llvm/IR/BasicBlock.h

//===- llvm/BasicBlock.h - Represent a basic block in the VM ----*- 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
//
//===----------------------------------------------------------------------===//
//
// This file contains the declaration of the BasicBlock class.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_IR_BASICBLOCK_H
#define LLVM_IR_BASICBLOCK_H

#include "llvm-c/Types.h"
#include "llvm/ADT/Twine.h"
#include "llvm/ADT/ilist.h"
#include "llvm/ADT/ilist_node.h"
#include "llvm/ADT/iterator.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/SymbolTableListTraits.h"
#include "llvm/IR/Value.h"
#include <cassert>
#include <cstddef>
#include <iterator>

namespace llvm {

class AssemblyAnnotationWriter;
class CallInst;
class Function;
class LandingPadInst;
class LLVMContext;
class Module;
class PHINode;
class ValueSymbolTable;

/// LLVM Basic Block Representation
///
/// This represents a single basic block in LLVM. A basic block is simply a
/// container of instructions that execute sequentially. Basic blocks are Values
/// because they are referenced by instructions such as branches and switch
/// tables. The type of a BasicBlock is "Type::LabelTy" because the basic block
/// represents a label to which a branch can jump.
///
/// A well formed basic block is formed of a list of non-terminating
/// instructions followed by a single terminator instruction. Terminator
/// instructions may not occur in the middle of basic blocks, and must terminate
/// the blocks. The BasicBlock class allows malformed basic blocks to occur
/// because it may be useful in the intermediate stage of constructing or
/// modifying a program. However, the verifier will ensure that basic blocks are
/// "well formed".
class BasicBlock final : public Value, // Basic blocks are data objects also
                         public ilist_node_with_parent<BasicBlock, Function> {
public:
  using InstListType = SymbolTableList<Instruction>;

private:
  friend class BlockAddress;
  friend class SymbolTableListTraits<BasicBlock>;

  InstListType InstList;
  Function *Parent;

  void setParent(Function *parent);

  /// Constructor.
  ///
  /// If the function parameter is specified, the basic block is automatically
  /// inserted at either the end of the function (if InsertBefore is null), or
  /// before the specified basic block.
  explicit BasicBlock(LLVMContext &C, const Twine &Name = "",
                      Function *Parent = nullptr,
                      BasicBlock *InsertBefore = nullptr);

public:
  BasicBlock(const BasicBlock &) = delete;
  BasicBlock &operator=(const BasicBlock &) = delete;
  ~BasicBlock();

  /// Get the context in which this basic block lives.
  LLVMContext &getContext() const;

  /// Instruction iterators...
  using iterator = InstListType::iterator;
  using const_iterator = InstListType::const_iterator;
  using reverse_iterator = InstListType::reverse_iterator;
  using const_reverse_iterator = InstListType::const_reverse_iterator;

  // These functions and classes need access to the instruction list.
  friend void Instruction::removeFromParent();
  friend iplist<Instruction>::iterator Instruction::eraseFromParent();
  friend BasicBlock::iterator Instruction::insertInto(BasicBlock *BB,
                                                      BasicBlock::iterator It);
  friend class llvm::SymbolTableListTraits<llvm::Instruction>;
  friend class llvm::ilist_node_with_parent<llvm::Instruction, llvm::BasicBlock>;

  /// Creates a new BasicBlock.
  ///
  /// If the Parent parameter is specified, the basic block is automatically
  /// inserted at either the end of the function (if InsertBefore is 0), or
  /// before the specified basic block.
  static BasicBlock *Create(LLVMContext &Context, const Twine &Name = "",
                            Function *Parent = nullptr,
                            BasicBlock *InsertBefore = nullptr) {
    return new BasicBlock(Context, Name, Parent, InsertBefore);
  }

  /// Return the enclosing method, or null if none.
  const Function *getParent() const { return Parent; }
        Function *getParent()       { return Parent; }

  /// Return the module owning the function this basic block belongs to, or
  /// nullptr if the function does not have a module.
  ///
  /// Note: this is undefined behavior if the block does not have a parent.
  const Module *getModule() const;
  Module *getModule() {
    return const_cast<Module *>(
                            static_cast<const BasicBlock *>(this)->getModule());
  }

  /// Returns the terminator instruction if the block is well formed or null
  /// if the block is not well formed.
  const Instruction *getTerminator() const LLVM_READONLY {
    if (InstList.empty() || !InstList.back().isTerminator())
      return nullptr;
    return &InstList.back();
  }
  Instruction *getTerminator() {
    return const_cast<Instruction *>(
        static_cast<const BasicBlock *>(this)->getTerminator());
  }

  /// Returns the call instruction calling \@llvm.experimental.deoptimize
  /// prior to the terminating return instruction of this basic block, if such
  /// a call is present.  Otherwise, returns null.
  const CallInst *getTerminatingDeoptimizeCall() const;
  CallInst *getTerminatingDeoptimizeCall() {
    return const_cast<CallInst *>(
         static_cast<const BasicBlock *>(this)->getTerminatingDeoptimizeCall());
  }

  /// Returns the call instruction calling \@llvm.experimental.deoptimize
  /// that is present either in current basic block or in block that is a unique
  /// successor to current block, if such call is present. Otherwise, returns null.
  const CallInst *getPostdominatingDeoptimizeCall() const;
  CallInst *getPostdominatingDeoptimizeCall() {
    return const_cast<CallInst *>(
         static_cast<const BasicBlock *>(this)->getPostdominatingDeoptimizeCall());
  }

  /// Returns the call instruction marked 'musttail' prior to the terminating
  /// return instruction of this basic block, if such a call is present.
  /// Otherwise, returns null.
  const CallInst *getTerminatingMustTailCall() const;
  CallInst *getTerminatingMustTailCall() {
    return const_cast<CallInst *>(
           static_cast<const BasicBlock *>(this)->getTerminatingMustTailCall());
  }

  /// Returns a pointer to the first instruction in this block that is not a
  /// PHINode instruction.
  ///
  /// When adding instructions to the beginning of the basic block, they should
  /// be added before the returned value, not before the first instruction,
  /// which might be PHI. Returns 0 is there's no non-PHI instruction.
  const Instruction* getFirstNonPHI() const;
  Instruction* getFirstNonPHI() {
    return const_cast<Instruction *>(
                       static_cast<const BasicBlock *>(this)->getFirstNonPHI());
  }

  /// Returns a pointer to the first instruction in this block that is not a
  /// PHINode or a debug intrinsic, or any pseudo operation if \c SkipPseudoOp
  /// is true.
  const Instruction *getFirstNonPHIOrDbg(bool SkipPseudoOp = true) const;
  Instruction *getFirstNonPHIOrDbg(bool SkipPseudoOp = true) {
    return const_cast<Instruction *>(
        static_cast<const BasicBlock *>(this)->getFirstNonPHIOrDbg(
            SkipPseudoOp));
  }

  /// Returns a pointer to the first instruction in this block that is not a
  /// PHINode, a debug intrinsic, or a lifetime intrinsic, or any pseudo
  /// operation if \c SkipPseudoOp is true.
  const Instruction *
  getFirstNonPHIOrDbgOrLifetime(bool SkipPseudoOp = true) const;
  Instruction *getFirstNonPHIOrDbgOrLifetime(bool SkipPseudoOp = true) {
    return const_cast<Instruction *>(
        static_cast<const BasicBlock *>(this)->getFirstNonPHIOrDbgOrLifetime(
            SkipPseudoOp));
  }

  /// Returns an iterator to the first instruction in this block that is
  /// suitable for inserting a non-PHI instruction.
  ///
  /// In particular, it skips all PHIs and LandingPad instructions.
  const_iterator getFirstInsertionPt() const;
  iterator getFirstInsertionPt() {
    return static_cast<const BasicBlock *>(this)
                                          ->getFirstInsertionPt().getNonConst();
  }

  /// Returns an iterator to the first instruction in this block that is
  /// not a PHINode, a debug intrinsic, a static alloca or any pseudo operation.
  const_iterator getFirstNonPHIOrDbgOrAlloca() const;
  iterator getFirstNonPHIOrDbgOrAlloca() {
    return static_cast<const BasicBlock *>(this)
        ->getFirstNonPHIOrDbgOrAlloca()
        .getNonConst();
  }

  /// Returns the first potential AsynchEH faulty instruction
  /// currently it checks for loads/stores (which may dereference a null
  /// pointer) and calls/invokes (which may propagate exceptions)
  const Instruction* getFirstMayFaultInst() const;
  Instruction* getFirstMayFaultInst() {
      return const_cast<Instruction*>(
          static_cast<const BasicBlock*>(this)->getFirstMayFaultInst());
  }

  /// Return a const iterator range over the instructions in the block, skipping
  /// any debug instructions. Skip any pseudo operations as well if \c
  /// SkipPseudoOp is true.
  iterator_range<filter_iterator<BasicBlock::const_iterator,
                                 std::function<bool(const Instruction &)>>>
  instructionsWithoutDebug(bool SkipPseudoOp = true) const;

  /// Return an iterator range over the instructions in the block, skipping any
  /// debug instructions. Skip and any pseudo operations as well if \c
  /// SkipPseudoOp is true.
  iterator_range<
      filter_iterator<BasicBlock::iterator, std::function<bool(Instruction &)>>>
  instructionsWithoutDebug(bool SkipPseudoOp = true);

  /// Return the size of the basic block ignoring debug instructions
  filter_iterator<BasicBlock::const_iterator,
                  std::function<bool(const Instruction &)>>::difference_type
  sizeWithoutDebug() const;

  /// Unlink 'this' from the containing function, but do not delete it.
  void removeFromParent();

  /// Unlink 'this' from the containing function and delete it.
  ///
  // \returns an iterator pointing to the element after the erased one.
  SymbolTableList<BasicBlock>::iterator eraseFromParent();

  /// Unlink this basic block from its current function and insert it into
  /// the function that \p MovePos lives in, right before \p MovePos.
  inline void moveBefore(BasicBlock *MovePos) {
    moveBefore(MovePos->getIterator());
  }
  void moveBefore(SymbolTableList<BasicBlock>::iterator MovePos);

  /// Unlink this basic block from its current function and insert it
  /// right after \p MovePos in the function \p MovePos lives in.
  void moveAfter(BasicBlock *MovePos);

  /// Insert unlinked basic block into a function.
  ///
  /// Inserts an unlinked basic block into \c Parent.  If \c InsertBefore is
  /// provided, inserts before that basic block, otherwise inserts at the end.
  ///
  /// \pre \a getParent() is \c nullptr.
  void insertInto(Function *Parent, BasicBlock *InsertBefore = nullptr);

  /// Return the predecessor of this block if it has a single predecessor
  /// block. Otherwise return a null pointer.
  const BasicBlock *getSinglePredecessor() const;
  BasicBlock *getSinglePredecessor() {
    return const_cast<BasicBlock *>(
                 static_cast<const BasicBlock *>(this)->getSinglePredecessor());
  }

  /// Return the predecessor of this block if it has a unique predecessor
  /// block. Otherwise return a null pointer.
  ///
  /// Note that unique predecessor doesn't mean single edge, there can be
  /// multiple edges from the unique predecessor to this block (for example a
  /// switch statement with multiple cases having the same destination).
  const BasicBlock *getUniquePredecessor() const;
  BasicBlock *getUniquePredecessor() {
    return const_cast<BasicBlock *>(
                 static_cast<const BasicBlock *>(this)->getUniquePredecessor());
  }

  /// Return true if this block has exactly N predecessors.
  bool hasNPredecessors(unsigned N) const;

  /// Return true if this block has N predecessors or more.
  bool hasNPredecessorsOrMore(unsigned N) const;

  /// Return the successor of this block if it has a single successor.
  /// Otherwise return a null pointer.
  ///
  /// This method is analogous to getSinglePredecessor above.
  const BasicBlock *getSingleSuccessor() const;
  BasicBlock *getSingleSuccessor() {
    return const_cast<BasicBlock *>(
                   static_cast<const BasicBlock *>(this)->getSingleSuccessor());
  }

  /// Return the successor of this block if it has a unique successor.
  /// Otherwise return a null pointer.
  ///
  /// This method is analogous to getUniquePredecessor above.
  const BasicBlock *getUniqueSuccessor() const;
  BasicBlock *getUniqueSuccessor() {
    return const_cast<BasicBlock *>(
                   static_cast<const BasicBlock *>(this)->getUniqueSuccessor());
  }

  /// Print the basic block to an output stream with an optional
  /// AssemblyAnnotationWriter.
  void print(raw_ostream &OS, AssemblyAnnotationWriter *AAW = nullptr,
             bool ShouldPreserveUseListOrder = false,
             bool IsForDebug = false) const;

  //===--------------------------------------------------------------------===//
  /// Instruction iterator methods
  ///
  inline iterator                begin()       { return InstList.begin(); }
  inline const_iterator          begin() const { return InstList.begin(); }
  inline iterator                end  ()       { return InstList.end();   }
  inline const_iterator          end  () const { return InstList.end();   }

  inline reverse_iterator        rbegin()       { return InstList.rbegin(); }
  inline const_reverse_iterator  rbegin() const { return InstList.rbegin(); }
  inline reverse_iterator        rend  ()       { return InstList.rend();   }
  inline const_reverse_iterator  rend  () const { return InstList.rend();   }

  inline size_t                   size() const { return InstList.size();  }
  inline bool                    empty() const { return InstList.empty(); }
  inline const Instruction      &front() const { return InstList.front(); }
  inline       Instruction      &front()       { return InstList.front(); }
  inline const Instruction       &back() const { return InstList.back();  }
  inline       Instruction       &back()       { return InstList.back();  }

  /// Iterator to walk just the phi nodes in the basic block.
  template <typename PHINodeT = PHINode, typename BBIteratorT = iterator>
  class phi_iterator_impl
      : public iterator_facade_base<phi_iterator_impl<PHINodeT, BBIteratorT>,
                                    std::forward_iterator_tag, PHINodeT> {
    friend BasicBlock;

    PHINodeT *PN;

    phi_iterator_impl(PHINodeT *PN) : PN(PN) {}

  public:
    // Allow default construction to build variables, but this doesn't build
    // a useful iterator.
    phi_iterator_impl() = default;

    // Allow conversion between instantiations where valid.
    template <typename PHINodeU, typename BBIteratorU,
              typename = std::enable_if_t<
                  std::is_convertible<PHINodeU *, PHINodeT *>::value>>
    phi_iterator_impl(const phi_iterator_impl<PHINodeU, BBIteratorU> &Arg)
        : PN(Arg.PN) {}

    bool operator==(const phi_iterator_impl &Arg) const { return PN == Arg.PN; }

    PHINodeT &operator*() const { return *PN; }

    using phi_iterator_impl::iterator_facade_base::operator++;
    phi_iterator_impl &operator++() {
      assert(PN && "Cannot increment the end iterator!");
      PN = dyn_cast<PHINodeT>(std::next(BBIteratorT(PN)));
      return *this;
    }
  };
  using phi_iterator = phi_iterator_impl<>;
  using const_phi_iterator =
      phi_iterator_impl<const PHINode, BasicBlock::const_iterator>;

  /// Returns a range that iterates over the phis in the basic block.
  ///
  /// Note that this cannot be used with basic blocks that have no terminator.
  iterator_range<const_phi_iterator> phis() const {
    return const_cast<BasicBlock *>(this)->phis();
  }
  iterator_range<phi_iterator> phis();

private:
  /// Return the underlying instruction list container.
  /// This is deliberately private because we have implemented an adequate set
  /// of functions to modify the list, including BasicBlock::splice(),
  /// BasicBlock::erase(), Instruction::insertInto() etc.
  const InstListType &getInstList() const { return InstList; }
  InstListType &getInstList() { return InstList; }

  /// Returns a pointer to a member of the instruction list.
  /// This is private on purpose, just like `getInstList()`.
  static InstListType BasicBlock::*getSublistAccess(Instruction *) {
    return &BasicBlock::InstList;
  }

public:
  /// Returns a pointer to the symbol table if one exists.
  ValueSymbolTable *getValueSymbolTable();

  /// Methods for support type inquiry through isa, cast, and dyn_cast.
  static bool classof(const Value *V) {
    return V->getValueID() == Value::BasicBlockVal;
  }

  /// Cause all subinstructions to "let go" of all the references that said
  /// subinstructions are maintaining.
  ///
  /// This allows one to 'delete' a whole class at a time, even though there may
  /// be circular references... first all references are dropped, and all use
  /// counts go to zero.  Then everything is delete'd for real.  Note that no
  /// operations are valid on an object that has "dropped all references",
  /// except operator delete.
  void dropAllReferences();

  /// Update PHI nodes in this BasicBlock before removal of predecessor \p Pred.
  /// Note that this function does not actually remove the predecessor.
  ///
  /// If \p KeepOneInputPHIs is true then don't remove PHIs that are left with
  /// zero or one incoming values, and don't simplify PHIs with all incoming
  /// values the same.
  void removePredecessor(BasicBlock *Pred, bool KeepOneInputPHIs = false);

  bool canSplitPredecessors() const;

  /// Split the basic block into two basic blocks at the specified instruction.
  ///
  /// If \p Before is true, splitBasicBlockBefore handles the
  /// block splitting. Otherwise, execution proceeds as described below.
  ///
  /// Note that all instructions BEFORE the specified iterator
  /// stay as part of the original basic block, an unconditional branch is added
  /// to the original BB, and the rest of the instructions in the BB are moved
  /// to the new BB, including the old terminator.  The newly formed basic block
  /// is returned. This function invalidates the specified iterator.
  ///
  /// Note that this only works on well formed basic blocks (must have a
  /// terminator), and \p 'I' must not be the end of instruction list (which
  /// would cause a degenerate basic block to be formed, having a terminator
  /// inside of the basic block).
  ///
  /// Also note that this doesn't preserve any passes. To split blocks while
  /// keeping loop information consistent, use the SplitBlock utility function.
  BasicBlock *splitBasicBlock(iterator I, const Twine &BBName = "",
                              bool Before = false);
  BasicBlock *splitBasicBlock(Instruction *I, const Twine &BBName = "",
                              bool Before = false) {
    return splitBasicBlock(I->getIterator(), BBName, Before);
  }

  /// Split the basic block into two basic blocks at the specified instruction
  /// and insert the new basic blocks as the predecessor of the current block.
  ///
  /// This function ensures all instructions AFTER and including the specified
  /// iterator \p I are part of the original basic block. All Instructions
  /// BEFORE the iterator \p I are moved to the new BB and an unconditional
  /// branch is added to the new BB. The new basic block is returned.
  ///
  /// Note that this only works on well formed basic blocks (must have a
  /// terminator), and \p 'I' must not be the end of instruction list (which
  /// would cause a degenerate basic block to be formed, having a terminator
  /// inside of the basic block).  \p 'I' cannot be a iterator for a PHINode
  /// with multiple incoming blocks.
  ///
  /// Also note that this doesn't preserve any passes. To split blocks while
  /// keeping loop information consistent, use the SplitBlockBefore utility
  /// function.
  BasicBlock *splitBasicBlockBefore(iterator I, const Twine &BBName = "");
  BasicBlock *splitBasicBlockBefore(Instruction *I, const Twine &BBName = "") {
    return splitBasicBlockBefore(I->getIterator(), BBName);
  }

  /// Transfer all instructions from \p FromBB to this basic block at \p ToIt.
  void splice(BasicBlock::iterator ToIt, BasicBlock *FromBB) {
    splice(ToIt, FromBB, FromBB->begin(), FromBB->end());
  }

  /// Transfer one instruction from \p FromBB at \p FromIt to this basic block
  /// at \p ToIt.
  void splice(BasicBlock::iterator ToIt, BasicBlock *FromBB,
              BasicBlock::iterator FromIt) {
    auto FromItNext = std::next(FromIt);
    // Single-element splice is a noop if destination == source.
    if (ToIt == FromIt || ToIt == FromItNext)
      return;
    splice(ToIt, FromBB, FromIt, FromItNext);
  }

  /// Transfer a range of instructions that belong to \p FromBB from \p
  /// FromBeginIt to \p FromEndIt, to this basic block at \p ToIt.
  void splice(BasicBlock::iterator ToIt, BasicBlock *FromBB,
              BasicBlock::iterator FromBeginIt,
              BasicBlock::iterator FromEndIt);

  /// Erases a range of instructions from \p FromIt to (not including) \p ToIt.
  /// \Returns \p ToIt.
  BasicBlock::iterator erase(BasicBlock::iterator FromIt, BasicBlock::iterator ToIt);

  /// Returns true if there are any uses of this basic block other than
  /// direct branches, switches, etc. to it.
  bool hasAddressTaken() const {
    return getBasicBlockBits().BlockAddressRefCount != 0;
  }

  /// Update all phi nodes in this basic block to refer to basic block \p New
  /// instead of basic block \p Old.
  void replacePhiUsesWith(BasicBlock *Old, BasicBlock *New);

  /// Update all phi nodes in this basic block's successors to refer to basic
  /// block \p New instead of basic block \p Old.
  void replaceSuccessorsPhiUsesWith(BasicBlock *Old, BasicBlock *New);

  /// Update all phi nodes in this basic block's successors to refer to basic
  /// block \p New instead of to it.
  void replaceSuccessorsPhiUsesWith(BasicBlock *New);

  /// Return true if this basic block is an exception handling block.
  bool isEHPad() const { return getFirstNonPHI()->isEHPad(); }

  /// Return true if this basic block is a landing pad.
  ///
  /// Being a ``landing pad'' means that the basic block is the destination of
  /// the 'unwind' edge of an invoke instruction.
  bool isLandingPad() const;

  /// Return the landingpad instruction associated with the landing pad.
  const LandingPadInst *getLandingPadInst() const;
  LandingPadInst *getLandingPadInst() {
    return const_cast<LandingPadInst *>(
                    static_cast<const BasicBlock *>(this)->getLandingPadInst());
  }

  /// Return true if it is legal to hoist instructions into this block.
  bool isLegalToHoistInto() const;

  /// Return true if this is the entry block of the containing function.
  /// This method can only be used on blocks that have a parent function.
  bool isEntryBlock() const;

  std::optional<uint64_t> getIrrLoopHeaderWeight() const;

  /// Returns true if the Order field of child Instructions is valid.
  bool isInstrOrderValid() const {
    return getBasicBlockBits().InstrOrderValid;
  }

  /// Mark instruction ordering invalid. Done on every instruction insert.
  void invalidateOrders() {
    validateInstrOrdering();
    BasicBlockBits Bits = getBasicBlockBits();
    Bits.InstrOrderValid = false;
    setBasicBlockBits(Bits);
  }

  /// Renumber instructions and mark the ordering as valid.
  void renumberInstructions();

  /// Asserts that instruction order numbers are marked invalid, or that they
  /// are in ascending order. This is constant time if the ordering is invalid,
  /// and linear in the number of instructions if the ordering is valid. Callers
  /// should be careful not to call this in ways that make common operations
  /// O(n^2). For example, it takes O(n) time to assign order numbers to
  /// instructions, so the order should be validated no more than once after
  /// each ordering to ensure that transforms have the same algorithmic
  /// complexity when asserts are enabled as when they are disabled.
  void validateInstrOrdering() const;

private:
#if defined(_AIX) && (!defined(__GNUC__) || defined(__clang__))
// Except for GCC; by default, AIX compilers store bit-fields in 4-byte words
// and give the `pack` pragma push semantics.
#define BEGIN_TWO_BYTE_PACK() _Pragma("pack(2)")
#define END_TWO_BYTE_PACK() _Pragma("pack(pop)")
#else
#define BEGIN_TWO_BYTE_PACK()
#define END_TWO_BYTE_PACK()
#endif

  BEGIN_TWO_BYTE_PACK()
  /// Bitfield to help interpret the bits in Value::SubclassData.
  struct BasicBlockBits {
    unsigned short BlockAddressRefCount : 15;
    unsigned short InstrOrderValid : 1;
  };
  END_TWO_BYTE_PACK()

#undef BEGIN_TWO_BYTE_PACK
#undef END_TWO_BYTE_PACK

  /// Safely reinterpret the subclass data bits to a more useful form.
  BasicBlockBits getBasicBlockBits() const {
    static_assert(sizeof(BasicBlockBits) == sizeof(unsigned short),
                  "too many bits for Value::SubclassData");
    unsigned short ValueData = getSubclassDataFromValue();
    BasicBlockBits AsBits;
    memcpy(&AsBits, &ValueData, sizeof(AsBits));
    return AsBits;
  }

  /// Reinterpret our subclass bits and store them back into Value.
  void setBasicBlockBits(BasicBlockBits AsBits) {
    unsigned short D;
    memcpy(&D, &AsBits, sizeof(D));
    Value::setValueSubclassData(D);
  }

  /// Increment the internal refcount of the number of BlockAddresses
  /// referencing this BasicBlock by \p Amt.
  ///
  /// This is almost always 0, sometimes one possibly, but almost never 2, and
  /// inconceivably 3 or more.
  void AdjustBlockAddressRefCount(int Amt) {
    BasicBlockBits Bits = getBasicBlockBits();
    Bits.BlockAddressRefCount += Amt;
    setBasicBlockBits(Bits);
    assert(Bits.BlockAddressRefCount < 255 && "Refcount wrap-around");
  }

  /// Shadow Value::setValueSubclassData with a private forwarding method so
  /// that any future subclasses cannot accidentally use it.
  void setValueSubclassData(unsigned short D) {
    Value::setValueSubclassData(D);
  }
};

// Create wrappers for C Binding types (see CBindingWrapping.h).
DEFINE_SIMPLE_CONVERSION_FUNCTIONS(BasicBlock, LLVMBasicBlockRef)

/// Advance \p It while it points to a debug instruction and return the result.
/// This assumes that \p It is not at the end of a block.
BasicBlock::iterator skipDebugIntrinsics(BasicBlock::iterator It);

#ifdef NDEBUG
/// In release builds, this is a no-op. For !NDEBUG builds, the checks are
/// implemented in the .cpp file to avoid circular header deps.
inline void BasicBlock::validateInstrOrdering() const {}
#endif

} // end namespace llvm

#endif // LLVM_IR_BASICBLOCK_H