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

[Ramkumar Ramachandra via llvm-commits]

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@@ -1349,6 +1349,31 @@ 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();
+      }
+      Type *I1Ty = IntegerType::getInt1Ty(Plan.getContext());
+      ElementCount MaxVF =
+          *max_element(Plan.vectorFactors(), ElementCount::isKnownLT) *
----------------
artagnon wrote:

Ah, this was due to a comment dropped in the rebase: we create separate VPlans for the fixed-vector and scalable-vector cases, and rely on this property. >From LoopVectorizationPlanner::plan:

```cpp
  buildVPlans(*VPlan1, ElementCount::getFixed(1), MaxFactors.FixedVF);
  buildVPlans(*VPlan1, ElementCount::getScalable(1), MaxFactors.ScalableVF);
```

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