[clang] [llvm] [LV] Mask off possibly aliasing vector lanes (PR #100579)

[Sam Tebbs via cfe-commits]

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@@ -8520,7 +8561,29 @@ LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(VFRange &Range) {
   // When not folding the tail, we know that the induction increment will not
   // overflow.
   bool HasNUW = Style == TailFoldingStyle::None;
-  addCanonicalIVRecipes(*Plan, Legal->getWidestInductionType(), HasNUW, DL);
+
+  VPValue *AliasMask = nullptr;
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SamTebbs33 wrote:

Currently the mask is only generated when tail predication is enabled, in which case it perhaps could be done with a transform since all the vector loads and stores will be masked anyway. However the goal is to generate the mask even when tail predication is disabled, to capture more cases, and so the transform would have to handle swapping the non-masked loads and stores to masked variants, which I don't think is best done with a transform since the original VPlan does a good job of that anyway.

I'd also argue that it's a good idea to get the masking as we want it to be from the beginning rather than setting up an active lane mask and building on top of it later on.

https://github.com/llvm/llvm-project/pull/100579