[Diffusion] rL244063: Add missing atomic libcall support.

[Richard Smith via cfe-commits]

rsmith added a subscriber: rsmith.
rsmith raised a concern with this commit.

/cfe/trunk/lib/CodeGen/CGAtomic.cpp:941-943 According to the GCC documentation for libatomic:

> for i = __atomic_add_fetch (ptr, j, model) the code generated will be
>
>        tmp = __atomic_fetch_add (ptr, j, model);
>        i = tmp + j;

We shouldn't be using an undocumented part of GCC's libatomic here -- GCC doesn't use these functions, and compiler-rt's atomic.c does not provide them.

Users:
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  rsmith (Auditor)

http://reviews.llvm.org/rL244063