[PATCH] D58373: [Dominators] Avoid potentially quadratic std::is_permutation

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added a comment.

In D58373#1402235 <https://reviews.llvm.org/D58373#1402235>, @kuhar wrote:

> `Roots.size()` is only > 1 for the PostDominatorTree, and I'd expect it to be small (< 100) on virtually all functions. `is_permulation` was used in the original code because it doesn't allocate and should be cheaper for small trees.
>
> Do you have some statistics on the size of Roots? At what size does SmallPtrSet start to outperform `is_permuatation`?


Thanks for the empirical data (<100). I find SmallPtrSet<void *, 4> starts to outperform `std::is_permutation` when there are about 16 elements on my machine. It may be 2x slower (should be larger, but I put it in a loop and the loop itself has some costs) when there is only one root. So the benefit to use a `SmallPtrSet` here is just to filter out some unlikely cases (>1000 roots) and to clear up my doubts when I see that `verifyRoots` claims to be O(N)...


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