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

[Elvis Wang via llvm-commits]

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@@ -1598,14 +1598,26 @@ static void narrowToSingleScalarRecipes(VPlan &Plan) {
                "Not consecutive memory recipes shouldn't be reversed");
         VPValue *Mask = WidenStoreR->getMask();
 
-        // Only convert the scatter to a scalar store if it is unmasked.
-        // TODO: Support converting scatter masked by the header mask to scalar
-        // store.
-        if (Mask)
+        // Convert the scatter to a scalar store if it is unmasked or header
+        // masked.
+        if (Mask && !vputils::isHeaderMask(Mask, Plan))
           continue;
 
-        auto *Extract = new VPInstruction(VPInstruction::ExtractLastLane,
-                                          {WidenStoreR->getOperand(1)});
+        VPInstruction *Extract;
+        if (!Mask) {
+          Extract = new VPInstruction(VPInstruction::ExtractLastLane,
+                                      {WidenStoreR->getOperand(1)});
+        } else {
+          // If the mask is the header mask, this mask contains at least one
+          // active lane. So it is safe to convert the scatter to a scalar
+          // store.
+          assert(vputils::isHeaderMask(Mask, Plan) &&
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ElvisWang123 wrote:

Removed, thanks!

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