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

[David Sherwood 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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david-arm wrote:

I think we have to be careful about this transformation because it's making the assumption that there are no holes in the mask. For example, you'd presumably get something unexpected if you transform LastActiveLane(11100111000) -> FirstActiveLane(00011000111).

It might require adding documentation to the opcodes saying that holes are not permitted when using the LastActiveLane opcode. I don't know if there is a way to add an assert here that the mask doesn't contain holes? Presumably it should only be used when the mask has originally come get.active.lane.mask?

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