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The crux of the issue is that LCSSA doesn't preserve stateful alias analyses. Before r200067, LICM didn't cause LCSSA to run in the LTO pass manager, where LICM runs essentially without any of the other loop passes. As a consequence the globalmodref-aa pass run before that loop pass manager was able to survive the loop pass manager and be used by DSE to eliminate stores in the function called from the loop body in Adobe-C++/loop_unroll (and similar patterns in other benchmarks). When LICM was taught to preserve LCSSA it had to require it as well. This caused it to be run in the loop pass manager and because it did not preserve AA, the stateful AA was lost. Most of LLVM's AA isn't stateful and so this didn't manifest in most cases. Also, in most cases LCSSA was already running, and so there was no interesting change. The real kicker is that LCSSA by its definition (injecting PHI nodes only) trivially preserves AA! All we need to do is mark it, and then everything goes back to working as intended. It probably was blocking some other weird cases of stateful AA but the only one I have is a 1000-line IR test case from loop_unroll, so I don't really have a good test case here. Hopefully this fixes the regressions on performance that have been seen since that revision. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201104 91177308-0d34-0410-b5e6-96231b3b80d8
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