[libc++][Android] Always redirect <stdatomic.h> to <atomic>

[rprichard]

When targeting Android, enable the <stdatomic.h> to redirection for C++ language dialects before C++23, because this redirection historically existed for Android's C++ toolchain. Currently, the Android toolchain achieves this redirection with a stdatomic.h in the sysroot (from Bionic) that defines the C API either directly or by aliasing std identifiers from . The Android team's LLVM build scripts then copy Bionic's header over the one in the Clang resource directory.

Hopefully, by moving Android's redirection into LLVM itself, the Android stdatomic.h situation can be simplified eventually.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Ryan Prichard (rprichard)

Changes

When targeting Android, enable the <stdatomic.h> to <atomic> redirection for C++ language dialects before C++23, because this redirection historically existed for Android's C++ toolchain. Currently, the Android toolchain achieves this redirection with a stdatomic.h in the sysroot (from Bionic) that defines the C API either directly or by aliasing std identifiers from <atomic>. The Android team's LLVM build scripts then copy Bionic's header over the one in the Clang resource directory.

Hopefully, by moving Android's redirection into LLVM itself, the Android stdatomic.h situation can be simplified eventually.

---

Full diff: https://github.com/llvm/llvm-project/pull/143036.diff

5 Files Affected:

• (modified) libcxx/include/atomic (+4-1)
• (modified) libcxx/include/stdatomic.h (+4-1)
• (modified) libcxx/test/libcxx/atomics/atomics.syn/compatible_with_stdatomic.compile.pass.cpp (+4-1)
• (modified) libcxx/test/libcxx/atomics/atomics.syn/incompatible_with_stdatomic.verify.cpp (+3)
• (modified) libcxx/test/libcxx/atomics/stdatomic.h.syn/dont_hijack_header.cxx23.compile.pass.cpp (+3)

diff --git a/libcxx/include/atomic b/libcxx/include/atomic
index 75af5de33ca4c..fba7040f6900c 100644
--- a/libcxx/include/atomic
+++ b/libcxx/include/atomic
@@ -598,7 +598,10 @@ template <class T>
 #    define _LIBCPP_STDATOMIC_H_HAS_DEFINITELY_BEEN_INCLUDED 0
 #  endif
 
-#  if _LIBCPP_STD_VER < 23 && _LIBCPP_STDATOMIC_H_HAS_DEFINITELY_BEEN_INCLUDED
+// The Android LLVM toolchain has historically allowed combining the <atomic>
+// and <stdatomic.h> headers in dialects before C++23, so for backwards
+// compatibility, preserve that ability when targeting Android.
+#  if _LIBCPP_STD_VER < 23 && !defined(__ANDROID__) && _LIBCPP_STDATOMIC_H_HAS_DEFINITELY_BEEN_INCLUDED
 #    error <atomic> is incompatible with <stdatomic.h> before C++23. Please compile with -std=c++23.
 #  endif
 
diff --git a/libcxx/include/stdatomic.h b/libcxx/include/stdatomic.h
index 2991030eee456..dc1a955b69cb6 100644
--- a/libcxx/include/stdatomic.h
+++ b/libcxx/include/stdatomic.h
@@ -126,7 +126,10 @@ using std::atomic_signal_fence                         // see below
 #    pragma GCC system_header
 #  endif
 
-#  if defined(__cplusplus) && _LIBCPP_STD_VER >= 23
+// The Android LLVM toolchain has historically allowed combining the <atomic>
+// and <stdatomic.h> headers in dialects before C++23, so for backwards
+// compatibility, preserve that ability when targeting Android.
+#  if defined(__cplusplus) && (_LIBCPP_STD_VER >= 23 || defined(__ANDROID__))
 
 #    include <atomic>
 #    include <version>
diff --git a/libcxx/test/libcxx/atomics/atomics.syn/compatible_with_stdatomic.compile.pass.cpp b/libcxx/test/libcxx/atomics/atomics.syn/compatible_with_stdatomic.compile.pass.cpp
index 349dc51aaa0e6..c997e4b792459 100644
--- a/libcxx/test/libcxx/atomics/atomics.syn/compatible_with_stdatomic.compile.pass.cpp
+++ b/libcxx/test/libcxx/atomics/atomics.syn/compatible_with_stdatomic.compile.pass.cpp
@@ -7,7 +7,10 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: no-threads
-// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// On Android, libc++'s <stdatomic.h> header always redirects to <atomic>, even before C++23.
+// UNSUPPORTED: c++03
+// UNSUPPORTED: (c++11 || c++14 || c++17 || c++20) && !android
 
 // This test verifies that <stdatomic.h> redirects to <atomic>.
 
diff --git a/libcxx/test/libcxx/atomics/atomics.syn/incompatible_with_stdatomic.verify.cpp b/libcxx/test/libcxx/atomics/atomics.syn/incompatible_with_stdatomic.verify.cpp
index a788ea32dddc8..7d14fa308bf06 100644
--- a/libcxx/test/libcxx/atomics/atomics.syn/incompatible_with_stdatomic.verify.cpp
+++ b/libcxx/test/libcxx/atomics/atomics.syn/incompatible_with_stdatomic.verify.cpp
@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // REQUIRES: c++03 || c++11 || c++14 || c++17 || c++20
 
+// On Android, libc++'s <stdatomic.h> header always redirects to <atomic>, even before C++23.
+// XFAIL: android
+
 // No diagnostic gets emitted when we build with modules.
 // XFAIL: clang-modules-build
 
diff --git a/libcxx/test/libcxx/atomics/stdatomic.h.syn/dont_hijack_header.cxx23.compile.pass.cpp b/libcxx/test/libcxx/atomics/stdatomic.h.syn/dont_hijack_header.cxx23.compile.pass.cpp
index a8a99e6937f31..5c966c6bca6e2 100644
--- a/libcxx/test/libcxx/atomics/stdatomic.h.syn/dont_hijack_header.cxx23.compile.pass.cpp
+++ b/libcxx/test/libcxx/atomics/stdatomic.h.syn/dont_hijack_header.cxx23.compile.pass.cpp
@@ -21,6 +21,9 @@
 // doesn't work at all if we don't use the <stdatomic.h> provided by libc++ in C++23 and above.
 // XFAIL: (c++11 || c++14 || c++17 || c++20) && gcc
 
+// On Android, libc++'s <stdatomic.h> header always redirects to <atomic>, even before C++23.
+// XFAIL: android
+
 #include <atomic>
 #include <stdatomic.h>
 #include <type_traits>

[rprichard]

Some more context:

• Prior to this diagnostic fix, [libc++] Fix diagnostic for <stdatomic.h> before C++23 #83351 (comment), the Android LLVM team's build of LLVM allowed mixing and <stdatomic.h> in a fragile way -- the previous libc++ diagnostic didn't fire because Bionic's stdatomic.h doesn't define kill_dependency. After the libc++ diagnostic was improved, it broke our C++23 stdatomic.h backport.
• libc++ tried to enable the backport for all C++ dialects, [libc++] Enable C++ stdatomic.h for all C++ versions #95498, but that broke other things, in particular _Atomic as a type-qualifier. It was reverted in [libc++] Don't implement <stdatomic.h> before C++23 #123130.

I'm trying to make the Android situation less clumsy, by making the upstream LLVM headers acceptable to use as-is, for the Android toolchain team's build of LLVM.

I think our team would actually prefer a way to enable the backport even when we target non-Android, but I don't know if this desire warrants adding a config flag (e.g. to __config_site).

I'm wondering if upstream libc++ has any thoughts on restoring the backport, but only when targeting Android.

[enh-google]

why is c++03 on a separate line now?

[enh-google]

(the config option so we can have this for Android host builds too sounds even better, but at least this lets us not hurt app developers and keep any pain "just" to OS developers...)