[libc++] Build with -fsized-deallocation

[ldionne]

This patch makes libc++ build with -fsized-deallocation. That flag is
enabled by default in recent versions of Clang, so this patch will make
libc++ forward-compatible with ToT Clang.

[llvmbot]

@llvm/pr-subscribers-libcxxabi

Author: Louis Dionne (ldionne)

Changes

This patch makes libc++ and libc++abi build with -fsized-deallocation. That flag is enabled by default in recent versions of Clang, so this patch will make these runtimes forward-compatible with ToT Clang.

---

Full diff: https://github.com/llvm/llvm-project/pull/96217.diff

2 Files Affected:

• (modified) libcxx/CMakeLists.txt (+4)
• (modified) libcxxabi/CMakeLists.txt (+4)

diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index 4b927017f8c2a..f0946b6022280 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -553,6 +553,10 @@ function(cxx_add_basic_build_flags target)
     target_add_compile_flags_if_supported(${target} PRIVATE -fvisibility=hidden)
   endif()
 
+  # Build with -fsized-deallocation, which is default in recent versions of Clang.
+  # TODO(LLVM 21): This can be dropped once we only support Clang >= 19.
+  target_compile_options(${target} PRIVATE -fsized-deallocation)
+
   # Let the library headers know they are currently being used to build the
   # library.
   target_compile_definitions(${target} PRIVATE -D_LIBCPP_BUILDING_LIBRARY)
diff --git a/libcxxabi/CMakeLists.txt b/libcxxabi/CMakeLists.txt
index 52ba52f3439fb..d45b104b41e32 100644
--- a/libcxxabi/CMakeLists.txt
+++ b/libcxxabi/CMakeLists.txt
@@ -247,6 +247,10 @@ endif()
 add_compile_flags("${LIBCXXABI_ADDITIONAL_COMPILE_FLAGS}")
 add_library_flags("${LIBCXXABI_ADDITIONAL_LIBRARIES}")
 
+# Build with -fsized-deallocation, which is default in recent versions of Clang.
+# TODO(LLVM 21): This can be dropped once we only support Clang >= 19.
+add_compile_flags(-fsized-deallocation)
+
 # Configure compiler. Must happen after setting the target flags.
 include(config-ix)
 

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Louis Dionne (ldionne)

Changes

This patch makes libc++ and libc++abi build with -fsized-deallocation. That flag is enabled by default in recent versions of Clang, so this patch will make these runtimes forward-compatible with ToT Clang.

---

Full diff: https://github.com/llvm/llvm-project/pull/96217.diff

2 Files Affected:

• (modified) libcxx/CMakeLists.txt (+4)
• (modified) libcxxabi/CMakeLists.txt (+4)

diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index 4b927017f8c2a..f0946b6022280 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -553,6 +553,10 @@ function(cxx_add_basic_build_flags target)
     target_add_compile_flags_if_supported(${target} PRIVATE -fvisibility=hidden)
   endif()
 
+  # Build with -fsized-deallocation, which is default in recent versions of Clang.
+  # TODO(LLVM 21): This can be dropped once we only support Clang >= 19.
+  target_compile_options(${target} PRIVATE -fsized-deallocation)
+
   # Let the library headers know they are currently being used to build the
   # library.
   target_compile_definitions(${target} PRIVATE -D_LIBCPP_BUILDING_LIBRARY)
diff --git a/libcxxabi/CMakeLists.txt b/libcxxabi/CMakeLists.txt
index 52ba52f3439fb..d45b104b41e32 100644
--- a/libcxxabi/CMakeLists.txt
+++ b/libcxxabi/CMakeLists.txt
@@ -247,6 +247,10 @@ endif()
 add_compile_flags("${LIBCXXABI_ADDITIONAL_COMPILE_FLAGS}")
 add_library_flags("${LIBCXXABI_ADDITIONAL_LIBRARIES}")
 
+# Build with -fsized-deallocation, which is default in recent versions of Clang.
+# TODO(LLVM 21): This can be dropped once we only support Clang >= 19.
+add_compile_flags(-fsized-deallocation)
+
 # Configure compiler. Must happen after setting the target flags.
 include(config-ix)
 

[EricWF]

I don't think this change should affect anything. Can we drop it and see what happens?

The size-deallocation functions should always be present in the library, and we shouldn't be calling new/delete from inside the library (I think?)

[ldionne]

The size-deallocation functions should always be present in the library,

I don't think that's true, libc++abi doesn't define operator new in all configurations. In some configurations it is provided by libc++ instead.

and we shouldn't be calling new/delete from inside the library (I think?)

However that should definitely be correct, so I think you're right I could drop that part of the change. I'll try that.

[EricWF]

Lets see how the bots come back, they should break hopefully.

[EricWF]

Fascinating. It appears the readelf: Warning: Unrecognized form: 0x23 messages in the output are related to sized deallocation.

I assumed they were due to the upgrade, but it appears not.

Nevermind, the readelf messages are always present, I've just never noticed them.

[EricWF]

Also maybe we should move the ABI checking to a single build configuration. Since most of the configurations build libc++ /libc++abi in the same way, they should all have the same results.

That way breakages like this are less disruptive?

[ldionne]

Looks like the CI failures are Android via an out-of-memory issue (which looks like a flake), and the bootstrapping build failure that we need this patch in order to fix. Merging.