[VPlan] Add getSingleUser helper (NFC).

Add helper to make it easier to retrieve the single user of a VPUser.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -4226,18 +4226,16 @@ VectorizationFactor LoopVectorizationPlanner::selectVectorizationFactor() {
           // Selects are only modelled in the legacy cost model for safe
           // divisors.
           case Instruction::Select: {
-            VPValue *VPV = VPI->getVPSingleValue();
-            if (VPV->getNumUsers() == 1) {
-              if (auto *WR = dyn_cast<VPWidenRecipe>(*VPV->user_begin())) {
-                switch (WR->getOpcode()) {
-                case Instruction::UDiv:
-                case Instruction::SDiv:
-                case Instruction::URem:
-                case Instruction::SRem:
-                  continue;
-                default:
-                  break;
-                }
+            if (auto *WR =
+                    dyn_cast_or_null<VPWidenRecipe>(VPI->getSingleUser())) {
+              switch (WR->getOpcode()) {
+              case Instruction::UDiv:
+              case Instruction::SDiv:
+              case Instruction::URem:
+              case Instruction::SRem:
+                continue;
+              default:
+                break;
               }
             }
             C += VPI->cost(VF, CostCtx);
@@ -6976,11 +6974,10 @@ static bool planContainsAdditionalSimplifications(VPlan &Plan,
   // the more accurate VPlan-based cost model.
   for (VPRecipeBase &R : *Plan.getVectorPreheader()) {
     auto *VPI = dyn_cast<VPInstruction>(&R);
-    if (!VPI || VPI->getOpcode() != Instruction::Select ||
-        VPI->getNumUsers() != 1)
+    if (!VPI || VPI->getOpcode() != Instruction::Select)
       continue;
 
-    if (auto *WR = dyn_cast<VPWidenRecipe>(*VPI->user_begin())) {
+    if (auto *WR = dyn_cast_or_null<VPWidenRecipe>(VPI->getSingleUser())) {
       switch (WR->getOpcode()) {
       case Instruction::UDiv:
       case Instruction::SDiv:

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -580,10 +580,13 @@ void VPlanTransforms::removeDeadRecipes(VPlan &Plan) {
 
       // Check if R is a dead VPPhi <-> update cycle and remove it.
       auto *PhiR = dyn_cast<VPPhi>(&R);
-      if (!PhiR || PhiR->getNumOperands() != 2 || PhiR->getNumUsers() != 1)
+      if (!PhiR || PhiR->getNumOperands() != 2)
+        continue;
+      VPUser *PhiUser = PhiR->getSingleUser();
+      if (!PhiUser)
         continue;
       VPValue *Incoming = PhiR->getOperand(1);
-      if (*PhiR->user_begin() != Incoming->getDefiningRecipe() ||
+      if (PhiUser != Incoming->getDefiningRecipe() ||
           Incoming->getNumUsers() != 1)
         continue;
       PhiR->replaceAllUsesWith(PhiR->getOperand(0));
@@ -1307,7 +1310,7 @@ static void simplifyRecipe(VPSingleDefRecipe *Def, VPTypeAnalysis &TypeInfo) {
       isa<VPPhi>(X)) {
     auto *Phi = cast<VPPhi>(X);
     if (Phi->getOperand(1) != Def && match(Phi->getOperand(0), m_ZeroInt()) &&
-        Phi->getNumUsers() == 1 && (*Phi->user_begin() == Def)) {
+        Phi->getSingleUser() == Def) {
       Phi->setOperand(0, Y);
       Def->replaceAllUsesWith(Phi);
       return;
@@ -1592,10 +1595,11 @@ static bool optimizeVectorInductionWidthForTCAndVFUF(VPlan &Plan,
 
     // Currently only handle cases where the single user is a header-mask
     // comparison with the backedge-taken-count.
-    if (!match(*WideIV->user_begin(),
-               m_ICmp(m_Specific(WideIV),
-                      m_Broadcast(
-                          m_Specific(Plan.getOrCreateBackedgeTakenCount())))))
+    VPUser *SingleUser = WideIV->getSingleUser();
+    if (!SingleUser ||
+        !match(SingleUser, m_ICmp(m_Specific(WideIV),
+                                  m_Broadcast(m_Specific(
+                                      Plan.getOrCreateBackedgeTakenCount())))))
       continue;
 
     // Update IV operands and comparison bound to use new narrower type.
@@ -1607,7 +1611,7 @@ static bool optimizeVectorInductionWidthForTCAndVFUF(VPlan &Plan,
     auto *NewBTC = new VPWidenCastRecipe(
         Instruction::Trunc, Plan.getOrCreateBackedgeTakenCount(), NewIVTy);
     Plan.getVectorPreheader()->appendRecipe(NewBTC);
-    auto *Cmp = cast<VPInstruction>(*WideIV->user_begin());
+    auto *Cmp = cast<VPInstruction>(WideIV->getSingleUser());
     Cmp->setOperand(1, NewBTC);
 
     MadeChange = true;

llvm/lib/Transforms/Vectorize/VPlanValue.h

@@ -150,6 +150,13 @@ class LLVM_ABI_FOR_TEST VPValue {
 
   bool hasOneUse() const { return getNumUsers() == 1; }
 
+  /// Return the single user of this value, or nullptr if there is not exactly
+  /// one user.
+  VPUser *getSingleUser() { return hasOneUse() ? *user_begin() : nullptr; }
+  const VPUser *getSingleUser() const {
+    return hasOneUse() ? *user_begin() : nullptr;
+  }
+
   void replaceAllUsesWith(VPValue *New);
 
   /// Go through the uses list for this VPValue and make each use point to \p