[llvm] [LV] Convert scatter w/uniform addr and mask being header mask to scalar store. (PR #172799)

[Florian Hahn via llvm-commits]

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@@ -1926,8 +1926,33 @@ static void narrowToSingleScalarRecipes(VPlan &Plan) {
            vp_depth_first_deep(Plan.getEntry()))) {
     for (VPRecipeBase &R : make_early_inc_range(reverse(*VPBB))) {
       if (!isa<VPWidenRecipe, VPWidenGEPRecipe, VPReplicateRecipe,
-               VPWidenIntrinsicRecipe>(&R))
+               VPWidenIntrinsicRecipe, VPWidenStoreRecipe>(&R))
         continue;
+
+      // Narrow header masked scatter to scalar store.
+      auto *WidenStoreR = dyn_cast<VPWidenStoreRecipe>(&R);
+      if (WidenStoreR) {
+        if (!vputils::isUniformAcrossVFsAndUFs(WidenStoreR->getAddr()) ||
+            WidenStoreR->isConsecutive())
+          continue;
+        VPValue *Mask = WidenStoreR->getMask();
+        if (!Mask || !vputils::isHeaderMask(Mask, Plan))
+          continue;
+
+        VPBuilder Builder(WidenStoreR);
+        VPInstruction *LastActiveLane = Builder.createLastActiveLane(Mask);
+        VPInstruction *Extract = Builder.createNaryOp(
+            VPInstruction::ExtractLane,
+            {LastActiveLane, WidenStoreR->getStoredValue()});
+        auto *ScalarStore = new VPReplicateRecipe(
+            &WidenStoreR->getIngredient(), {Extract, WidenStoreR->getAddr()},
+            /*IsSingleScalar*/ true, /*Mask*/ nullptr, {},
+            /*Metadata*/ *WidenStoreR);
+        ScalarStore->insertBefore(WidenStoreR);
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fhahn wrote:

I've not looked at the test-suite diff closely, but I think there are cases where we won't be able to sink, e.g. if we have loads/stores which me alias?

https://github.com/llvm/llvm-project/pull/172799