[PATCH] D116688: [SLP]Excluded external uses from the reprdering estimation.

[Alexey Bataev via Phabricator via llvm-commits]

ABataev added a comment.

In D116688#3224141 <https://reviews.llvm.org/D116688#3224141>, @vporpo wrote:

> I don't fully understand why you are completely removing the cost of external uses. This is modeling the additional extract instructions that will be needed if we decide to proceed with that specific operand order. This is useful when you have to break ties, for example if we to choose between instructions of the same opcode, one with external uses and the other without. In that case we would prefer to vectorize the instructions without uses. Perhaps the current implementation is causing issues because the external cost is always subtracted (line 1233) and is not just used as a tie breaker only when the costs are the same.
> If I understand correctly the splat cost is orthogonal to the the external uses? Can't we have both?
>
> Also could you split the MainAlt changes into a separate patch?

Sure, I will split the patch (already did it for the first part).

There are several reasons.

1. External uses. It will affect the cost in any case (does not matter in which lane the scalar is used) so we can just ignore it.
2. In-tree use or use in graph. Currently it is too pessimistic. We subtract 1 for each such use and the total cost of the lane becomes NumLanes less, though in the worst case it should be just 1-2 less because we end up just with a single shuffle, not with the gather of extracts. Better just to ignore all these uses and just count number of unique scalar to see if we can vectorize the lane after removing reused scalars.


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