llvm / clang and thread_local storage problems

[Ken Cunningham]

I've run into this again tonight.

I'm using, at this moment, clang-3.7 / llvm-3.7 with macports-created libc++ and libc++abi.


Every once in a while, a port I'm trying to create or build will error out due to this:

error: thread-local storage is not supported for the current target

/opt/local/var/macports/build/_opt_myports_devel_glbinding/glbinding/work/build/source/glbinding/include/glbinding/glbinding_features.h:240:36: note: expanded from macro 'GLBINDING_THREAD_LOCAL'
#    define GLBINDING_THREAD_LOCAL __thread
                                   ^

In this case, I'm trying to create this port <https://github.com/cginternals/glbinding> which is a dev library for openGL.


As I understand it, thread_local storage is a feature of cxx11 that should be - but apparently isn't - supported by clang / llvm.

This page <http://clang.llvm.org/cxx_status.html#n2659> says it is enabled in clang-3.3, if your c++ runtime provides __cxa_thread_atexit, like libc++abi 3.6 or later.
I have   libcxxabi @3.7.0_1+universal (active) platform='darwin 10' archs='i386 x86_64'

there's a thread on stackoverflow about it:

<http://stackoverflow.com/questions/28094794/why-does-apple-clang-disallow-c11-thread-local-when-official-clang-supports>

which has this quote:

For Xcode 7.x and earlier, here is an answer from 2014 from an Apple engineer on the Apple Developer Forum:

We don't support the thread_local implementation from the open-source Clang because we believe we can provide a higher-performance implementation for our platforms using various features in the dynamic linker. Such an implementation would be ABI-incompatible with the implementation in the open-source Clang, so we won't support thread_local until we get an implementation we can live with for the foreseeable future.

There are many smart people on this list who know a lot more about this kind of thing than I ever will.

Anyone offer any insight into this issue?

best,

Ken