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

[Ramkumar Ramachandra via llvm-commits]

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@@ -1338,6 +1338,26 @@ static VPIRValue *tryToFoldLiveIns(VPSingleDefRecipe &R,
     case Instruction::ExtractElement:
       assert(!Ops[0]->getType()->isVectorTy() && "Live-ins should be scalar");
       return Ops[0];
+    case VPInstruction::ActiveLaneMask: {
+      if (!isa<ConstantInt>(Ops[0]) || !isa<ConstantInt>(Ops[1]) ||
+          Plan.hasScalableVF())
+        return nullptr;
+      uint64_t Base = cast<ConstantInt>(Ops[0])->getZExtValue();
+      uint64_t Limit = cast<ConstantInt>(Ops[1])->getZExtValue();
+      uint64_t Multiplier = cast<ConstantInt>(Ops[2])->getZExtValue();
+      Type *I1Ty = Type::getInt1Ty(Plan.getContext());
+      VPBuilder Builder(&R);
+      unsigned MaxVF =
+          max_element(Plan.vectorFactors(), ElementCount::isKnownLT)
+              ->getFixedValue();
+      unsigned MinVF =
+          min_element(Plan.vectorFactors(), ElementCount::isKnownLT)
+              ->getFixedValue();
+      if (Base + MaxVF * Multiplier < Limit)
+        return ConstantInt::getTrue(I1Ty);
+      if (Base + MinVF * Multiplier > Limit)
+        return ConstantInt::getFalse(I1Ty);
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artagnon wrote:

Patch re-done, although it's more of a hobby task now than anything: the real optimization benefit is negligible :)

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