[llvm] [LV] Use ExtractLane(LastActiveLane, V) live outs when tail-folding. (PR #149042)

[Luke Lau via llvm-commits]

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@@ -3342,6 +3370,34 @@ void VPlanTransforms::convertToConcreteRecipes(VPlan &Plan) {
         ToRemove.push_back(Expr);
       }
 
+      // Expand LastActiveLane into Not + FirstActiveLane + Sub.
+      auto *LastActiveL = dyn_cast<VPInstruction>(&R);
+      if (LastActiveL &&
+          LastActiveL->getOpcode() == VPInstruction::LastActiveLane) {
+        // Create Not(Mask) for all operands.
+        SmallVector<VPValue *, 2> NotMasks;
+        for (VPValue *Op : LastActiveL->operands()) {
+          VPValue *NotMask = Builder.createNot(Op, LastActiveL->getDebugLoc());
+          NotMasks.push_back(NotMask);
+        }
+
+        // Create FirstActiveLane on the inverted masks.
+        VPValue *FirstInactiveLane = Builder.createNaryOp(
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lukel97 wrote:

I'm also realising now that we need to model the use of the header mask to prevent removeDeadRecipes from stripping it if e.g. it's EVL tail folded and all other uses of it are removed. 

https://github.com/llvm/llvm-project/pull/149042