[llvm] [AArch64][CostModel] Reduce cost of wider than legal get.active.lane.mask (PR #163786)

[David Sherwood via llvm-commits]

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@@ -957,10 +957,24 @@ AArch64TTIImpl::getIntrinsicInstrCost(const IntrinsicCostAttributes &ICA,
     return TyL.first + ExtraCost;
   }
   case Intrinsic::get_active_lane_mask: {
-    auto *RetTy = dyn_cast<FixedVectorType>(ICA.getReturnType());
-    if (RetTy) {
-      EVT RetVT = getTLI()->getValueType(DL, RetTy);
-      EVT OpVT = getTLI()->getValueType(DL, ICA.getArgTypes()[0]);
+    auto RetTy = cast<VectorType>(ICA.getReturnType());
+    EVT RetVT = getTLI()->getValueType(DL, RetTy);
+    EVT OpVT = getTLI()->getValueType(DL, ICA.getArgTypes()[0]);
+    if (RetTy->isScalableTy()) {
+      // When SVE2p1 or SME2 is available, get_active_lane_mask will lower
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david-arm wrote:

nit: Should this say "get_active_lane_mask *may* lower"? If I understand correctly we only use the while pair when we know the result will be split into parts?

https://github.com/llvm/llvm-project/pull/163786