[PATCH] D56913: decoupling Freestanding atomic<T> from libatomic.a

[Olivier Giroux via Phabricator]

__simt__ created this revision.
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Hello,

I volunteered to write a first patch to make use of a Freestanding configuration in libcxx. The goal of this patch is to decouple non-lock-free atomic<T> from platform libraries, like libatomic.a, when a Freestanding implementation is selected. This plays a big role in the accelerator-integrated library that we are working on.

This patch is based on the GitHub monorepo as of today.

Copy/pasting how this works from my email thread to -dev:

1. Creates a single __cxx_atomic_* layer which is an adaptation of either __c11_atomic_*, preferentially, or else __atomic_*. (For __atomic_*, it’s a rename of the layer that’s there now. For __c11_atomic_*, it’s a new but boring wrapper that just forwards calls.)

2. Creates a __cxx_atomic_type<> template that is an alias to what should wrap the _Tp inside of an atomic<T> / atomic_flag. Defaults to the same _Atomic(_Tp) as today, either from C11 or from the adaptation layer.

3. Under #ifdef LIBCXX_FREESTANDING, introduce __cxx_locked_atomic_type<> with an implementation of __cxx_atomic_*, and redirect __cxx_atomic_type<> based on always-lock-freedom. a. In C++17 mode, it uses __atomic_is_always_lock_free. b. Otherwise, it uses the C lock-free macros as shown in wg21.link/p0152.

Thanks for looking this over,

Olivier


Repository:
  rCXX libc++

https://reviews.llvm.org/D56913

Files:
  libcxx/include/atomic

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