Fix non-determinisms

GVNSink used to order instructions based on their pointer values and was prone to
non-determinism because of that. Using DFSNumber for ordering all the values
fixes the non-determinism

Author: hiraditya

llvm/lib/Transforms/Scalar/GVNSink.cpp

@@ -222,20 +222,29 @@ raw_ostream &operator<<(raw_ostream &OS, const SinkingInstructionCandidate &C) {
 class ModelledPHI {
   SmallVector<Value *, 4> Values;
   SmallVector<BasicBlock *, 4> Blocks;
+  DominatorTree *DT;
 
 public:
   ModelledPHI() = default;
 
-  ModelledPHI(const PHINode *PN) {
+  ModelledPHI(const PHINode *PN, DominatorTree *DT) : DT(DT) {
     // BasicBlock comes first so we sort by basic block pointer order, then by value pointer order.
-    SmallVector<std::pair<BasicBlock *, Value *>, 4> Ops;
+    using OpsType = std::pair<BasicBlock *, Value *>;
+    SmallVector<OpsType, 4> Ops;
     for (unsigned I = 0, E = PN->getNumIncomingValues(); I != E; ++I)
       Ops.push_back({PN->getIncomingBlock(I), PN->getIncomingValue(I)});
-    llvm::sort(Ops);
+
+    auto DFSOrder = [DT](OpsType O1, OpsType O2) {
+      return DT->getNode(O1.first) < DT->getNode(O2.first);
+    };
+    // Sort by DFSNumber to have a deterministic order.
+    llvm::sort(Ops, DFSOrder);
+
     for (auto &P : Ops) {
       Blocks.push_back(P.first);
       Values.push_back(P.second);
     }
+    verifyModelledPHI();
   }
 
   /// Create a dummy ModelledPHI that will compare unequal to any other ModelledPHI
@@ -247,19 +256,37 @@ class ModelledPHI {
     return M;
   }
 
+  void verifyModelledPHI() {
+    assert(Values.size() > 1 && Blocks.size() > 1 &&
+           "Modelling PHI with less than 2 values");
+    auto DFSOrder = [this](const BasicBlock *BB1, const BasicBlock *BB2) {
+      return this->DT->getNode(BB1) < this->DT->getNode(BB2);
+    };
+    assert(llvm::is_sorted(Blocks, DFSOrder));
+    int C = 0;
+    llvm::for_each(Values, [&C, this](const Value* V) {
+      const Instruction *I = cast<Instruction>(V);
+      assert(I->getParent() == this->Blocks[C++]);
+    });
+  }
   /// Create a PHI from an array of incoming values and incoming blocks.
-  template <typename VArray, typename BArray>
-  ModelledPHI(const VArray &V, const BArray &B) {
+  ModelledPHI(SmallVectorImpl<Instruction *> &V,
+              SmallSetVector<BasicBlock *, 4> &B, DominatorTree *DT)
+      : DT(DT) {
+    // The order of Values and Blocks are already as per their DFSNumbers.
     llvm::copy(V, std::back_inserter(Values));
     llvm::copy(B, std::back_inserter(Blocks));
+    verifyModelledPHI();
   }
 
   /// Create a PHI from [I[OpNum] for I in Insts].
-  template <typename BArray>
-  ModelledPHI(ArrayRef<Instruction *> Insts, unsigned OpNum, const BArray &B) {
+  ModelledPHI(ArrayRef<Instruction *> Insts, unsigned OpNum,
+              SmallSetVector<BasicBlock *, 4> &B, DominatorTree *DT)
+      : DT(DT) {
     llvm::copy(B, std::back_inserter(Blocks));
     for (auto *I : Insts)
       Values.push_back(I->getOperand(OpNum));
+    verifyModelledPHI();
   }
 
   /// Restrict the PHI's contents down to only \c NewBlocks.
@@ -297,7 +324,8 @@ class ModelledPHI {
 
   // Hash functor
   unsigned hash() const {
-      return (unsigned)hash_combine_range(Values.begin(), Values.end());
+    // Is deterministic because Values are saved in DFSOrder.
+    return (unsigned)hash_combine_range(Values.begin(), Values.end());
   }
 
   bool operator==(const ModelledPHI &Other) const {
@@ -566,7 +594,7 @@ class ValueTable {
 
 class GVNSink {
 public:
-  GVNSink() = default;
+  GVNSink(DominatorTree *DT) : DT(DT) {}
 
   bool run(Function &F) {
     LLVM_DEBUG(dbgs() << "GVNSink: running on function @" << F.getName()
@@ -583,6 +611,7 @@ class GVNSink {
 
 private:
   ValueTable VN;
+  DominatorTree *DT;
 
   bool shouldAvoidSinkingInstruction(Instruction *I) {
     // These instructions may change or break semantics if moved.
@@ -603,7 +632,7 @@ class GVNSink {
   void analyzeInitialPHIs(BasicBlock *BB, ModelledPHISet &PHIs,
                           SmallPtrSetImpl<Value *> &PHIContents) {
     for (PHINode &PN : BB->phis()) {
-      auto MPHI = ModelledPHI(&PN);
+      auto MPHI = ModelledPHI(&PN, DT);
       PHIs.insert(MPHI);
       for (auto *V : MPHI.getValues())
         PHIContents.insert(V);
@@ -691,7 +720,7 @@ GVNSink::analyzeInstructionForSinking(LockstepReverseIterator &LRI,
   }
 
   // The sunk instruction's results.
-  ModelledPHI NewPHI(NewInsts, ActivePreds);
+  ModelledPHI NewPHI(NewInsts, ActivePreds, DT);
 
   // Does sinking this instruction render previous PHIs redundant?
   if (NeededPHIs.erase(NewPHI))
@@ -728,7 +757,7 @@ GVNSink::analyzeInstructionForSinking(LockstepReverseIterator &LRI,
     return std::nullopt;
 
   for (unsigned OpNum = 0, E = I0->getNumOperands(); OpNum != E; ++OpNum) {
-    ModelledPHI PHI(NewInsts, OpNum, ActivePreds);
+    ModelledPHI PHI(NewInsts, OpNum, ActivePreds, DT);
     if (PHI.areAllIncomingValuesSame())
       continue;
     if (!canReplaceOperandWithVariable(I0, OpNum))
@@ -774,7 +803,11 @@ unsigned GVNSink::sinkBB(BasicBlock *BBEnd) {
   }
   if (Preds.size() < 2)
     return 0;
-  llvm::sort(Preds);
+  auto DFSOrder = [this](const BasicBlock *BB1, const BasicBlock *BB2) {
+    return this->DT->getNode(BB1) < this->DT->getNode(BB2);
+  };
+  // Sort by DFSNumber to have a deterministic order.
+  llvm::sort(Preds, DFSOrder);
 
   unsigned NumOrigPreds = Preds.size();
   // We can only sink instructions through unconditional branches.
@@ -886,8 +919,12 @@ void GVNSink::sinkLastInstruction(ArrayRef<BasicBlock *> Blocks,
 } // end anonymous namespace
 
 PreservedAnalyses GVNSinkPass::run(Function &F, FunctionAnalysisManager &AM) {
-  GVNSink G;
+  auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
+  GVNSink G(&DT);
   if (!G.run(F))
     return PreservedAnalyses::all();
+
+  // PHI nodes get inserted which haven't been added to the Dominator Tree.
+  // FIXME: Update DominatorTree to account for sunk instructions.
   return PreservedAnalyses::none();
 }

llvm/test/Transforms/GVNSink/int_sideeffect.ll

@@ -28,3 +28,27 @@ if.end:
   ret float %phi
 }
 
+; CHECK-LABEL: scalarsSinkingReverse
+; CHECK-NOT: fmul
+; CHECK: = phi
+; CHECK: = fmul
+define float @scalarsSinkingReverse(float %d, float %m, float %a, i1 %cmp) {
+entry:
+  br i1 %cmp, label %if.then, label %if.else
+
+if.then:
+  %add = fadd float %m, %a
+  %mul1 = fmul float %add, %d
+  br label %if.end
+
+if.else:
+  call void @llvm.sideeffect()
+  %sub = fsub float %m, %a
+  %mul0 = fmul float %sub, %d
+  br label %if.end
+
+if.end:
+  %phi = phi float [ %mul1, %if.then ], [ %mul0, %if.else ]
+  ret float %phi
+}
+