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Summary: The goal is to eventually remove all the code related to getInsertFencesForAtomic in SelectionDAGBuilder as it is wrong (designed for ARM, not really portable, works mostly by accident because the backends are overly conservative), and repeats the same logic that goes in emitLeading/TrailingFence. In this patch, I make AtomicExpandPass insert the fences as it knows better where to put them. Because this requires getting the fences and not just passing an IRBuilder around, I had to change the return type of emitLeading/TrailingFence. This code only triggers on ARM for now. Because it is earlier in the pipeline than SelectionDAGBuilder, it triggers and lowers atomic accesses to atomic so SelectionDAGBuilder does not add barriers anymore on ARM. If this patch is accepted I plan to implement emitLeading/TrailingFence for all backends that setInsertFencesForAtomic(true), which will allow both making them less conservative and simplifying SelectionDAGBuilder once they are all using this interface. This should not cause any functionnal change so the existing tests are used and not modified. Test Plan: make check-all, benefits from existing tests of atomics on ARM Reviewers: jfb, t.p.northover Subscribers: aemerson, llvm-commits Differential Revision: http://reviews.llvm.org/D5179 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218329 91177308-0d34-0410-b5e6-96231b3b80d8