lib/HLSL/DxilNoops.cpp

///////////////////////////////////////////////////////////////////////////////
//                                                                           //
// DxilNoops.cpp                                                             //
// Copyright (C) Microsoft Corporation. All rights reserved.                 //
// This file is distributed under the University of Illinois Open Source     //
// License. See LICENSE.TXT for details.                                     //
//                                                                           //
// Passes to insert dx.noops() and replace them with llvm.donothing()        //
//                                                                           //
///////////////////////////////////////////////////////////////////////////////
//
// Here is how dx.preserve and dx.noop work.
//
// For example, the following HLSL code:
//
//     float foo(float y) {
//        float x = 10;
//        x = 20;
//        x += y;
//        return x;
//     }
//
//     float main() : SV_Target {
//       float ret = foo(10);
//       return ret;
//     }
//
// Ordinarily, it gets lowered as:
//
//     dx.op.storeOutput(3.0)
//
// Intermediate steps at "x = 20;", "x += y;", "return x", and
// even the call to "foo()" are lost.
//
// But with with Preserve and Noop:
//
//     void call dx.noop()           // float ret = foo(10);
//       %y = dx.preserve(10.0, 10.0)  // argument: y=10
//       %x0 = dx.preserve(10.0, 10.0) // float x = 10;
//       %x1 = dx.preserve(20.0, %x0)  // x = 20;
//       %x2 = fadd %x1, %y            // x += y;
//       void call dx.noop()           // return x
//     %ret = dx.preserve(%x2, %x2)   // ret = returned from foo()
//     dx.op.storeOutput(%ret)
//
// All the intermediate transformations are visible and could be
// made inspectable in the debugger.
//
// The reason why dx.preserve takes 2 arguments is so that the previous
// value of a variable does not get cleaned up by DCE. For example:
//
//    float x = ...;
//    do_some_stuff_with(x);
//    do_some_other_stuff(); // At this point, x's last values
//                           // are dead and register allocators
//                           // are free to reuse its location during
//                           // call this code.
//                           // So until x is assigned a new value below
//                           // x could become unavailable.
//                           //
//                           // The second parameter in dx.preserve
//                           // keeps x's previous value alive.
//
//    x = ...; // Assign something else
//
//
// When emitting proper DXIL, dx.noop and dx.preserve are lowered to
// ordinary LLVM instructions that do not affect the semantic of the
// shader, but can be used by a debugger or backend generator if they
// know what to look for.
//
// We generate two special internal constant global vars:
//
//      @dx.preserve.value = internal constant i1 false
//      @dx.nothing = internal constant i32 0
//
// "call dx.noop()" is lowered to "load @dx.nothing"
//
// "... = call dx.preserve(%cur_val, %last_val)" is lowered to:
//
//    %p = load @dx.preserve.value
//    ... = select i1 %p, %last_val, %cur_val
//
// Since %p is guaranteed to be false, the select is guaranteed
// to return %cur_val.
//

#include "dxc/HLSL/DxilNoops.h"
#include "dxc/DXIL/DxilConstants.h"
#include "dxc/DXIL/DxilMetadataHelper.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Analysis/DxilValueCache.h"
#include "llvm/IR/DIBuilder.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/raw_os_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"

#include <unordered_set>

using namespace llvm;

static Function *GetOrCreateNoopF(Module &M) {
  LLVMContext &Ctx = M.getContext();
  FunctionType *FT = FunctionType::get(Type::getVoidTy(Ctx), false);
  Function *NoopF = cast<Function>(M.getOrInsertFunction(hlsl::kNoopName, FT));
  NoopF->addFnAttr(Attribute::AttrKind::Convergent);
  return NoopF;
}

static Constant *GetConstGep(Constant *Ptr, unsigned Idx0, unsigned Idx1) {
  Type *i32Ty = Type::getInt32Ty(Ptr->getContext());
  Constant *Indices[] = {ConstantInt::get(i32Ty, Idx0),
                         ConstantInt::get(i32Ty, Idx1)};
  return ConstantExpr::getGetElementPtr(nullptr, Ptr, Indices);
}

struct Store_Info {
  Instruction *StoreOrMC = nullptr;
  Value *Source = nullptr; // Alloca, GV, or Argument
  bool AllowLoads = false;
};

static void FindAllStores(Value *Ptr, std::vector<Store_Info> *Stores,
                          std::vector<Value *> &WorklistStorage,
                          std::unordered_set<Value *> &SeenStorage) {
  assert(isa<Argument>(Ptr) || isa<AllocaInst>(Ptr) ||
         isa<GlobalVariable>(Ptr));

  WorklistStorage.clear();
  WorklistStorage.push_back(Ptr);
  // Don't clear Seen Storage because two pointers can be involved with the same
  // memcpy. Clearing it can get the memcpy added twice.

  unsigned StartIdx = Stores->size();
  bool AllowLoad = false;
  while (WorklistStorage.size()) {
    Value *V = WorklistStorage.back();
    WorklistStorage.pop_back();
    SeenStorage.insert(V);

    if (isa<BitCastOperator>(V) || isa<GEPOperator>(V) ||
        isa<GlobalVariable>(V) || isa<AllocaInst>(V) || isa<Argument>(V)) {
      for (User *U : V->users()) {
        MemCpyInst *MC = nullptr;
        // Allow load if MC reads from pointer
        if ((MC = dyn_cast<MemCpyInst>(U)) && MC->getSource() == V) {
          AllowLoad = true;
        } else if (isa<LoadInst>(U)) {
          AllowLoad = true;
        }
        // Add to worklist if we haven't seen it before.
        else {
          if (!SeenStorage.count(U))
            WorklistStorage.push_back(U);
        }
      }
    } else if (StoreInst *Store = dyn_cast<StoreInst>(V)) {
      Store_Info Info;
      Info.StoreOrMC = Store;
      Info.Source = Ptr;
      Stores->push_back(Info);
    } else if (MemCpyInst *MC = dyn_cast<MemCpyInst>(V)) {
      Store_Info Info;
      Info.StoreOrMC = MC;
      Info.Source = Ptr;
      Stores->push_back(Info);
    }
  }

  if (isa<GlobalVariable>(Ptr)) {
    AllowLoad = true;
  }

  if (AllowLoad) {
    Store_Info *ptr = Stores->data();
    for (unsigned i = StartIdx; i < Stores->size(); i++)
      ptr[i].AllowLoads = true;
  }
}

static User *GetUniqueUser(Value *V) {
  if (V->user_begin() != V->user_end()) {
    if (std::next(V->user_begin()) == V->user_end())
      return *V->user_begin();
  }
  return nullptr;
}

static Value *GetOrCreatePreserveCond(Function *F) {
  assert(!F->isDeclaration());

  Module *M = F->getParent();
  GlobalVariable *GV = M->getGlobalVariable(hlsl::kPreserveName, true);
  if (!GV) {
    Type *i32Ty = Type::getInt32Ty(M->getContext());
    Type *i32ArrayTy = ArrayType::get(i32Ty, 1);

    unsigned int Values[1] = {0};
    Constant *InitialValue =
        llvm::ConstantDataArray::get(M->getContext(), Values);

    GV = new GlobalVariable(*M, i32ArrayTy, true,
                            llvm::GlobalValue::InternalLinkage, InitialValue,
                            hlsl::kPreserveName);
  }

  for (User *U : GV->users()) {
    GEPOperator *Gep = cast<GEPOperator>(U);
    for (User *GepU : Gep->users()) {
      LoadInst *LI = cast<LoadInst>(GepU);
      if (LI->getParent()->getParent() == F) {
        return GetUniqueUser(LI);
      }
    }
  }

  BasicBlock *BB = &F->getEntryBlock();
  Instruction *InsertPt = &BB->front();
  while (isa<AllocaInst>(InsertPt) || isa<DbgInfoIntrinsic>(InsertPt))
    InsertPt = InsertPt->getNextNode();

  IRBuilder<> B(InsertPt);

  Constant *Gep = GetConstGep(GV, 0, 0);
  LoadInst *Load = B.CreateLoad(Gep);
  return B.CreateTrunc(Load, B.getInt1Ty());
}

bool hlsl::IsNop(llvm::Instruction *I) {
  CallInst *CI = dyn_cast<CallInst>(I);
  if (!CI)
    return false;

  Function *F = CI->getCalledFunction();
  return F && F->getName() == hlsl::kNoopName;
}

static bool IsPreserveLoad(llvm::Instruction *I) {
  LoadInst *Load = dyn_cast<LoadInst>(I);
  if (!Load)
    return false;

  GEPOperator *GEP = dyn_cast<GEPOperator>(Load->getPointerOperand());
  if (!GEP)
    return false;

  GlobalVariable *GV = dyn_cast<GlobalVariable>(GEP->getPointerOperand());

  return GV &&
         GV->getLinkage() == GlobalVariable::LinkageTypes::InternalLinkage &&
         GV->getName() == hlsl::kPreserveName;
}

static bool IsPreserveTrunc(llvm::Instruction *I) {
  TruncInst *Trunc = dyn_cast<TruncInst>(I);
  if (!Trunc)
    return false;

  Instruction *Load = dyn_cast<Instruction>(Trunc->getOperand(0));
  if (!Load)
    return false;
  return IsPreserveLoad(Load);
}

bool hlsl::IsPreserve(llvm::Instruction *I) {
  SelectInst *S = dyn_cast<SelectInst>(I);
  if (!S)
    return false;

  Instruction *Trunc = dyn_cast<Instruction>(S->getCondition());
  if (!Trunc)
    return false;
  return IsPreserveTrunc(Trunc);
}

bool hlsl::IsPreserveRelatedValue(llvm::Instruction *I) {
  return IsPreserveLoad(I) || IsPreserveTrunc(I) || hlsl::IsPreserve(I);
}

static Function *GetOrCreatePreserveF(Module *M, Type *Ty) {
  std::string str = hlsl::kPreservePrefix;
  raw_string_ostream os(str);
  Ty->print(os);
  os.flush();

  FunctionType *FT = FunctionType::get(Ty, {Ty, Ty}, false);
  Function *PreserveF = cast<Function>(M->getOrInsertFunction(str, FT));
  PreserveF->addFnAttr(Attribute::AttrKind::ReadNone);
  PreserveF->addFnAttr(Attribute::AttrKind::NoUnwind);
  return PreserveF;
}

static Instruction *CreatePreserve(Value *V, Value *LastV,
                                   Instruction *InsertPt) {
  assert(V->getType() == LastV->getType());
  Type *Ty = V->getType();
  Function *PreserveF = GetOrCreatePreserveF(InsertPt->getModule(), Ty);
  return CallInst::Create(PreserveF, ArrayRef<Value *>{V, LastV}, "", InsertPt);
}

static void LowerPreserveToSelect(CallInst *CI) {
  Value *V = CI->getArgOperand(0);
  Value *LastV = CI->getArgOperand(1);

  if (LastV == V)
    LastV = UndefValue::get(V->getType());

  Value *Cond = GetOrCreatePreserveCond(CI->getParent()->getParent());
  SelectInst *Select = SelectInst::Create(Cond, LastV, V, "", CI);
  Select->setDebugLoc(CI->getDebugLoc());
  CI->replaceAllUsesWith(Select);
  CI->eraseFromParent();
}

static void InsertNoopAt(Instruction *I) {
  Module &M = *I->getModule();
  Function *NoopF = GetOrCreateNoopF(M);
  CallInst *Noop = CallInst::Create(NoopF, {}, I);
  Noop->setDebugLoc(I->getDebugLoc());
}

static void InsertPreserve(bool AllowLoads, StoreInst *Store) {
  Value *V = Store->getValueOperand();

  IRBuilder<> B(Store);
  Value *Last_Value = nullptr;
  // If there's never any loads for this memory location,
  // don't generate a load.
  if (AllowLoads) {
    Last_Value = B.CreateLoad(Store->getPointerOperand());
  } else {
    Last_Value = UndefValue::get(V->getType());
  }

  Instruction *Preserve = CreatePreserve(V, Last_Value, Store);
  Preserve->setDebugLoc(Store->getDebugLoc());
  Store->replaceUsesOfWith(V, Preserve);
}

//==========================================================
// Insertion pass
//
// This pass inserts dx.noop and dx.preserve where we want
// to preserve line mapping or perserve some intermediate
// values.

struct DxilInsertPreserves : public ModulePass {
  static char ID;
  DxilInsertPreserves(bool AllowPreserves = false)
      : ModulePass(ID), AllowPreserves(AllowPreserves) {
    initializeDxilInsertPreservesPass(*PassRegistry::getPassRegistry());
  }
  bool AllowPreserves = false;

  // Function overrides that resolve options when used for DxOpt
  void applyOptions(PassOptions O) override {
    GetPassOptionBool(O, "AllowPreserves", &AllowPreserves, false);
  }
  void dumpConfig(raw_ostream &OS) override {
    ModulePass::dumpConfig(OS);
    OS << ",AllowPreserves=" << AllowPreserves;
  }

  bool runOnModule(Module &M) override {

    std::vector<Store_Info> Stores;
    std::vector<Value *> WorklistStorage;
    std::unordered_set<Value *> SeenStorage;

    for (GlobalVariable &GV : M.globals()) {
      if (GV.getLinkage() != GlobalValue::LinkageTypes::InternalLinkage ||
          GV.getType()->getPointerAddressSpace() ==
              hlsl::DXIL::kTGSMAddrSpace) {
        continue;
      }

      for (User *U : GV.users()) {
        if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
          InsertNoopAt(LI);
        }
      }

      FindAllStores(&GV, &Stores, WorklistStorage, SeenStorage);
    }

    bool Changed = false;
    for (Function &F : M) {
      if (F.isDeclaration())
        continue;

      // Collect Stores on Allocas in function
      BasicBlock *Entry = &*F.begin();
      for (Instruction &I : *Entry) {
        AllocaInst *AI = dyn_cast<AllocaInst>(&I);
        if (!AI)
          continue;
        // Skip temp allocas
        if (!AI->getMetadata(hlsl::DxilMDHelper::kDxilTempAllocaMDName))
          FindAllStores(AI, &Stores, WorklistStorage, SeenStorage);
      }

      // Collect Stores on pointer Arguments in function
      for (Argument &Arg : F.args()) {
        if (Arg.getType()->isPointerTy())
          FindAllStores(&Arg, &Stores, WorklistStorage, SeenStorage);
      }

      // For every real function call, insert a nop
      // so we can put a breakpoint there.
      for (User *U : F.users()) {
        if (CallInst *CI = dyn_cast<CallInst>(U)) {
          InsertNoopAt(CI);
        }
      }

      // Insert nops for void return statements
      for (BasicBlock &BB : F) {
        ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
        if (Ret)
          InsertNoopAt(Ret);
      }
    }

    // Insert preserves or noops for these stores
    for (Store_Info &Info : Stores) {
      if (StoreInst *Store = dyn_cast<StoreInst>(Info.StoreOrMC)) {
        Value *V = Store->getValueOperand();

        if (this->AllowPreserves && V && !V->getType()->isAggregateType() &&
            !V->getType()->isPointerTy()) {
          InsertPreserve(Info.AllowLoads, Store);
          Changed = true;
        } else {
          InsertNoopAt(Store);
          Changed = true;
        }
      } else if (MemCpyInst *MC = cast<MemCpyInst>(Info.StoreOrMC)) {
        // TODO: Do something to preserve pointer's previous value.
        InsertNoopAt(MC);
        Changed = true;
      }
    }

    return Changed;
  }

  StringRef getPassName() const override { return "Dxil Insert Preserves"; }
};

char DxilInsertPreserves::ID;

Pass *llvm::createDxilInsertPreservesPass(bool AllowPreserves) {
  return new DxilInsertPreserves(AllowPreserves);
}

INITIALIZE_PASS(DxilInsertPreserves, "dxil-insert-preserves",
                "Dxil Insert Preserves", false, false)

//==========================================================
// Lower dx.preserve to select
//
// This pass replaces all dx.preserve calls to select
//

namespace {

class DxilPreserveToSelect : public ModulePass {
public:
  static char ID;

  SmallDenseMap<Type *, Function *> PreserveFunctions;

  DxilPreserveToSelect() : ModulePass(ID) {
    initializeDxilPreserveToSelectPass(*PassRegistry::getPassRegistry());
  }

  bool runOnModule(Module &M) override {
    bool Changed = false;
    for (auto fit = M.getFunctionList().begin(),
              end = M.getFunctionList().end();
         fit != end;) {
      Function *F = &*(fit++);
      if (!F->isDeclaration())
        continue;

      if (F->getName().startswith(hlsl::kPreservePrefix)) {
        for (auto uit = F->user_begin(), end = F->user_end(); uit != end;) {
          User *U = *(uit++);
          CallInst *CI = cast<CallInst>(U);
          LowerPreserveToSelect(CI);
        }

        F->eraseFromParent();
        Changed = true;
      }
    }

    return Changed;
  }
  StringRef getPassName() const override {
    return "Dxil Lower Preserves to Selects";
  }
};

char DxilPreserveToSelect::ID;
} // namespace

Pass *llvm::createDxilPreserveToSelectPass() {
  return new DxilPreserveToSelect();
}

INITIALIZE_PASS(DxilPreserveToSelect, "dxil-preserves-to-select",
                "Dxil Preserves To Select", false, false)

//==========================================================
// output Argument debug info rewrite
//
namespace {

class DxilRewriteOutputArgDebugInfo : public ModulePass {
public:
  static char ID;

  DxilRewriteOutputArgDebugInfo() : ModulePass(ID) {
    initializeDxilRewriteOutputArgDebugInfoPass(
        *PassRegistry::getPassRegistry());
  }

  bool runOnModule(Module &M) override {
    DITypeIdentifierMap EmptyMap;
    DIBuilder DIB(M);

    bool Changed = false;

    for (Function &F : M) {
      for (Argument &Arg : F.args()) {
        if (!Arg.getType()->isPointerTy())
          continue;

        DbgDeclareInst *Declare = llvm::FindAllocaDbgDeclare(&Arg);
        if (!Declare)
          continue;

        DILocalVariable *Var = Declare->getVariable();
        DIType *Ty = Var->getType().resolve(EmptyMap);

        DIExpression *Expr = Declare->getExpression();
        if (Expr->getNumElements() == 1 &&
            Expr->getElement(0) == dwarf::DW_OP_deref) {
          while (Ty && (Ty->getTag() == dwarf::DW_TAG_reference_type ||
                        Ty->getTag() == dwarf::DW_TAG_restrict_type)) {
            Ty = cast<DIDerivedType>(Ty)->getBaseType().resolve(EmptyMap);
          }

          if (Ty) {
            DILocalVariable *NewVar = DIB.createLocalVariable(
                dwarf::DW_TAG_arg_variable, Var->getScope(), Var->getName(),
                Var->getFile(), Var->getLine(), Ty, false, 0, Var->getArg());
            DIExpression *EmptyExpr = DIExpression::get(M.getContext(), {});
            DIB.insertDeclare(&Arg, NewVar, EmptyExpr, Declare->getDebugLoc(),
                              Declare);
            Declare->eraseFromParent();

            Changed = true;
          }
        }
      }
    }

    return Changed;
  }
  StringRef getPassName() const override {
    return "Dxil Rewrite Output Arg Debug Info";
  }
};

char DxilRewriteOutputArgDebugInfo::ID;
} // namespace

Pass *llvm::createDxilRewriteOutputArgDebugInfoPass() {
  return new DxilRewriteOutputArgDebugInfo();
}

INITIALIZE_PASS(DxilRewriteOutputArgDebugInfo,
                "dxil-rewrite-output-arg-debug-info",
                "Dxil Rewrite Output Arg Debug Info", false, false)

//==========================================================
// Reader pass
//

namespace {

class DxilReinsertNops : public ModulePass {
public:
  static char ID;

  DxilReinsertNops() : ModulePass(ID) {
    initializeDxilReinsertNopsPass(*PassRegistry::getPassRegistry());
  }

  // In various linking scenarios, the dx.nothing.a variable might be prefixed
  // and/or suffixed with something:
  //
  //   <library_name>.dx.nothing.a.<another_thing>
  //
  // This routine looks for the "dx.nothing.a" string inside of it, and as long
  // as it's used in the expected way:
  //
  // %0 = load i32, i32* getelementptr inbounds ([1 x i32], [1 x i32]*
  // @dx.nothing.a, i32 0, i32 0)
  //
  // ...it is deemed a valid nop.
  //
  static bool IsLegalNothingVarName(StringRef Name) {
    // There should be a single instance of the name in this GV.
    if (1 != Name.count(hlsl::kNothingName))
      return false;
    size_t Loc = Name.find(hlsl::kNothingName);
    StringRef Prefix = Name.substr(0, Loc);
    StringRef Suffix = Name.substr(Loc + Name.size());
    // There should be either no prefix or a prefix that ends with .
    if (!Prefix.empty() && !Prefix.endswith(".")) {
      return false;
    }
    // There should be either no suffix or a prefix that begins with with .
    if (!Suffix.empty() && !Suffix.startswith(".")) {
      return false;
    }
    return true;
  }

  bool runOnModule(Module &M) override {
    bool Changed = false;
    for (GlobalVariable &GV : M.globals()) {
      if (!IsLegalNothingVarName(GV.getName()))
        continue;

      const bool IsValidType = GV.getValueType()->isArrayTy() &&
                               GV.getValueType()->getArrayElementType() ==
                                   Type::getInt32Ty(M.getContext()) &&
                               GV.getValueType()->getArrayNumElements() == 1;
      if (!IsValidType)
        return false;

      for (User *GVU : GV.users()) {
        ConstantExpr *CE = dyn_cast<ConstantExpr>(GVU);
        if (!CE || CE->getOpcode() != Instruction::GetElementPtr)
          continue;

        for (auto it = CE->user_begin(), end = CE->user_end(); it != end;) {
          User *U = *(it++);
          LoadInst *LI = dyn_cast<LoadInst>(U);
          if (!LI)
            continue;
          InsertNoopAt(LI);
          LI->eraseFromParent();
          Changed = true;
        }
      }
    }

    return Changed;
  }
  StringRef getPassName() const override { return "Dxil Reinsert Nops"; }
};

char DxilReinsertNops::ID;
} // namespace
Pass *llvm::createDxilReinsertNopsPass() { return new DxilReinsertNops(); }

INITIALIZE_PASS(DxilReinsertNops, "dxil-reinsert-nops", "Dxil Reinsert Nops",
                false, false)

//==========================================================
// Finalize pass
//

namespace {

class DxilFinalizePreserves : public ModulePass {
public:
  static char ID;
  GlobalVariable *NothingGV = nullptr;

  DxilFinalizePreserves() : ModulePass(ID) {
    initializeDxilFinalizePreservesPass(*PassRegistry::getPassRegistry());
  }

  Instruction *GetFinalNoopInst(Module &M, Instruction *InsertBefore) {
    Type *i32Ty = Type::getInt32Ty(M.getContext());
    if (!NothingGV) {
      NothingGV = M.getGlobalVariable(hlsl::kNothingName);
      if (!NothingGV) {
        Type *i32ArrayTy = ArrayType::get(i32Ty, 1);

        unsigned int Values[1] = {0};
        Constant *InitialValue =
            llvm::ConstantDataArray::get(M.getContext(), Values);

        NothingGV = new GlobalVariable(M, i32ArrayTy, true,
                                       llvm::GlobalValue::InternalLinkage,
                                       InitialValue, hlsl::kNothingName);
      }
    }

    Constant *Gep = GetConstGep(NothingGV, 0, 0);
    return new llvm::LoadInst(Gep, nullptr, InsertBefore);
  }

  bool LowerPreserves(Module &M);
  bool LowerNoops(Module &M);
  bool runOnModule(Module &M) override;
  StringRef getPassName() const override { return "Dxil Finalize Preserves"; }
};

char DxilFinalizePreserves::ID;
} // namespace

// Fix undefs in the dx.preserve -> selects
bool DxilFinalizePreserves::LowerPreserves(Module &M) {
  bool Changed = false;

  GlobalVariable *GV = M.getGlobalVariable(hlsl::kPreserveName, true);
  if (GV) {
    for (User *U : GV->users()) {
      GEPOperator *Gep = cast<GEPOperator>(U);
      for (User *GepU : Gep->users()) {
        LoadInst *LI = cast<LoadInst>(GepU);
        assert(LI->user_begin() != LI->user_end() &&
               std::next(LI->user_begin()) == LI->user_end());
        Instruction *I = cast<Instruction>(*LI->user_begin());

        for (User *UU : I->users()) {

          SelectInst *P = cast<SelectInst>(UU);
          Value *PrevV = P->getTrueValue();
          Value *CurV = P->getFalseValue();

          if (isa<UndefValue>(PrevV) || isa<Constant>(PrevV)) {
            P->setOperand(1, CurV);
            Changed = true;
          }
        }
      }
    }
  }

  return Changed;
}

// Replace all @dx.noop's with load @dx.nothing.value
bool DxilFinalizePreserves::LowerNoops(Module &M) {
  bool Changed = false;

  Function *NoopF = nullptr;
  for (Function &F : M) {
    if (!F.isDeclaration())
      continue;
    if (F.getName() == hlsl::kNoopName) {
      NoopF = &F;
    }
  }

  if (NoopF) {
    for (auto It = NoopF->user_begin(), E = NoopF->user_end(); It != E;) {
      User *U = *(It++);
      CallInst *CI = cast<CallInst>(U);

      Instruction *Nop = GetFinalNoopInst(M, CI);
      Nop->setDebugLoc(CI->getDebugLoc());

      CI->eraseFromParent();
      Changed = true;
    }

    assert(NoopF->user_empty() && "dx.noop calls must be all removed now");
    NoopF->eraseFromParent();
  }

  return Changed;
}

// Replace all preserves and nops
bool DxilFinalizePreserves::runOnModule(Module &M) {
  bool Changed = false;

  Changed |= LowerPreserves(M);
  Changed |= LowerNoops(M);

  return Changed;
}

Pass *llvm::createDxilFinalizePreservesPass() {
  return new DxilFinalizePreserves();
}

INITIALIZE_PASS(DxilFinalizePreserves, "dxil-finalize-preserves",
                "Dxil Finalize Preserves", false, false)