[VPlan] Remove unique_ptr from VPBranchOnRecipeMask (NFC).

[fhahn]

Summary

We can remove a dynamic memory allocation, by checking the number of
operands: no operands = all true, 1 operand = mask.

[Ayal]

Perhaps clearer to introduce getMask() as in other recipes, and check if it returns null?

[fhahn]

In D80110#2041354, @Ayal wrote:

Perhaps clearer to introduce getMask() as in other recipes, and check if it returns null?

Thanks Ayal, I've added & used a getMask() method in the committed version!

[Ayal]

In D80110#2044051, @fhahn wrote:

In D80110#2041354, @Ayal wrote:

Perhaps clearer to introduce getMask() as in other recipes, and check if it returns null?

Thanks Ayal, I've added & used a getMask() method in the committed version!

LGTM :-)
Could have inlined { VPValue *BlockInMask = getMask(); if (BlockInMask) }

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

 
 void VPBranchOnMaskRecipe::execute(VPTransformState &State) {
   assert(State.Instance && "Branch on Mask works only on single instance.");
 
   unsigned Part = State.Instance->Part;
   unsigned Lane = State.Instance->Lane;
 
   Value *ConditionBit = nullptr;
-  if (!User) // Block in mask is all-one.
-    ConditionBit = State.Builder.getTrue();
-  else {
-    VPValue *BlockInMask = User->getOperand(0);
+  VPValue *BlockInMask = getMask();
+  if (BlockInMask) {
     ConditionBit = State.get(BlockInMask, Part);
     if (ConditionBit->getType()->isVectorTy())
       ConditionBit = State.Builder.CreateExtractElement(
           ConditionBit, State.Builder.getInt32(Lane));
-  }
+  } else // Block in mask is all-one.
+    ConditionBit = State.Builder.getTrue();
 
   // Replace the temporary unreachable terminator with a new conditional branch,
   // whose two destinations will be set later when they are created.
   auto *CurrentTerminator = State.CFG.PrevBB->getTerminator();
   assert(isa<UnreachableInst>(CurrentTerminator) &&
          "Expected to replace unreachable terminator with conditional branch.");
   auto *CondBr = BranchInst::Create(State.CFG.PrevBB, nullptr, ConditionBit);
   CondBr->setSuccessor(0, nullptr);

llvm/lib/Transforms/Vectorize/VPlan.h

public:
 
   /// Print the recipe.
   void print(raw_ostream &O, const Twine &Indent,
              VPSlotTracker &SlotTracker) const override;
 };
 
 /// A recipe for generating conditional branches on the bits of a mask.
 class VPBranchOnMaskRecipe : public VPRecipeBase {
-  std::unique_ptr<VPUser> User;
+  VPUser User;
 
 public:
   VPBranchOnMaskRecipe(VPValue *BlockInMask) : VPRecipeBase(VPBranchOnMaskSC) {
     if (BlockInMask) // nullptr means all-one mask.
-      User.reset(new VPUser({BlockInMask}));
+      User.addOperand(BlockInMask);
   }
 
   /// Method to support type inquiry through isa, cast, and dyn_cast.
   static inline bool classof(const VPRecipeBase *V) {
     return V->getVPRecipeID() == VPRecipeBase::VPBranchOnMaskSC;
   }
 
   /// Generate the extraction of the appropriate bit from the block mask and the
   /// conditional branch.
   void execute(VPTransformState &State) override;
 
   /// Print the recipe.
   void print(raw_ostream &O, const Twine &Indent,
              VPSlotTracker &SlotTracker) const override {
     O << " +\n" << Indent << "\"BRANCH-ON-MASK ";
-    if (User)
-      User->getOperand(0)->print(O, SlotTracker);
+    if (VPValue *Mask = getMask())
+      Mask->print(O, SlotTracker);
     else
       O << " All-One";
     O << "\\l\"";
   }
+
+  /// Return the mask used by this recipe. Note that a full mask is represented
+  /// by a nullptr.
+  VPValue *getMask() const {
+    assert(User.getNumOperands() <= 1 && "should have either 0 or 1 operands");
+    // Mask is optional.
+    return User.getNumOperands() == 1 ? User.getOperand(0) : nullptr;
+  }
 };
 
 /// VPPredInstPHIRecipe is a recipe for generating the phi nodes needed when
 /// control converges back from a Branch-on-Mask. The phi nodes are needed in
 /// order to merge values that are set under such a branch and feed their uses.
 /// The phi nodes can be scalar or vector depending on the users of the value.
 /// This recipe works in concert with VPBranchOnMaskRecipe.
 class VPPredInstPHIRecipe : public VPRecipeBase {