clang-13: fix for SnowLeopard

[kencu (Ken)]

FYI clang-13 / flang-13 / mlir-13 builds without trouble on SnowLeopard if libcxx has been upgraded to the version that supports thread_local_storage:

To rebuild libcxx with full thread_local support, please install clang-9.0 or any macports-clang >= clang-5.0 and then rebuild libcxx like this:

sudo port -v -n upgrade --force --enforce-variants libcxx +emulated_tls

and if the attached patch is used.

$ port -v installed clang-13
The following ports are currently installed:
  clang-13 @13.0.0_0+analyzer+defaultlibcxx+libstdcxx (active) requested_variants='' platform='darwin 10' archs='x86_64' date='2021-10-27T09:39:39-0700'

[devernay (Frédéric Devernay)]

Great!

Any reason why +emulated_tls should not be the default (and maybe not even a variant) on Snow Leopard, since a modern clang is pretty much a requirement anyway?

[kencu (Ken)]

We have not been able to automatically bootstrap from bare metal SnowLeopard to a full TLS-supporting libc++ to date.

It is still a two step process -- install libcxx, install clang-9.0 or so, then reinstall libcxx with emulated_tls.

I had an idea to use a bootstrap self-contained version of gcc7 or gcc10 to build clang-something (maybe clang-5.0?) against libstdc++, which is what Ubuntu does, and use that clang as the bootstrap, to build libcxx (with emulated TLS) in one go, and then build clang-9.0, etc against that libc++. Just never got around to it.

Another option -- to download and install a prebuilt libc++ with emulated_tls enabled, and then use that to bootstrap to a new one that we build, was less desirable.

[mojca (Mojca Miklavec)]

Ken, would you be willing to add that patch to the git tree (even if the build itself is kept disabled for now)?

[kencu (Ken)]

sure can.

[kencu (Ken)]

In f42838cad0d35f2a2d48e70e5b22d96f7769a0f5/macports-ports (master):

clang-13: fix and allow build on 10.6

closes: #63707