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formation phase and into the initial walk of the basic blocks. We essentially pre-merge all blocks where unanalyzable fallthrough exists, as we won't be able to update the terminators effectively after any reorderings. This is quite a bit more principled as there may be CFGs where the second half of the unanalyzable pair has some analyzable predecessor that gets placed first. Then it may get placed next, implicitly breaking the unanalyzable branch even though we never even looked at the part that isn't analyzable. I've included a test case that triggers this (thanks Benjamin yet again!), and I'm hoping to synthesize some more general ones as I dig into related issues. Also, to make this new scheme work we have to be able to handle branches into the middle of a chain, so add this check. We always fallback on the incoming ordering. Finally, this starts to really underscore a known limitation of the current implementation -- we don't consider broken predecessors when merging successors. This can caused major missed opportunities, and is something I'm planning on looking at next (modulo more bug reports). git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144994 91177308-0d34-0410-b5e6-96231b3b80d8
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