
+#   define _LIBUNWIND_VERSION
+#endif
+
 #if defined(__SEH__) && !defined(__USING_SJLJ_EXCEPTIONS__)
 #include <windows.h>
 #include <winnt.h>
 
 extern "C" EXCEPTION_DISPOSITION _GCC_specific_handler(PEXCEPTION_RECORD,
                                                        void *, PCONTEXT,
                                                        PDISPATCHER_CONTEXT,
                                                        _Unwind_Personality_Fn);
static _Unwind_Reason_Code continue_unwind(_Unwind_Exception* unwind_exception,
   case _URC_END_OF_STACK:
     return _URC_END_OF_STACK;
   default:
     return _URC_FAILURE;
   }
 }
 
 // ARM register names
-#if !defined(LIBCXXABI_USE_LLVM_UNWINDER)
+#if !defined(_LIBUNWIND_VERSION)
 static const uint32_t REG_UCB = 12;  // Register to save _Unwind_Control_Block
 #endif
 static const uint32_t REG_SP = 13;
 
 static void save_results_to_barrier_cache(_Unwind_Exception* unwind_exception,
                                           const scan_results& results)
 {
     unwind_exception->barrier_cache.bitpattern[0] = (uint32_t)results.adjustedPtr;
__gxx_personality_v0(_Unwind_State state,
                      _Unwind_Exception* unwind_exception,
                      _Unwind_Context* context)
 {
     if (unwind_exception == 0 || context == 0)
         return _URC_FATAL_PHASE1_ERROR;
 
     bool native_exception = __isOurExceptionClass(unwind_exception);
 
-#if !defined(LIBCXXABI_USE_LLVM_UNWINDER)
+#if !defined(_LIBUNWIND_VERSION)
     // Copy the address of _Unwind_Control_Block to r12 so that
     // _Unwind_GetLanguageSpecificData() and _Unwind_GetRegionStart() can
     // return correct address.
     _Unwind_SetGR(context, REG_UCB, reinterpret_cast<uint32_t>(unwind_exception));
 #endif
 
     // Check the undocumented force unwinding behavior
     bool is_force_unwinding = state & _US_FORCE_UNWIND;

libunwind/include/__libunwind_config.h

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef ____LIBUNWIND_CONFIG_H__
 #define ____LIBUNWIND_CONFIG_H__
 
+#define _LIBUNWIND_VERSION 15000
+
 #if defined(__arm__) && !defined(__USING_SJLJ_EXCEPTIONS__) && \
     !defined(__ARM_DWARF_EH__)
 #define _LIBUNWIND_ARM_EHABI
 #endif
 
 #define _LIBUNWIND_HIGHEST_DWARF_REGISTER_X86       8
 #define _LIBUNWIND_HIGHEST_DWARF_REGISTER_X86_64    32
 #define _LIBUNWIND_HIGHEST_DWARF_REGISTER_PPC       112