[PATCH] D57779: [SLP] Add support for throttling.

[Dinar Temirbulatov via Phabricator via llvm-commits]

dtemirbulatov added a comment.

In D57779#1509166 <https://reviews.llvm.org/D57779#1509166>, @vporpo wrote:

> I think the first throttling patch should implement a very simple and fast algorithm for finding the cut:
>
> 1. Add new fields to TreeEntry for Cost, ExtractCost and PredecessorsCost.
> 2. During getTreeCost() set the TE.Cost and TE.ExtractCost (as you did in an earlier version of the patch if I am not mistaken)
> 3. Do a single top-down traversal of the tree in reverse postorder and set the TE.PredecessorsCost equal to the cost of all the predecessor's costs until TE. While doing so, you can compare the cost of cutting just below TE by comparing the gather cost of TE versus the Cost + PredecessorsCost. This is very fast as you only need to visit each TreeEntry node once, so the complexity is linear to the size of the tree.
>
>   For example, in slp-throttle.ll the bundle that needs to be scalarized [%add19, %sub22] has costs of Cost=1, ExtractCost = 0, PredecessorsCost=1 (because of bundle [%mul18, undef]). Cutting below the bundle has a cost of +1, while keeping it vectorized has a cost of +2 (Cost=1 + PredecessorsCost=1).
>
>   This should be good-enough for most simple cases. We can improve it later, if needed, with follow-up patches. What do you think?


yes, Looks good.


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