312975 2026-04-22 01:14:06 -0700 The libatomic check in WebKitCompilerFlags.cmake does not always work correctly 2026-04-24 00:59:37 -0700 1 1 1 Unclassified WebKit CMake Other Unspecified Unspecified RESOLVED FIXED P2 Normal --- 313214 1 berto berto aperez cgarcia mcatanzaro oldest_to_newest 2203086 0 berto 2026-04-22 01:14:06 -0700 The WebKitCompilerFlags.cmake file contains a test to detect whether the compiler needs to link against libatomic. https://github.com/WebKit/WebKit/blob/wpewebkit-2.53.1/Source/cmake/WebKitCompilerFlags.cmake#L448 The compiler tries to build this test code with the standard flags, and failure indicates that it needs to be linked against libatomic. However in some cases this test fails to detect if libatomic is needed because the atomic code gets optimized away by the compiler. Here's the expected behavior: $ clang++ --version Debian clang version 19.1.7 (3+b1) Target: arm-unknown-linux-gnueabihf Thread model: posix InstalledDir: /usr/lib/llvm-19/bin $ clang++ -o test.o -c test.cc $ clang++ -o test test.o /usr/bin/arm-linux-gnueabihf-ld: test.o: in function `std::atomic<std::optional<double> >::load(std::memory_order) const': test.cc:(.text._ZNKSt6atomicISt8optionalIdEE4loadESt12memory_order[_ZNKSt6atomicISt8optionalIdEE4loadESt12memory_order]+0x68): undefined reference to `__atomic_load' [...] $ nm test.o | grep atomic 00000000 W _ZNKSt6atomicISt8optionalIdEE4loadESt12memory_order 00000000 W _ZNSt6atomicISt8optionalIdEE5storeES1_St12memory_order 00000000 W _ZNSt6atomicISt8optionalIdEEC2Ev 00000000 W _ZNSt6atomicISt8optionalIdEEaSES1_ U __atomic_load U __atomic_store And now with -O2: $ clang++ -o test.o -c -O2 test.cc $ clang++ -o test test.o $ nm test.o | grep atomic ----------------- The problem seems to be that 'std::atomic<std::optional<double>> d' is a variable local to the main() function, and it gets optimized away with -O2. Turning it into a global variable fixes this. $ clang++ -o test.o -c -O2 test.cc $ clang++ -o test test.o /usr/bin/arm-linux-gnueabihf-ld: test.o: in function `main': test.cc:(.text+0x34): undefined reference to `__atomic_store' /usr/bin/arm-linux-gnueabihf-ld: test.cc:(.text+0xc8): undefined reference to `__atomic_load' clang++: error: linker command failed with exit code 1 (use -v to see invocation) 2203087 1 berto 2026-04-22 01:16:48 -0700 Pull request: https://github.com/WebKit/WebKit/pull/63310 2203907 2 ews-feeder 2026-04-23 15:41:30 -0700 Committed 311900@main (d65258118b1e): <https://commits.webkit.org/311900@main> Reviewed commits have been landed. Closing PR #63310 and removing active labels.