Does clang++ do threadsafe initialisation of local static variables?

Question

Does clang++ do threadsafe initialisation of local static variables?

GCC does, unless you turn it off with the misleadingly named -fno-threadsafe-statics. I'm using Apple clang version 2.0 (tags/Apple/clang-139) (based on LLVM 2.9svn).

Answer 1

Yes, same as with GCC (so can also be turned off with -fno-threadsafe-statics)

Answer 2

Yes, although Apple's libcppabi implements this in a suboptimal way: with a global mutex which harms performance if many threads are using many local statics.

Source code: http://www.opensource.apple.com/source/libcppabi/libcppabi-26/src/cxa_guard.cxx

Bug report: https://bugreports.qt.io/browse/QTBUG-33967 (which led Qt to implement this differently when libcppabi is used).

I hope making more people aware of this will result in Apple fixing their code one day...