[PATCH] D38586: SLPVectorizer.cpp: Avoid std::stable_sort() due to asymmetric comparator

[Davide Italiano via Phabricator via llvm-commits]

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The issue here is that properlyDominates only captures the parent->child relation in the dom, and siblings in the {post}dominator tree can appear in any order.
I don't think stupid sort is the best way forward, as it seems to be badly O(N^2) in the number of nodes in the dom (i.e. the number of reachable blocks in the CFG).
You may want to instead tighten the check to have a deterministic ordering for siblings).
For example, if you do an RPO walk of the CFG for your function F, you can assign an integer (unique) starting from zero, everytime you visit a new block.
This is is a start, and it's O(N), but you want probably try to be smarter and piggy-back on some other BB scan (like the one happening before this sorting, although I haven't checked the details).


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  rL LLVM

https://reviews.llvm.org/D38586