[PATCH] [Power] Use AtomicExpandPass for fence insertion, and use lwsync where appropriate

[Bill Schmidt]

I've reviewed this against what gcc does, and your emitLeadingFence matches exactly.  As you've noted, what you're doing for emitTrailingFence is conservative.  GCC generates an isync in rs6000.c:rs6000_post_atomic_barrier() for these cases.

In short, the patch LGTM, and you can improve it later if desired.

http://reviews.llvm.org/D5180