[llvm] [LV] Convert uniform-address unmasked scatters to scalar store. (PR #166114)

[Florian Hahn via llvm-commits]

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@@ -1400,14 +1400,46 @@ static void narrowToSingleScalarRecipes(VPlan &Plan) {
   for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
            vp_depth_first_shallow(Plan.getVectorLoopRegion()->getEntry()))) {
     for (VPRecipeBase &R : make_early_inc_range(reverse(*VPBB))) {
-      if (!isa<VPWidenRecipe, VPWidenSelectRecipe, VPReplicateRecipe>(&R))
+      if (!isa<VPWidenRecipe, VPWidenSelectRecipe, VPReplicateRecipe,
+               VPWidenStoreRecipe>(&R))
         continue;
       auto *RepR = dyn_cast<VPReplicateRecipe>(&R);
       if (RepR && (RepR->isSingleScalar() || RepR->isPredicated()))
         continue;
 
-      auto *RepOrWidenR = cast<VPSingleDefRecipe>(&R);
-      if (RepR && isa<StoreInst>(RepR->getUnderlyingInstr()) &&
+      // Convert scatters with a uniform address that is unmasked into an
+      // extract-last-element + scalar store.
+      // TODO: Add a profitability check comparing the cost of a scatter vs.
+      // extract + scalar store.
+      auto *WidenStoreR = dyn_cast<VPWidenStoreRecipe>(&R);
+      if (WidenStoreR && vputils::isSingleScalar(WidenStoreR->getAddr()) &&
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fhahn wrote:

Is the reason we create a scatter initial, but can simplify it here due to some other VPlan transformation? 

https://github.com/llvm/llvm-project/pull/166114