[PATCH] D87408: [NFC] EliminateDuplicatePHINodes(): drop DenseMap-driven CSE in favor of quadratic algorithmn

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

In D87408#2264270 <https://reviews.llvm.org/D87408#2264270>, @nikic wrote:

> The problem here is that this is O(n^2) in the number of phi nodes, rather than O(n log n). So this will be faster for average inputs, but potentially much slower for degenerate cases. That said, I can't say that I've encountered "block with ten thousand phi nodes" as a problem before.

Yes.

> It might still make sense to limit this heuristically, e.g. by limiting the inner loop to at most 100 iterations (which may fail to CSE some phi nodes in degenerate cases, but will avoid quadratic blowup.)

Before doing that, there is one more thing i guess i should try first - it doesn't make sense to reprocess *all* phis,
we should partition them by the type, and only reprocess the group in which we CSE'd.


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