[PATCH] D81584: [BreakCritEdges] Support preserving loop-simplify form with indirectbr. (WIP)

[Eli Friedman via Phabricator via llvm-commits]

efriedma added a comment.

> Agreed. I think that would go well with D81582 <https://reviews.llvm.org/D81582>, i.e. have a PreserveLoopSimplify option, that guarantees loop-simplify form is preserved or nullptr is returned if it's not possible

We can kill off the call to SplitBlockPredecessors, and add an option to bail out if preserving loop-simplify form would require transforming other edges, sure.  (I'd rather not condition it on whether the other edges are IndirectBr edges; I'd like to avoid obscure IndirectBr edge cases where possible.)

> Also, do you agree that preserving loop-simplify form by moving instructions to a different BB is more trouble than it is worth?

This doesn't seem worse than calling SplitBlockPredecessors(), in terms of transforming the CFG in unpredictable ways.



================
Comment at: llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp:324
+          DT->applyUpdates({{DominatorTree::Insert, NewBB, ExitCont},
+                            {DominatorTree::Delete, NewBB, DestBB}});
         }
----------------
fhahn wrote:
> efriedma wrote:
> > Missing LoopInfo update?
> I don't think we need to do anything about LI here, as the block should already be added to LI by `SplitBlock` and we only adjust the successor to another successor in the same loop I think.
Okay, that makes sense.


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  https://reviews.llvm.org/D81584/new/

https://reviews.llvm.org/D81584