[NFC][VPlan] Use VPUser to store block's predicate

[fhahn]

Thanks for taking a stab at this after D95382!

llvm/lib/Transforms/Vectorize/VPlan.h

584

The functions here are essentially the same as for CondBit, right? Could you factor out the logic into private helpers, that take a reference to VPUser?

[a.elovikov]

Move common implementation of get/set methods into helpers

[fhahn]

LGTM, thanks!

llvm/lib/Transforms/Vectorize/VPlan.h

 //===- VPlan.h - Represent A Vectorizer Plan --------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
class VPBlockBase {
   /// List of successor blocks.
   SmallVector<VPBlockBase *, 1> Successors;
 
   /// Successor selector managed by a VPUser. For blocks with zero or one
   /// successors, there is no operand. Otherwise there is exactly one operand
   /// which is the branch condition.
   VPUser CondBitUser;
 
-  /// Current block predicate - null if the block does not need a predicate.
-  VPValue *Predicate = nullptr;
+  /// If the block is predicated, its predicate is stored as an operand of this
+  /// VPUser to maintain the def-use relations. Otherwise there is no operand
+  /// here.
+  VPUser PredicateUser;
 
   /// VPlan containing the block. Can only be set on the entry block of the
   /// plan.
   VPlan *Plan = nullptr;
 
   /// Add \p Successor as the last successor to this block.
   void appendSuccessor(VPBlockBase *Successor) {
     assert(Successor && "Cannot add nullptr successor!");
if (!CV) {
       return;
     }
     if (CondBitUser.getNumOperands() == 1)
       CondBitUser.setOperand(0, CV);
     else
       CondBitUser.addOperand(CV);
   }
 
-  VPValue *getPredicate() { return Predicate; }
+  VPValue *getPredicate() {
+    if (PredicateUser.getNumOperands())
+      return PredicateUser.getOperand(0);
+    return nullptr;
+  }
 
-  const VPValue *getPredicate() const { return Predicate; }
+  const VPValue *getPredicate() const {
+    if (PredicateUser.getNumOperands())
+      return PredicateUser.getOperand(0);
+    return nullptr;
+  }
 
-  void setPredicate(VPValue *Pred) { Predicate = Pred; }
+  void setPredicate(VPValue *Pred) {
+    if (!Pred) {
+      if (PredicateUser.getNumOperands() == 1)
+        PredicateUser.removeLastOperand();
+      return;
+    }
+    if (PredicateUser.getNumOperands() == 1)
+      PredicateUser.setOperand(0, Pred);
+    else
+      PredicateUser.addOperand(Pred);
+  }
 
   /// Set a given VPBlockBase \p Successor as the single successor of this
   /// VPBlockBase. This VPBlockBase is not added as predecessor of \p Successor.
   /// This VPBlockBase must have no successors.
   void setOneSuccessor(VPBlockBase *Successor) {
     assert(Successors.empty() && "Setting one successor when others exist.");
     appendSuccessor(Successor);
   }