[PATCH] [libcxx] Add __atomic header that handles needed atomic operations.

[Jonathan Roelofs]

================
Comment at: include/__atomic:85
@@ +84,3 @@
+{
+    *__dest = __val;
+}
----------------
EricWF wrote:
> jroelofs wrote:
> > Should these fall back on the __sync_* ones instead of just dropping the atomicness? It seems a little precarious to silently default to non-atomic implementations for these (unless this is for a single-threaded build of the library).
> Yes and no. The only atomic operations needed in the headers are relaxed atomic loads. AFAIK all loads are relaxed atomic loads on x86 so not using atomic builtins isn't a big deal (its what we already do).
> 
> The rest of the atomic operations happen within the dylib. I agree we don't want to drop the atomicness on these unless it is a single threaded build.
> 
> What we really should do is limit the libc++ dylib build to compilers that provide the required atomic intrinsics (single-threaded builds not withstanding). If the compiler is not supported then the build should fail. For GCC and Clang the required versions would be 4.7 and 3.3 (possibly lower) respectively.
> 
> Since most of the stuff in `__atomic` is only used in the dylib build perhaps it should be moved out of the shipped headers. I'll figure out how to handle that tomorrow.
> AFAIK all loads are relaxed atomic loads on x86 so not using atomic builtins isn't a big deal

What about PPC-darwin?

> What we really should do is limit the libc++ dylib build to compilers that provide the required atomic intrinsics (single-threaded builds not withstanding)

Agreed.

http://reviews.llvm.org/D10406

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