[libcxx-commits] [compiler-rt] [libcxxabi] [llvm] [libcxx] [runtimes] Use LLVM libunwind from libc++abi and compiler-rt by default (PR #77687)

Wed Jan 10 13:09:04 PST 2024

llvmbot wrote:




@llvm/pr-subscribers-libcxx

Author: Louis Dionne (ldionne)

<details>
<summary>Changes</summary>

I recently came across LIBCXXABI_USE_LLVM_UNWINDER and was surprised to notice it was disabled by default. Since we build libunwind by default and ship it in the LLVM toolchain, it would seem to make sense that libc++ and libc++abi rely on libunwind for unwinding instead of using the system-provided unwinding library (if any).

Most importantly, using the system unwinder implies that libc++abi is ABI compatible with that system unwinder, which is not necessarily the case. Hence, it makes a lot more sense to instead default to using the known-to-be-compatible LLVM unwinder, and let vendors manually select a different unwinder if desired.

Differential Revision: https://reviews.llvm.org/D150897
Fixes #77662
rdar://120801778

---
Full diff: https://github.com/llvm/llvm-project/pull/77687.diff


19 Files Affected:

- (modified) .github/workflows/libcxx-build-and-test.yaml (-1) 
- (modified) compiler-rt/CMakeLists.txt (+1-1) 
- (modified) libcxx/CMakeLists.txt (+1-1) 
- (modified) libcxx/cmake/caches/AArch64.cmake (-1) 
- (modified) libcxx/cmake/caches/AIX.cmake (-1) 
- (modified) libcxx/cmake/caches/Apple.cmake (+1) 
- (modified) libcxx/cmake/caches/Armv7Arm.cmake (-1) 
- (modified) libcxx/cmake/caches/Armv7M-picolibc.cmake (-1) 
- (modified) libcxx/cmake/caches/Armv7Thumb-no-exceptions.cmake (-1) 
- (modified) libcxx/cmake/caches/Armv8Arm.cmake (-1) 
- (modified) libcxx/cmake/caches/Armv8Thumb-no-exceptions.cmake (-1) 
- (modified) libcxx/cmake/caches/Generic-merged.cmake (-1) 
- (modified) libcxx/cmake/caches/Generic-msan.cmake (+1) 
- (modified) libcxx/cmake/caches/Generic-tsan.cmake (+1) 
- (modified) libcxx/cmake/caches/MinGW.cmake (-1) 
- (modified) libcxx/docs/BuildingLibcxx.rst (+1-1) 
- (modified) libcxx/docs/ReleaseNotes/18.rst (+7) 
- (modified) libcxx/utils/ci/run-buildbot (-5) 
- (modified) libcxxabi/CMakeLists.txt (+1-1) 


``````````diff

diff --git a/.github/workflows/libcxx-build-and-test.yaml b/.github/workflows/libcxx-build-and-test.yaml
index 985790a0ee2361..1666a687aa5d04 100644
--- a/.github/workflows/libcxx-build-and-test.yaml
+++ b/.github/workflows/libcxx-build-and-test.yaml
@@ -163,7 +163,6 @@ jobs:
           'generic-no-rtti',
           'generic-optimized-speed',
           'generic-static',
-          'generic-with_llvm_unwinder',
           # TODO Find a better place for the benchmark and bootstrapping builds to live. They're either very expensive
           # or don't provide much value since the benchmark run results are too noise on the bots.
           'benchmarks',
diff --git a/compiler-rt/CMakeLists.txt b/compiler-rt/CMakeLists.txt
index bbb4e8d7c333e4..ff6e4347b80b39 100644
--- a/compiler-rt/CMakeLists.txt
+++ b/compiler-rt/CMakeLists.txt
@@ -229,7 +229,7 @@ set(CXXLIBS none default libstdc++ libc++)
 set_property(CACHE SANITIZER_TEST_CXX PROPERTY STRINGS ;${CXXLIBS})
 handle_default_cxx_lib(SANITIZER_TEST_CXX)
 
-option(COMPILER_RT_USE_LLVM_UNWINDER "Use the LLVM unwinder." OFF)
+option(COMPILER_RT_USE_LLVM_UNWINDER "Use the LLVM unwinder." ON)
 cmake_dependent_option(COMPILER_RT_ENABLE_STATIC_UNWINDER
   "Statically link the LLVM unwinder." OFF
   "COMPILER_RT_USE_LLVM_UNWINDER" OFF)
diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index 75cb63222da35c..cf197d8c224143 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -274,7 +274,7 @@ option(LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS
    libc++abi. Doing otherwise is an ODR violation." OFF)
 # Build libc++abi with libunwind. We need this option to determine whether to
 # link with libunwind or libgcc_s while running the test cases.
-option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
+option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." ON)
 
 # Target options --------------------------------------------------------------
 option(LIBCXX_BUILD_32_BITS "Build 32 bit multilib libc++. This option is not supported anymore when building the runtimes. Please specify a full triple instead." ${LLVM_BUILD_32_BITS})
diff --git a/libcxx/cmake/caches/AArch64.cmake b/libcxx/cmake/caches/AArch64.cmake
index fa802d3de63f04..d813273584b0f0 100644
--- a/libcxx/cmake/caches/AArch64.cmake
+++ b/libcxx/cmake/caches/AArch64.cmake
@@ -1,2 +1 @@
-set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
 set(CMAKE_CXX_COMPILER_TARGET "aarch64-linux-gnu" CACHE STRING "")
diff --git a/libcxx/cmake/caches/AIX.cmake b/libcxx/cmake/caches/AIX.cmake
index 31d9a2d4b72751..c01aa5b14df065 100644
--- a/libcxx/cmake/caches/AIX.cmake
+++ b/libcxx/cmake/caches/AIX.cmake
@@ -13,6 +13,5 @@ set(LIBCXX_ENABLE_STATIC OFF CACHE BOOL "")
 set(LIBCXXABI_ENABLE_SHARED ON CACHE BOOL "")
 set(LIBCXXABI_ENABLE_STATIC OFF CACHE BOOL "")
 set(LIBCXX_CXX_ABI libcxxabi CACHE STRING "")
-set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
 set(LIBUNWIND_ENABLE_SHARED ON CACHE BOOL "")
 set(LIBUNWIND_ENABLE_STATIC OFF CACHE BOOL "")
diff --git a/libcxx/cmake/caches/Apple.cmake b/libcxx/cmake/caches/Apple.cmake
index 804eccd3a5dc5e..cec13c08acf107 100644
--- a/libcxx/cmake/caches/Apple.cmake
+++ b/libcxx/cmake/caches/Apple.cmake
@@ -14,6 +14,7 @@ set(LIBCXXABI_HERMETIC_STATIC_LIBRARY ON CACHE BOOL "")
 
 set(LIBCXXABI_ENABLE_ASSERTIONS OFF CACHE BOOL "")
 set(LIBCXXABI_ENABLE_FORGIVING_DYNAMIC_CAST ON CACHE BOOL "")
+set(LIBCXXABI_USE_LLVM_UNWINDER OFF CACHE BOOL "") # libunwind is built separately
 
 set(LIBCXX_TEST_CONFIG "apple-libc++-shared.cfg.in" CACHE STRING "")
 set(LIBCXXABI_TEST_CONFIG "apple-libc++abi-shared.cfg.in" CACHE STRING "")
diff --git a/libcxx/cmake/caches/Armv7Arm.cmake b/libcxx/cmake/caches/Armv7Arm.cmake
index 4d18d08fefcd22..e60d43f33dde7f 100644
--- a/libcxx/cmake/caches/Armv7Arm.cmake
+++ b/libcxx/cmake/caches/Armv7Arm.cmake
@@ -1,4 +1,3 @@
-set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
 set(CMAKE_CXX_COMPILER_TARGET "armv7l-linux-gnueabihf" CACHE STRING "")
 set(CMAKE_CXX_FLAGS "-marm" CACHE STRING "")
 set(CMAKE_C_FLAGS "-marm" CACHE STRING "")
diff --git a/libcxx/cmake/caches/Armv7M-picolibc.cmake b/libcxx/cmake/caches/Armv7M-picolibc.cmake
index 3ab80b960ed44a..b5f9089308d22e 100644
--- a/libcxx/cmake/caches/Armv7M-picolibc.cmake
+++ b/libcxx/cmake/caches/Armv7M-picolibc.cmake
@@ -19,7 +19,6 @@ set(LIBCXXABI_ENABLE_STATIC ON CACHE BOOL "")
 set(LIBCXXABI_ENABLE_STATIC_UNWINDER ON CACHE BOOL "")
 set(LIBCXXABI_ENABLE_THREADS OFF CACHE BOOL "")
 set(LIBCXXABI_USE_COMPILER_RT ON CACHE BOOL "")
-set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
 set(LIBCXX_ENABLE_EXCEPTIONS ON CACHE BOOL "")
 set(LIBCXX_ENABLE_FILESYSTEM OFF CACHE STRING "")
 set(LIBCXX_ENABLE_MONOTONIC_CLOCK OFF CACHE BOOL "")
diff --git a/libcxx/cmake/caches/Armv7Thumb-no-exceptions.cmake b/libcxx/cmake/caches/Armv7Thumb-no-exceptions.cmake
index 71173af106b637..70b619f8ec31f3 100644
--- a/libcxx/cmake/caches/Armv7Thumb-no-exceptions.cmake
+++ b/libcxx/cmake/caches/Armv7Thumb-no-exceptions.cmake
@@ -1,4 +1,3 @@
-set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
 set(CMAKE_CXX_COMPILER_TARGET "armv7l-linux-gnueabihf" CACHE STRING "")
 set(CMAKE_CXX_FLAGS "-mthumb" CACHE STRING "")
 set(CMAKE_C_FLAGS "-mthumb" CACHE STRING "")
diff --git a/libcxx/cmake/caches/Armv8Arm.cmake b/libcxx/cmake/caches/Armv8Arm.cmake
index 5055582fdafc0d..a2894716035304 100644
--- a/libcxx/cmake/caches/Armv8Arm.cmake
+++ b/libcxx/cmake/caches/Armv8Arm.cmake
@@ -1,4 +1,3 @@
-set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
 set(CMAKE_CXX_COMPILER_TARGET "armv8l-linux-gnueabihf" CACHE STRING "")
 set(CMAKE_CXX_FLAGS "-marm" CACHE STRING "")
 set(CMAKE_C_FLAGS "-marm" CACHE STRING "")
diff --git a/libcxx/cmake/caches/Armv8Thumb-no-exceptions.cmake b/libcxx/cmake/caches/Armv8Thumb-no-exceptions.cmake
index 316edd31490665..b4c7dfca279cd3 100644
--- a/libcxx/cmake/caches/Armv8Thumb-no-exceptions.cmake
+++ b/libcxx/cmake/caches/Armv8Thumb-no-exceptions.cmake
@@ -1,4 +1,3 @@
-set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
 set(CMAKE_CXX_COMPILER_TARGET "armv8l-linux-gnueabihf" CACHE STRING "")
 set(CMAKE_CXX_FLAGS "-mthumb" CACHE STRING "")
 set(CMAKE_C_FLAGS "-mthumb" CACHE STRING "")
diff --git a/libcxx/cmake/caches/Generic-merged.cmake b/libcxx/cmake/caches/Generic-merged.cmake
index b2e02f73105557..7ebb8026236ddf 100644
--- a/libcxx/cmake/caches/Generic-merged.cmake
+++ b/libcxx/cmake/caches/Generic-merged.cmake
@@ -5,7 +5,6 @@ set(LIBCXX_ENABLE_STATIC_ABI_LIBRARY ON CACHE BOOL "")
 set(LIBCXX_STATICALLY_LINK_ABI_IN_SHARED_LIBRARY ON CACHE BOOL "")
 
 set(LIBCXXABI_ENABLE_SHARED OFF CACHE BOOL "")
-set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
 set(LIBCXXABI_ENABLE_STATIC_UNWINDER ON CACHE BOOL "")
 set(LIBCXXABI_STATICALLY_LINK_UNWINDER_IN_STATIC_LIBRARY ON CACHE BOOL "")
 
diff --git a/libcxx/cmake/caches/Generic-msan.cmake b/libcxx/cmake/caches/Generic-msan.cmake
index 7c948f51642dd4..9e50255fe4ff6b 100644
--- a/libcxx/cmake/caches/Generic-msan.cmake
+++ b/libcxx/cmake/caches/Generic-msan.cmake
@@ -1 +1,2 @@
 set(LLVM_USE_SANITIZER "MemoryWithOrigins" CACHE STRING "")
+set(LIBCXXABI_USE_LLVM_UNWINDER OFF CACHE BOOL "") # MSAN is compiled against the system unwinder, which leads to false positives
diff --git a/libcxx/cmake/caches/Generic-tsan.cmake b/libcxx/cmake/caches/Generic-tsan.cmake
index a4b599e3e5094b..c42c1bb8e72221 100644
--- a/libcxx/cmake/caches/Generic-tsan.cmake
+++ b/libcxx/cmake/caches/Generic-tsan.cmake
@@ -1 +1,2 @@
 set(LLVM_USE_SANITIZER "Thread" CACHE STRING "")
+set(LIBCXXABI_USE_LLVM_UNWINDER OFF CACHE BOOL "") # TSAN is compiled against the system unwinder, which leads to false positives
diff --git a/libcxx/cmake/caches/MinGW.cmake b/libcxx/cmake/caches/MinGW.cmake
index a5927096d8ff96..09e6ea5c1bab20 100644
--- a/libcxx/cmake/caches/MinGW.cmake
+++ b/libcxx/cmake/caches/MinGW.cmake
@@ -1,5 +1,4 @@
 set(LIBCXX_CXX_ABI libcxxabi CACHE STRING "")
-set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
 
 set(LIBCXXABI_ENABLE_SHARED OFF CACHE BOOL "")
 set(LIBCXX_ENABLE_STATIC_ABI_LIBRARY ON CACHE BOOL "")
diff --git a/libcxx/docs/BuildingLibcxx.rst b/libcxx/docs/BuildingLibcxx.rst
index 0b4d4205286512..f2304c9796b540 100644
--- a/libcxx/docs/BuildingLibcxx.rst
+++ b/libcxx/docs/BuildingLibcxx.rst
@@ -337,7 +337,7 @@ ABI Library Specific Options
 
 .. option:: LIBCXXABI_USE_LLVM_UNWINDER:BOOL
 
-  **Default**: ``OFF``
+  **Default**: ``ON``
 
   Build and use the LLVM unwinder. Note: This option can only be used when
   libc++abi is the C++ ABI library used.
diff --git a/libcxx/docs/ReleaseNotes/18.rst b/libcxx/docs/ReleaseNotes/18.rst
index 5df6242e52317a..478e87f8888332 100644
--- a/libcxx/docs/ReleaseNotes/18.rst
+++ b/libcxx/docs/ReleaseNotes/18.rst
@@ -191,3 +191,10 @@ Build System Changes
   passing ``-Dexecutor=...`` to ``llvm-lit``. Alternatively, this flag can be made persistent in the generated test
   configuration file by passing ``-DLIBCXX_TEST_PARAMS=executor=...``. This also applies to the ``LIBUWIND_EXECTOR``
   and ``LIBCXXABI_EXECUTOR`` CMake variables. LLVM 19 will completely remove support for the ``*_EXECUTOR`` variables.
+
+- ``LIBCXXABI_USE_LLVM_UNWINDER`` and ``COMPILER_RT_USE_LLVM_UNWINDER`` switched defaults from ``OFF`` to ``ON``.
+  This means that by default, libc++abi and compiler-rt will link against the LLVM provided ``libunwind`` library
+  instead of the system-provided unwinding library. If you are building the LLVM runtimes with the goal of shipping
+  them so that they can interoperate with other system-provided libraries that might be using a different unwinding
+  library (such as ``libgcc_s``), you should pass ``LIBCXXABI_USE_LLVM_UNWINDER=OFF`` and ``COMPILER_RT_USE_LLVM_UNWINDER=OFF``
+  to make sure the system-provided unwinding library is used by the LLVM runtimes.
diff --git a/libcxx/utils/ci/run-buildbot b/libcxx/utils/ci/run-buildbot
index ed2bc2a14f17d6..0ff028743bc1a3 100755
--- a/libcxx/utils/ci/run-buildbot
+++ b/libcxx/utils/ci/run-buildbot
@@ -439,11 +439,6 @@ generic-hardening-mode-debug)
     check-runtimes
     check-abi-list
 ;;
-generic-with_llvm_unwinder)
-    clean
-    generate-cmake -DLIBCXXABI_USE_LLVM_UNWINDER=ON
-    check-runtimes
-;;
 #
 # Module builds
 #
diff --git a/libcxxabi/CMakeLists.txt b/libcxxabi/CMakeLists.txt
index efe830bd2ad665..c62b05bf2feafc 100644
--- a/libcxxabi/CMakeLists.txt
+++ b/libcxxabi/CMakeLists.txt
@@ -46,7 +46,7 @@ option(LIBCXXABI_ENABLE_EXCEPTIONS
 option(LIBCXXABI_ENABLE_ASSERTIONS "Enable assertions independent of build mode." ON)
 option(LIBCXXABI_ENABLE_PEDANTIC "Compile with pedantic enabled." OFF)
 option(LIBCXXABI_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
-option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
+option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." ON)
 option(LIBCXXABI_ENABLE_STATIC_UNWINDER "Statically link the LLVM unwinder." OFF)
 option(LIBCXXABI_USE_COMPILER_RT "Use compiler-rt instead of libgcc" OFF)
 option(LIBCXXABI_ENABLE_THREADS "Build with threads enabled" ON)

``````````

</details>


https://github.com/llvm/llvm-project/pull/77687
