[PATCH] D79037: [StructurizeCFG] Fix region nodes ordering

[Sameer Sahasrabuddhe via Phabricator via llvm-commits]

sameerds added a comment.

I have bad news. This fix does not help with the test needs-fr-ule.ll, i.e., the resulting CFG is incorret if I remove the unify-loop-exits pass. The fix-irreducible pass produces an inner loop with the header "irr.guard" and a latch "while.cond47". This is inside an outer loop with header "while.cond" and many backedges. When I run the structurizer with the current fix on this graph, the backedge of the inner loop is lost. Instead we see a spurious loop between two new blocks Flow5 and Flow4, which is governed by incorrect boolean constants when in fact they should have been %Pred variables. That information is lost along the edge Flow8 to Flow5.

Now I am kicking myself for not remembering this earlier. I had encountered the same problem, which is why we introduced the unify-loop-exits pass. Things are not so easy with nested loops. It's not enough to process all inner blocks before outer blocks. The ordering of the outer blocks relative to the inner loop also matters, and there does not seem to be a practical way to preserve it.


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  https://reviews.llvm.org/D79037/new/

https://reviews.llvm.org/D79037