[llvm] [VPlan] Constant-fold ActiveLaneMask intrinsics (PR #208852)

[Ramkumar Ramachandra via llvm-commits]

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@@ -1355,6 +1355,34 @@ VPIRValue *vputils::tryToFoldLiveIns(VPSingleDefRecipe &R,
     case Instruction::ExtractElement:
       assert(!Ops[0]->getType()->isVectorTy() && "Live-ins should be scalar");
       return Ops[0];
+    case VPInstruction::ActiveLaneMask:
+    case VPInstruction::WideActiveLaneMask: {
+      uint64_t Multiplier = 1;
+      if (Opcode == VPInstruction::WideActiveLaneMask) {
+        // Optimizing WideALM can only happen after the Plan is unrolled.
+        if (!Plan.isUnrolled())
+          return nullptr;
+        Multiplier = cast<ConstantInt>(Ops[2])->getZExtValue();
+        Ops.pop_back();
+      }
+
+      // We rely on the fact that different VPlans are created for the
+      // fixed-vector and scalable-vector cases.
+      ElementCount MaxVF =
+          *max_element(Plan.vectorFactors(), ElementCount::isKnownLT) *
+          Multiplier;
+
+      Type *I1Ty = IntegerType::getInt1Ty(Plan.getContext());
+      if (auto *C = dyn_cast_if_present<Constant>(Folder.FoldIntrinsic(
+              Intrinsic::get_active_lane_mask, Ops,
+              VectorType::get(I1Ty, MaxVF), {}, Plan.getIRFunction())))
+        // We cannot handle vector constants that are not all-true or all-false,
+        // because they would not be collapsable to a scalar constant, that
+        // would be necessary for live-in simplification. We do not produce ALMs
+        // foldable to false at the moment.
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artagnon wrote:

Thanks for the additional tests!

https://github.com/llvm/llvm-project/pull/208852