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f45051c79a
Two observations: 1. Unavailability of DomTree makes it impossible to make `FoldBranchToCommonDest()` transform in certain cases, where the successor is dominated by predecessor, because we then don't have PHI's, and can't recreate them, well, without handrolling 'is dominated by' check, which doesn't really look like a great solution to me. 2. Avoiding invalidating DomTree in SimplifyCFG will decrease the number of `Dominator Tree Construction` by 5 (from 28 now, i.e. -18%) in `-O3` old-pm pipeline (as per `llvm/test/Other/opt-O3-pipeline.ll`) This might or might not be beneficial for compile time. So the plan is to make SimplifyCFG preserve DomTree, and then eventually make DomTree fully required and preserved by the pass. Now, SimplifyCFG is ~7KLOC. I don't think it will be nice to do all this uplifting in a single mega-commit, nor would it be possible to review it in any meaningful way. But, i believe, it should be possible to do this in smaller steps, introducing the new behavior, in an optional way, off-by-default, opt-in option, and gradually fixing transforms one-by-one and adding the flag to appropriate test coverage. Then, eventually, the default should be flipped, and eventually^2 the flag removed. And that is what is happening here - when the new off-by-default option is specified, DomTree is required and is claimed to be preserved, and SimplifyCFG-internal assertions verify that the DomTree is still OK. |
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