text: //===-- User.cpp - Implement the User class -------------------------------===//
  //
  //                     The LLVM Compiler Infrastructure
  //
  // This file is distributed under the University of Illinois Open Source
  // License. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  
  #include "llvm/IR/User.h"
  #include "llvm/IR/Constant.h"
  #include "llvm/IR/GlobalValue.h"
  #include "llvm/IR/Operator.h"
  
  namespace llvm {
  class BasicBlock;
  
  //===----------------------------------------------------------------------===//
  //                                 User Class
  //===----------------------------------------------------------------------===//
  
  void User::replaceUsesOfWith(Value *From, Value *To) {
    if (From == To) return;   // Duh what?
  
    assert((!isa<Constant>(this) || isa<GlobalValue>(this)) &&
           "Cannot call User::replaceUsesOfWith on a constant!");
  
    for (unsigned i = 0, E = getNumOperands(); i != E; ++i)
      if (getOperand(i) == From) {  // Is This operand is pointing to oldval?
        // The side effects of this setOperand call include linking to
        // "To", adding "this" to the uses list of To, and
        // most importantly, removing "this" from the use list of "From".
        setOperand(i, To); // Fix it now...
      }
  }
  
  //===----------------------------------------------------------------------===//
  //                         User allocHungoffUses Implementation
  //===----------------------------------------------------------------------===//
  
  void User::allocHungoffUses(unsigned N, bool IsPhi) {
    assert(HasHungOffUses && "alloc must have hung off uses");
  
    static_assert(alignof(Use) >= alignof(Use::UserRef),
                  "Alignment is insufficient for 'hung-off-uses' pieces");
    static_assert(alignof(Use::UserRef) >= alignof(BasicBlock *),
                  "Alignment is insufficient for 'hung-off-uses' pieces");
  
    // Allocate the array of Uses, followed by a pointer (with bottom bit set) to
    // the User.
    size_t size = N * sizeof(Use) + sizeof(Use::UserRef);
    if (IsPhi)
      size += N * sizeof(BasicBlock *);
    Use *Begin = static_cast<Use*>(::operator new(size));
    Use *End = Begin + N;
    (void) new(End) Use::UserRef(const_cast<User*>(this), 1);
    setOperandList(Use::initTags(Begin, End));
  }
  
  void User::growHungoffUses(unsigned NewNumUses, bool IsPhi) {
    assert(HasHungOffUses && "realloc must have hung off uses");
  
    unsigned OldNumUses = getNumOperands();
  
    // We don't support shrinking the number of uses.  We wouldn't have enough
    // space to copy the old uses in to the new space.
    assert(NewNumUses > OldNumUses && "realloc must grow num uses");
  
    Use *OldOps = getOperandList();
    allocHungoffUses(NewNumUses, IsPhi);
    Use *NewOps = getOperandList();
  
    // Now copy from the old operands list to the new one.
    std::copy(OldOps, OldOps + OldNumUses, NewOps);
  
    // If this is a Phi, then we need to copy the BB pointers too.
    if (IsPhi) {
      auto *OldPtr =
          reinterpret_cast<char *>(OldOps + OldNumUses) + sizeof(Use::UserRef);
      auto *NewPtr =
          reinterpret_cast<char *>(NewOps + NewNumUses) + sizeof(Use::UserRef);
      std::copy(OldPtr, OldPtr + (OldNumUses * sizeof(BasicBlock *)), NewPtr);
    }
    Use::zap(OldOps, OldOps + OldNumUses, true);
  }
  
  
  // This is a private struct used by `User` to track the co-allocated descriptor
  // section.
  struct DescriptorInfo {
    intptr_t SizeInBytes;
  };
  
  ArrayRef<const uint8_t> User::getDescriptor() const {
    auto MutableARef = const_cast<User *>(this)->getDescriptor();
    return {MutableARef.begin(), MutableARef.end()};
  }
  
  MutableArrayRef<uint8_t> User::getDescriptor() {
    assert(HasDescriptor && "Don't call otherwise!");
    assert(!HasHungOffUses && "Invariant!");
  
    auto *DI = reinterpret_cast<DescriptorInfo *>(getIntrusiveOperands()) - 1;
    assert(DI->SizeInBytes != 0 && "Should not have had a descriptor otherwise!");
  
    return MutableArrayRef<uint8_t>(
        reinterpret_cast<uint8_t *>(DI) - DI->SizeInBytes, DI->SizeInBytes);
  }
  
  //===----------------------------------------------------------------------===//
  //                         User operator new Implementations
  //===----------------------------------------------------------------------===//
  
  void *User::allocateFixedOperandUser(size_t Size, unsigned Us,
                                       unsigned DescBytes) {
    assert(Us < (1u << NumUserOperandsBits) && "Too many operands");
  
    static_assert(sizeof(DescriptorInfo) % sizeof(void *) == 0, "Required below");
  
    unsigned DescBytesToAllocate =
        DescBytes == 0 ? 0 : (DescBytes + sizeof(DescriptorInfo));
    assert(DescBytesToAllocate % sizeof(void *) == 0 &&
           "We need this to satisfy alignment constraints for Uses");
  
    uint8_t *Storage = static_cast<uint8_t *>(
        ::operator new(Size + sizeof(Use) * Us + DescBytesToAllocate));
    Use *Start = reinterpret_cast<Use *>(Storage + DescBytesToAllocate);
    Use *End = Start + Us;
    User *Obj = reinterpret_cast<User*>(End);
    Obj->NumUserOperands = Us;
    Obj->HasHungOffUses = false;
    Obj->HasDescriptor = DescBytes != 0;
    Use::initTags(Start, End);
  
    if (DescBytes != 0) {
      auto *DescInfo = reinterpret_cast<DescriptorInfo *>(Storage + DescBytes);
      DescInfo->SizeInBytes = DescBytes;
    }
  
    return Obj;
  }
  
  void *User::operator new(size_t Size, unsigned Us) {
    return allocateFixedOperandUser(Size, Us, 0);
  }
  
  void *User::operator new(size_t Size, unsigned Us, unsigned DescBytes) {
    return allocateFixedOperandUser(Size, Us, DescBytes);
  }
  
  void *User::operator new(size_t Size) {
    // Allocate space for a single Use*
    void *Storage = ::operator new(Size + sizeof(Use *));
    Use **HungOffOperandList = static_cast<Use **>(Storage);
    User *Obj = reinterpret_cast<User *>(HungOffOperandList + 1);
    Obj->NumUserOperands = 0;
    Obj->HasHungOffUses = true;
    Obj->HasDescriptor = false;
    *HungOffOperandList = nullptr;
    return Obj;
  }
  
  //===----------------------------------------------------------------------===//
  //                         User operator delete Implementation
  //===----------------------------------------------------------------------===//
  
  void User::operator delete(void *Usr) {
    // Hung off uses use a single Use* before the User, while other subclasses
    // use a Use[] allocated prior to the user.
    User *Obj = static_cast<User *>(Usr);
    if (Obj->HasHungOffUses) {
      assert(!Obj->HasDescriptor && "not supported!");
  
      Use **HungOffOperandList = static_cast<Use **>(Usr) - 1;
      // drop the hung off uses.
      Use::zap(*HungOffOperandList, *HungOffOperandList + Obj->NumUserOperands,
               /* Delete */ true);
      ::operator delete(HungOffOperandList);
    } else if (Obj->HasDescriptor) {
      Use *UseBegin = static_cast<Use *>(Usr) - Obj->NumUserOperands;
      Use::zap(UseBegin, UseBegin + Obj->NumUserOperands, /* Delete */ false);
  
      auto *DI = reinterpret_cast<DescriptorInfo *>(UseBegin) - 1;
      uint8_t *Storage = reinterpret_cast<uint8_t *>(DI) - DI->SizeInBytes;
      ::operator delete(Storage);
    } else {
      Use *Storage = static_cast<Use *>(Usr) - Obj->NumUserOperands;
      Use::zap(Storage, Storage + Obj->NumUserOperands,
               /* Delete */ false);
      ::operator delete(Storage);
    }
  }
  
  } // End llvm namespace
  