RDFGraph.h

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//===- RDFGraph.h -----------------------------------------------*- 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
//
//===----------------------------------------------------------------------===//
//
// Target-independent, SSA-based data flow graph for register data flow (RDF)
// for a non-SSA program representation (e.g. post-RA machine code).
//
//
// *** Introduction
//
// The RDF graph is a collection of nodes, each of which denotes some element
// of the program. There are two main types of such elements: code and refe-
// rences. Conceptually, "code" is something that represents the structure
// of the program, e.g. basic block or a statement, while "reference" is an
// instance of accessing a register, e.g. a definition or a use. Nodes are
// connected with each other based on the structure of the program (such as
// blocks, instructions, etc.), and based on the data flow (e.g. reaching
// definitions, reached uses, etc.). The single-reaching-definition principle
// of SSA is generally observed, although, due to the non-SSA representation
// of the program, there are some differences between the graph and a "pure"
// SSA representation.
//
//
// *** Implementation remarks
//
// Since the graph can contain a large number of nodes, memory consumption
// was one of the major design considerations. As a result, there is a single
// base class NodeBase which defines all members used by all possible derived
// classes. The members are arranged in a union, and a derived class cannot
// add any data members of its own. Each derived class only defines the
// functional interface, i.e. member functions. NodeBase must be a POD,
// which implies that all of its members must also be PODs.
// Since nodes need to be connected with other nodes, pointers have been
// replaced with 32-bit identifiers: each node has an id of type NodeId.
// There are mapping functions in the graph that translate between actual
// memory addresses and the corresponding identifiers.
// A node id of 0 is equivalent to nullptr.
//
//
// *** Structure of the graph
//
// A code node is always a collection of other nodes. For example, a code
// node corresponding to a basic block will contain code nodes corresponding
// to instructions. In turn, a code node corresponding to an instruction will
// contain a list of reference nodes that correspond to the definitions and
// uses of registers in that instruction. The members are arranged into a
// circular list, which is yet another consequence of the effort to save
// memory: for each member node it should be possible to obtain its owner,
// and it should be possible to access all other members. There are other
// ways to accomplish that, but the circular list seemed the most natural.
//
// +- CodeNode -+
// |            | <---------------------------------------------------+
// +-+--------+-+                                                     |
//   |FirstM  |LastM                                                  |
//   |        +-------------------------------------+                 |
//   |                                              |                 |
//   V                                              V                 |
//  +----------+ Next +----------+ Next       Next +----------+ Next  |
//  |          |----->|          |-----> ... ----->|          |----->-+
//  +- Member -+      +- Member -+                 +- Member -+
//
// The order of members is such that related reference nodes (see below)
// should be contiguous on the member list.
//
// A reference node is a node that encapsulates an access to a register,
// in other words, data flowing into or out of a register. There are two
// major kinds of reference nodes: defs and uses. A def node will contain
// the id of the first reached use, and the id of the first reached def.
// Each def and use will contain the id of the reaching def, and also the
// id of the next reached def (for def nodes) or use (for use nodes).
// The "next node sharing the same reaching def" is denoted as "sibling".
// In summary:
// - Def node contains: reaching def, sibling, first reached def, and first
// reached use.
// - Use node contains: reaching def and sibling.
//
// +-- DefNode --+
// | R2 = ...    | <---+--------------------+
// ++---------+--+     |                    |
//  |Reached  |Reached |                    |
//  |Def      |Use     |                    |
//  |         |        |Reaching            |Reaching
//  |         V        |Def                 |Def
//  |      +-- UseNode --+ Sib  +-- UseNode --+ Sib       Sib
//  |      | ... = R2    |----->| ... = R2    |----> ... ----> 0
//  |      +-------------+      +-------------+
//  V
// +-- DefNode --+ Sib
// | R2 = ...    |----> ...
// ++---------+--+
//  |         |
//  |         |
// ...       ...
//
// To get a full picture, the circular lists connecting blocks within a
// function, instructions within a block, etc. should be superimposed with
// the def-def, def-use links shown above.
// To illustrate this, consider a small example in a pseudo-assembly:
// foo:
//   add r2, r0, r1   ; r2 = r0+r1
//   addi r0, r2, 1   ; r0 = r2+1
//   ret r0           ; return value in r0
//
// The graph (in a format used by the debugging functions) would look like:
//
//   DFG dump:[
//   f1: Function foo
//   b2: === %bb.0 === preds(0), succs(0):
//   p3: phi [d4<r0>(,d12,u9):]
//   p5: phi [d6<r1>(,,u10):]
//   s7: add [d8<r2>(,,u13):, u9<r0>(d4):, u10<r1>(d6):]
//   s11: addi [d12<r0>(d4,,u15):, u13<r2>(d8):]
//   s14: ret [u15<r0>(d12):]
//   ]
//
// The f1, b2, p3, etc. are node ids. The letter is prepended to indicate the
// kind of the node (i.e. f - function, b - basic block, p - phi, s - state-
// ment, d - def, u - use).
// The format of a def node is:
//   dN<R>(rd,d,u):sib,
// where
//   N   - numeric node id,
//   R   - register being defined
//   rd  - reaching def,
//   d   - reached def,
//   u   - reached use,
//   sib - sibling.
// The format of a use node is:
//   uN<R>[!](rd):sib,
// where
//   N   - numeric node id,
//   R   - register being used,
//   rd  - reaching def,
//   sib - sibling.
// Possible annotations (usually preceding the node id):
//   +   - preserving def,
//   ~   - clobbering def,
//   "   - shadow ref (follows the node id),
//   !   - fixed register (appears after register name).
//
// The circular lists are not explicit in the dump.
//
//
// *** Node attributes
//
// NodeBase has a member "Attrs", which is the primary way of determining
// the node's characteristics. The fields in this member decide whether
// the node is a code node or a reference node (i.e. node's "type"), then
// within each type, the "kind" determines what specifically this node
// represents. The remaining bits, "flags", contain additional information
// that is even more detailed than the "kind".
// CodeNode's kinds are:
// - Phi:   Phi node, members are reference nodes.
// - Stmt:  Statement, members are reference nodes.
// - Block: Basic block, members are instruction nodes (i.e. Phi or Stmt).
// - Func:  The whole function. The members are basic block nodes.
// RefNode's kinds are:
// - Use.
// - Def.
//
// Meaning of flags:
// - Preserving: applies only to defs. A preserving def is one that can
//   preserve some of the original bits among those that are included in
//   the register associated with that def. For example, if R0 is a 32-bit
//   register, but a def can only change the lower 16 bits, then it will
//   be marked as preserving.
// - Shadow: a reference that has duplicates holding additional reaching
//   defs (see more below).
// - Clobbering: applied only to defs, indicates that the value generated
//   by this def is unspecified. A typical example would be volatile registers
//   after function calls.
// - Fixed: the register in this def/use cannot be replaced with any other
//   register. A typical case would be a parameter register to a call, or
//   the register with the return value from a function.
// - Undef: the register in this reference the register is assumed to have
//   no pre-existing value, even if it appears to be reached by some def.
//   This is typically used to prevent keeping registers artificially live
//   in cases when they are defined via predicated instructions. For example:
//     r0 = add-if-true cond, r10, r11                (1)
//     r0 = add-if-false cond, r12, r13, implicit r0  (2)
//     ... = r0                                       (3)
//   Before (1), r0 is not intended to be live, and the use of r0 in (3) is
//   not meant to be reached by any def preceding (1). However, since the
//   defs in (1) and (2) are both preserving, these properties alone would
//   imply that the use in (3) may indeed be reached by some prior def.
//   Adding Undef flag to the def in (1) prevents that. The Undef flag
//   may be applied to both defs and uses.
// - Dead: applies only to defs. The value coming out of a "dead" def is
//   assumed to be unused, even if the def appears to be reaching other defs
//   or uses. The motivation for this flag comes from dead defs on function
//   calls: there is no way to determine if such a def is dead without
//   analyzing the target's ABI. Hence the graph should contain this info,
//   as it is unavailable otherwise. On the other hand, a def without any
//   uses on a typical instruction is not the intended target for this flag.
//
// *** Shadow references
//
// It may happen that a super-register can have two (or more) non-overlapping
// sub-registers. When both of these sub-registers are defined and followed
// by a use of the super-register, the use of the super-register will not
// have a unique reaching def: both defs of the sub-registers need to be
// accounted for. In such cases, a duplicate use of the super-register is
// added and it points to the extra reaching def. Both uses are marked with
// a flag "shadow". Example:
// Assume t0 is a super-register of r0 and r1, r0 and r1 do not overlap:
//   set r0, 1        ; r0 = 1
//   set r1, 1        ; r1 = 1
//   addi t1, t0, 1   ; t1 = t0+1
//
// The DFG:
//   s1: set [d2<r0>(,,u9):]
//   s3: set [d4<r1>(,,u10):]
//   s5: addi [d6<t1>(,,):, u7"<t0>(d2):, u8"<t0>(d4):]
//
// The statement s5 has two use nodes for t0: u7" and u9". The quotation
// mark " indicates that the node is a shadow.
//

#ifndef LLVM_LIB_TARGET_HEXAGON_RDFGRAPH_H
#define LLVM_LIB_TARGET_HEXAGON_RDFGRAPH_H

#include "RDFRegisters.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/MC/LaneBitmask.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/MathExtras.h"
#include <cassert>
#include <cstdint>
#include <cstring>
#include <map>
#include <set>
#include <unordered_map>
#include <utility>
#include <vector>

// RDF uses uint32_t to refer to registers. This is to ensure that the type
// size remains specific. In other places, registers are often stored using
// unsigned.
static_assert(sizeof(uint32_t) == sizeof(unsigned), "Those should be equal");

namespace llvm {

class MachineBasicBlock;
class MachineDominanceFrontier;
class MachineDominatorTree;
class MachineFunction;
class MachineInstr;
class MachineOperand;
class raw_ostream;
class TargetInstrInfo;
class TargetRegisterInfo;

namespace rdf {

  using NodeId = uint32_t;

  struct DataFlowGraph;

  struct NodeAttrs {
    enum : uint16_t {
      None          = 0x0000,   // Nothing

      // Types: 2 bits
      TypeMask      = 0x0003,
      Code          = 0x0001,   // 01, Container
      Ref           = 0x0002,   // 10, Reference

      // Kind: 3 bits
      KindMask      = 0x0007 << 2,
      Def           = 0x0001 << 2,  // 001
      Use           = 0x0002 << 2,  // 010
      Phi           = 0x0003 << 2,  // 011
      Stmt          = 0x0004 << 2,  // 100
      Block         = 0x0005 << 2,  // 101
      Func          = 0x0006 << 2,  // 110

      // Flags: 7 bits for now
      FlagMask      = 0x007F << 5,
      Shadow        = 0x0001 << 5,  // 0000001, Has extra reaching defs.
      Clobbering    = 0x0002 << 5,  // 0000010, Produces unspecified values.
      PhiRef        = 0x0004 << 5,  // 0000100, Member of PhiNode.
      Preserving    = 0x0008 << 5,  // 0001000, Def can keep original bits.
      Fixed         = 0x0010 << 5,  // 0010000, Fixed register.
      Undef         = 0x0020 << 5,  // 0100000, Has no pre-existing value.
      Dead          = 0x0040 << 5,  // 1000000, Does not define a value.
    };

    static uint16_t type(uint16_t T)  { return T & TypeMask; }
    static uint16_t kind(uint16_t T)  { return T & KindMask; }
    static uint16_t flags(uint16_t T) { return T & FlagMask; }

    static uint16_t set_type(uint16_t A, uint16_t T) {
      return (A & ~TypeMask) | T;
    }

    static uint16_t set_kind(uint16_t A, uint16_t K) {
      return (A & ~KindMask) | K;
    }

    static uint16_t set_flags(uint16_t A, uint16_t F) {
      return (A & ~FlagMask) | F;
    }

    // Test if A contains B.
    static bool contains(uint16_t A, uint16_t B) {
      if (type(A) != Code)
        return false;
      uint16_t KB = kind(B);
      switch (kind(A)) {
        case Func:
          return KB == Block;
        case Block:
          return KB == Phi || KB == Stmt;
        case Phi:
        case Stmt:
          return type(B) == Ref;
      }
      return false;
    }
  };

  struct BuildOptions {
    enum : unsigned {
      None          = 0x00,
      KeepDeadPhis  = 0x01,   // Do not remove dead phis during build.
    };
  };

  template <typename T> struct NodeAddr {
    NodeAddr() = default;
    NodeAddr(T A, NodeId I) : Addr(A), Id(I) {}

    // Type cast (casting constructor). The reason for having this class
    // instead of std::pair.
    template <typename S> NodeAddr(const NodeAddr<S> &NA)
      : Addr(static_cast<T>(NA.Addr)), Id(NA.Id) {}

    bool operator== (const NodeAddr<T> &NA) const {
      assert((Addr == NA.Addr) == (Id == NA.Id));
      return Addr == NA.Addr;
    }
    bool operator!= (const NodeAddr<T> &NA) const {
      return !operator==(NA);
    }

    T Addr = nullptr;
    NodeId Id = 0;
  };

  struct NodeBase;

  // Fast memory allocation and translation between node id and node address.
  // This is really the same idea as the one underlying the "bump pointer
  // allocator", the difference being in the translation. A node id is
  // composed of two components: the index of the block in which it was
  // allocated, and the index within the block. With the default settings,
  // where the number of nodes per block is 4096, the node id (minus 1) is:
  //
  // bit position:                11             0
  // +----------------------------+--------------+
  // | Index of the block         |Index in block|
  // +----------------------------+--------------+
  //
  // The actual node id is the above plus 1, to avoid creating a node id of 0.
  //
  // This method significantly improved the build time, compared to using maps
  // (std::unordered_map or DenseMap) to translate between pointers and ids.
  struct NodeAllocator {
    // Amount of storage for a single node.
    enum { NodeMemSize = 32 };

    NodeAllocator(uint32_t NPB = 4096)
        : NodesPerBlock(NPB), BitsPerIndex(Log2_32(NPB)),
          IndexMask((1 << BitsPerIndex)-1) {
      assert(isPowerOf2_32(NPB));
    }

    NodeBase *ptr(NodeId N) const {
      uint32_t N1 = N-1;
      uint32_t BlockN = N1 >> BitsPerIndex;
      uint32_t Offset = (N1 & IndexMask) * NodeMemSize;
      return reinterpret_cast<NodeBase*>(Blocks[BlockN]+Offset);
    }

    NodeId id(const NodeBase *P) const;
    NodeAddr<NodeBase*> New();
    void clear();

  private:
    void startNewBlock();
    bool needNewBlock();

    uint32_t makeId(uint32_t Block, uint32_t Index) const {
      // Add 1 to the id, to avoid the id of 0, which is treated as "null".
      return ((Block << BitsPerIndex) | Index) + 1;
    }

    const uint32_t NodesPerBlock;
    const uint32_t BitsPerIndex;
    const uint32_t IndexMask;
    char *ActiveEnd = nullptr;
    std::vector<char*> Blocks;
    using AllocatorTy = BumpPtrAllocatorImpl<MallocAllocator, 65536>;
    AllocatorTy MemPool;
  };

  using RegisterSet = std::set<RegisterRef>;

  struct TargetOperandInfo {
    TargetOperandInfo(const TargetInstrInfo &tii) : TII(tii) {}
    virtual ~TargetOperandInfo() = default;

    virtual bool isPreserving(const MachineInstr &In, unsigned OpNum) const;
    virtual bool isClobbering(const MachineInstr &In, unsigned OpNum) const;
    virtual bool isFixedReg(const MachineInstr &In, unsigned OpNum) const;

    const TargetInstrInfo &TII;
  };

  // Packed register reference. Only used for storage.
  struct PackedRegisterRef {
    RegisterId Reg;
    uint32_t MaskId;
  };

  struct LaneMaskIndex : private IndexedSet<LaneBitmask> {
    LaneMaskIndex() = default;

    LaneBitmask getLaneMaskForIndex(uint32_t K) const {
      return K == 0 ? LaneBitmask::getAll() : get(K);
    }

    uint32_t getIndexForLaneMask(LaneBitmask LM) {
      assert(LM.any());
      return LM.all() ? 0 : insert(LM);
    }

    uint32_t getIndexForLaneMask(LaneBitmask LM) const {
      assert(LM.any());
      return LM.all() ? 0 : find(LM);
    }
  };

  struct NodeBase {
  public:
    // Make sure this is a POD.
    NodeBase() = default;

    uint16_t getType()  const { return NodeAttrs::type(Attrs); }
    uint16_t getKind()  const { return NodeAttrs::kind(Attrs); }
    uint16_t getFlags() const { return NodeAttrs::flags(Attrs); }
    NodeId   getNext()  const { return Next; }

    uint16_t getAttrs() const { return Attrs; }
    void setAttrs(uint16_t A) { Attrs = A; }
    void setFlags(uint16_t F) { setAttrs(NodeAttrs::set_flags(getAttrs(), F)); }

    // Insert node NA after "this" in the circular chain.
    void append(NodeAddr<NodeBase*> NA);

    // Initialize all members to 0.
    void init() { memset(this, 0, sizeof *this); }

    void setNext(NodeId N) { Next = N; }

  protected:
    uint16_t Attrs;
    uint16_t Reserved;
    NodeId Next;                // Id of the next node in the circular chain.
    // Definitions of nested types. Using anonymous nested structs would make
    // this class definition clearer, but unnamed structs are not a part of
    // the standard.
    struct Def_struct  {
      NodeId DD, DU;          // Ids of the first reached def and use.
    };
    struct PhiU_struct  {
      NodeId PredB;           // Id of the predecessor block for a phi use.
    };
    struct Code_struct {
      void *CP;               // Pointer to the actual code.
      NodeId FirstM, LastM;   // Id of the first member and last.
    };
    struct Ref_struct {
      NodeId RD, Sib;         // Ids of the reaching def and the sibling.
      union {
        Def_struct Def;
        PhiU_struct PhiU;
      };
      union {
        MachineOperand *Op;   // Non-phi refs point to a machine operand.
        PackedRegisterRef PR; // Phi refs store register info directly.
      };
    };

    // The actual payload.
    union {
      Ref_struct Ref;
      Code_struct Code;
    };
  };
  // The allocator allocates chunks of 32 bytes for each node. The fact that
  // each node takes 32 bytes in memory is used for fast translation between
  // the node id and the node address.
  static_assert(sizeof(NodeBase) <= NodeAllocator::NodeMemSize,
        "NodeBase must be at most NodeAllocator::NodeMemSize bytes");

  using NodeList = SmallVector<NodeAddr<NodeBase *>, 4>;
  using NodeSet = std::set<NodeId>;

  struct RefNode : public NodeBase {
    RefNode() = default;

    RegisterRef getRegRef(const DataFlowGraph &G) const;

    MachineOperand &getOp() {
      assert(!(getFlags() & NodeAttrs::PhiRef));
      return *Ref.Op;
    }

    void setRegRef(RegisterRef RR, DataFlowGraph &G);
    void setRegRef(MachineOperand *Op, DataFlowGraph &G);

    NodeId getReachingDef() const {
      return Ref.RD;
    }
    void setReachingDef(NodeId RD) {
      Ref.RD = RD;
    }

    NodeId getSibling() const {
      return Ref.Sib;
    }
    void setSibling(NodeId Sib) {
      Ref.Sib = Sib;
    }

    bool isUse() const {
      assert(getType() == NodeAttrs::Ref);
      return getKind() == NodeAttrs::Use;
    }

    bool isDef() const {
      assert(getType() == NodeAttrs::Ref);
      return getKind() == NodeAttrs::Def;
    }

    template <typename Predicate>
    NodeAddr<RefNode*> getNextRef(RegisterRef RR, Predicate P, bool NextOnly,
        const DataFlowGraph &G);
    NodeAddr<NodeBase*> getOwner(const DataFlowGraph &G);
  };

  struct DefNode : public RefNode {
    NodeId getReachedDef() const {
      return Ref.Def.DD;
    }
    void setReachedDef(NodeId D) {
      Ref.Def.DD = D;
    }
    NodeId getReachedUse() const {
      return Ref.Def.DU;
    }
    void setReachedUse(NodeId U) {
      Ref.Def.DU = U;
    }

    void linkToDef(NodeId Self, NodeAddr<DefNode*> DA);
  };

  struct UseNode : public RefNode {
    void linkToDef(NodeId Self, NodeAddr<DefNode*> DA);
  };

  struct PhiUseNode : public UseNode {
    NodeId getPredecessor() const {
      assert(getFlags() & NodeAttrs::PhiRef);
      return Ref.PhiU.PredB;
    }
    void setPredecessor(NodeId B) {
      assert(getFlags() & NodeAttrs::PhiRef);
      Ref.PhiU.PredB = B;
    }
  };

  struct CodeNode : public NodeBase {
    template <typename T> T getCode() const {
      return static_cast<T>(Code.CP);
    }
    void setCode(void *C) {
      Code.CP = C;
    }

    NodeAddr<NodeBase*> getFirstMember(const DataFlowGraph &G) const;
    NodeAddr<NodeBase*> getLastMember(const DataFlowGraph &G) const;
    void addMember(NodeAddr<NodeBase*> NA, const DataFlowGraph &G);
    void addMemberAfter(NodeAddr<NodeBase*> MA, NodeAddr<NodeBase*> NA,
        const DataFlowGraph &G);
    void removeMember(NodeAddr<NodeBase*> NA, const DataFlowGraph &G);

    NodeList members(const DataFlowGraph &G) const;
    template <typename Predicate>
    NodeList members_if(Predicate P, const DataFlowGraph &G) const;
  };

  struct InstrNode : public CodeNode {
    NodeAddr<NodeBase*> getOwner(const DataFlowGraph &G);
  };

  struct PhiNode : public InstrNode {
    MachineInstr *getCode() const {
      return nullptr;
    }
  };

  struct StmtNode : public InstrNode {
    MachineInstr *getCode() const {
      return CodeNode::getCode<MachineInstr*>();
    }
  };

  struct BlockNode : public CodeNode {
    MachineBasicBlock *getCode() const {
      return CodeNode::getCode<MachineBasicBlock*>();
    }

    void addPhi(NodeAddr<PhiNode*> PA, const DataFlowGraph &G);
  };

  struct FuncNode : public CodeNode {
    MachineFunction *getCode() const {
      return CodeNode::getCode<MachineFunction*>();
    }

    NodeAddr<BlockNode*> findBlock(const MachineBasicBlock *BB,
        const DataFlowGraph &G) const;
    NodeAddr<BlockNode*> getEntryBlock(const DataFlowGraph &G);
  };

  struct DataFlowGraph {
    DataFlowGraph(MachineFunction &mf, const TargetInstrInfo &tii,
        const TargetRegisterInfo &tri, const MachineDominatorTree &mdt,
        const MachineDominanceFrontier &mdf, const TargetOperandInfo &toi);

    NodeBase *ptr(NodeId N) const;
    template <typename T> T ptr(NodeId N) const {
      return static_cast<T>(ptr(N));
    }

    NodeId id(const NodeBase *P) const;

    template <typename T> NodeAddr<T> addr(NodeId N) const {
      return { ptr<T>(N), N };
    }

    NodeAddr<FuncNode*> getFunc() const { return Func; }
    MachineFunction &getMF() const { return MF; }
    const TargetInstrInfo &getTII() const { return TII; }
    const TargetRegisterInfo &getTRI() const { return TRI; }
    const PhysicalRegisterInfo &getPRI() const { return PRI; }
    const MachineDominatorTree &getDT() const { return MDT; }
    const MachineDominanceFrontier &getDF() const { return MDF; }
    const RegisterAggr &getLiveIns() const { return LiveIns; }

    struct DefStack {
      DefStack() = default;

      bool empty() const { return Stack.empty() || top() == bottom(); }

    private:
      using value_type = NodeAddr<DefNode *>;
      struct Iterator {
        using value_type = DefStack::value_type;

        Iterator &up() { Pos = DS.nextUp(Pos); return *this; }
        Iterator &down() { Pos = DS.nextDown(Pos); return *this; }

        value_type operator*() const {
          assert(Pos >= 1);
          return DS.Stack[Pos-1];
        }
        const value_type *operator->() const {
          assert(Pos >= 1);
          return &DS.Stack[Pos-1];
        }
        bool operator==(const Iterator &It) const { return Pos == It.Pos; }
        bool operator!=(const Iterator &It) const { return Pos != It.Pos; }

      private:
        friend struct DefStack;

        Iterator(const DefStack &S, bool Top);

        // Pos-1 is the index in the StorageType object that corresponds to
        // the top of the DefStack.
        const DefStack &DS;
        unsigned Pos;
      };

    public:
      using iterator = Iterator;

      iterator top() const { return Iterator(*this, true); }
      iterator bottom() const { return Iterator(*this, false); }
      unsigned size() const;

      void push(NodeAddr<DefNode*> DA) { Stack.push_back(DA); }
      void pop();
      void start_block(NodeId N);
      void clear_block(NodeId N);

    private:
      friend struct Iterator;

      using StorageType = std::vector<value_type>;

      bool isDelimiter(const StorageType::value_type &P, NodeId N = 0) const {
        return (P.Addr == nullptr) && (N == 0 || P.Id == N);
      }

      unsigned nextUp(unsigned P) const;
      unsigned nextDown(unsigned P) const;

      StorageType Stack;
    };

    // Make this std::unordered_map for speed of accessing elements.
    // Map: Register (physical or virtual) -> DefStack
    using DefStackMap = std::unordered_map<RegisterId, DefStack>;

    void build(unsigned Options = BuildOptions::None);
    void pushAllDefs(NodeAddr<InstrNode*> IA, DefStackMap &DM);
    void markBlock(NodeId B, DefStackMap &DefM);
    void releaseBlock(NodeId B, DefStackMap &DefM);

    PackedRegisterRef pack(RegisterRef RR) {
      return { RR.Reg, LMI.getIndexForLaneMask(RR.Mask) };
    }
    PackedRegisterRef pack(RegisterRef RR) const {
      return { RR.Reg, LMI.getIndexForLaneMask(RR.Mask) };
    }
    RegisterRef unpack(PackedRegisterRef PR) const {
      return RegisterRef(PR.Reg, LMI.getLaneMaskForIndex(PR.MaskId));
    }

    RegisterRef makeRegRef(unsigned Reg, unsigned Sub) const;
    RegisterRef makeRegRef(const MachineOperand &Op) const;
    RegisterRef restrictRef(RegisterRef AR, RegisterRef BR) const;

    NodeAddr<RefNode*> getNextRelated(NodeAddr<InstrNode*> IA,
        NodeAddr<RefNode*> RA) const;
    NodeAddr<RefNode*> getNextImp(NodeAddr<InstrNode*> IA,
        NodeAddr<RefNode*> RA, bool Create);
    NodeAddr<RefNode*> getNextImp(NodeAddr<InstrNode*> IA,
        NodeAddr<RefNode*> RA) const;
    NodeAddr<RefNode*> getNextShadow(NodeAddr<InstrNode*> IA,
        NodeAddr<RefNode*> RA, bool Create);
    NodeAddr<RefNode*> getNextShadow(NodeAddr<InstrNode*> IA,
        NodeAddr<RefNode*> RA) const;

    NodeList getRelatedRefs(NodeAddr<InstrNode*> IA,
        NodeAddr<RefNode*> RA) const;

    NodeAddr<BlockNode*> findBlock(MachineBasicBlock *BB) const {
      return BlockNodes.at(BB);
    }

    void unlinkUse(NodeAddr<UseNode*> UA, bool RemoveFromOwner) {
      unlinkUseDF(UA);
      if (RemoveFromOwner)
        removeFromOwner(UA);
    }

    void unlinkDef(NodeAddr<DefNode*> DA, bool RemoveFromOwner) {
      unlinkDefDF(DA);
      if (RemoveFromOwner)
        removeFromOwner(DA);
    }

    // Some useful filters.
    template <uint16_t Kind>
    static bool IsRef(const NodeAddr<NodeBase*> BA) {
      return BA.Addr->getType() == NodeAttrs::Ref &&
             BA.Addr->getKind() == Kind;
    }

    template <uint16_t Kind>
    static bool IsCode(const NodeAddr<NodeBase*> BA) {
      return BA.Addr->getType() == NodeAttrs::Code &&
             BA.Addr->getKind() == Kind;
    }

    static bool IsDef(const NodeAddr<NodeBase*> BA) {
      return BA.Addr->getType() == NodeAttrs::Ref &&
             BA.Addr->getKind() == NodeAttrs::Def;
    }

    static bool IsUse(const NodeAddr<NodeBase*> BA) {
      return BA.Addr->getType() == NodeAttrs::Ref &&
             BA.Addr->getKind() == NodeAttrs::Use;
    }

    static bool IsPhi(const NodeAddr<NodeBase*> BA) {
      return BA.Addr->getType() == NodeAttrs::Code &&
             BA.Addr->getKind() == NodeAttrs::Phi;
    }

    static bool IsPreservingDef(const NodeAddr<DefNode*> DA) {
      uint16_t Flags = DA.Addr->getFlags();
      return (Flags & NodeAttrs::Preserving) && !(Flags & NodeAttrs::Undef);
    }

  private:
    void reset();

    RegisterSet getLandingPadLiveIns() const;

    NodeAddr<NodeBase*> newNode(uint16_t Attrs);
    NodeAddr<NodeBase*> cloneNode(const NodeAddr<NodeBase*> B);
    NodeAddr<UseNode*> newUse(NodeAddr<InstrNode*> Owner,
        MachineOperand &Op, uint16_t Flags = NodeAttrs::None);
    NodeAddr<PhiUseNode*> newPhiUse(NodeAddr<PhiNode*> Owner,
        RegisterRef RR, NodeAddr<BlockNode*> PredB,
        uint16_t Flags = NodeAttrs::PhiRef);
    NodeAddr<DefNode*> newDef(NodeAddr<InstrNode*> Owner,
        MachineOperand &Op, uint16_t Flags = NodeAttrs::None);
    NodeAddr<DefNode*> newDef(NodeAddr<InstrNode*> Owner,
        RegisterRef RR, uint16_t Flags = NodeAttrs::PhiRef);
    NodeAddr<PhiNode*> newPhi(NodeAddr<BlockNode*> Owner);
    NodeAddr<StmtNode*> newStmt(NodeAddr<BlockNode*> Owner,
        MachineInstr *MI);
    NodeAddr<BlockNode*> newBlock(NodeAddr<FuncNode*> Owner,
        MachineBasicBlock *BB);
    NodeAddr<FuncNode*> newFunc(MachineFunction *MF);

    template <typename Predicate>
    std::pair<NodeAddr<RefNode*>,NodeAddr<RefNode*>>
    locateNextRef(NodeAddr<InstrNode*> IA, NodeAddr<RefNode*> RA,
        Predicate P) const;

    using BlockRefsMap = std::map<NodeId, RegisterSet>;

    void buildStmt(NodeAddr<BlockNode*> BA, MachineInstr &In);
    void recordDefsForDF(BlockRefsMap &PhiM, NodeAddr<BlockNode*> BA);
    void buildPhis(BlockRefsMap &PhiM, RegisterSet &AllRefs,
        NodeAddr<BlockNode*> BA);
    void removeUnusedPhis();

    void pushClobbers(NodeAddr<InstrNode*> IA, DefStackMap &DM);
    void pushDefs(NodeAddr<InstrNode*> IA, DefStackMap &DM);
    template <typename T> void linkRefUp(NodeAddr<InstrNode*> IA,
        NodeAddr<T> TA, DefStack &DS);
    template <typename Predicate> void linkStmtRefs(DefStackMap &DefM,
        NodeAddr<StmtNode*> SA, Predicate P);
    void linkBlockRefs(DefStackMap &DefM, NodeAddr<BlockNode*> BA);

    void unlinkUseDF(NodeAddr<UseNode*> UA);
    void unlinkDefDF(NodeAddr<DefNode*> DA);

    void removeFromOwner(NodeAddr<RefNode*> RA) {
      NodeAddr<InstrNode*> IA = RA.Addr->getOwner(*this);
      IA.Addr->removeMember(RA, *this);
    }

    MachineFunction &MF;
    const TargetInstrInfo &TII;
    const TargetRegisterInfo &TRI;
    const PhysicalRegisterInfo PRI;
    const MachineDominatorTree &MDT;
    const MachineDominanceFrontier &MDF;
    const TargetOperandInfo &TOI;

    RegisterAggr LiveIns;
    NodeAddr<FuncNode*> Func;
    NodeAllocator Memory;
    // Local map:  MachineBasicBlock -> NodeAddr<BlockNode*>
    std::map<MachineBasicBlock*,NodeAddr<BlockNode*>> BlockNodes;
    // Lane mask map.
    LaneMaskIndex LMI;
  };  // struct DataFlowGraph

  template <typename Predicate>
  NodeAddr<RefNode*> RefNode::getNextRef(RegisterRef RR, Predicate P,
        bool NextOnly, const DataFlowGraph &G) {
    // Get the "Next" reference in the circular list that references RR and
    // satisfies predicate "Pred".
    auto NA = G.addr<NodeBase*>(getNext());

    while (NA.Addr != this) {
      if (NA.Addr->getType() == NodeAttrs::Ref) {
        NodeAddr<RefNode*> RA = NA;
        if (RA.Addr->getRegRef(G) == RR && P(NA))
          return NA;
        if (NextOnly)
          break;
        NA = G.addr<NodeBase*>(NA.Addr->getNext());
      } else {
        // We've hit the beginning of the chain.
        assert(NA.Addr->getType() == NodeAttrs::Code);
        NodeAddr<CodeNode*> CA = NA;
        NA = CA.Addr->getFirstMember(G);
      }
    }
    // Return the equivalent of "nullptr" if such a node was not found.
    return NodeAddr<RefNode*>();
  }

  template <typename Predicate>
  NodeList CodeNode::members_if(Predicate P, const DataFlowGraph &G) const {
    NodeList MM;
    auto M = getFirstMember(G);
    if (M.Id == 0)
      return MM;

    while (M.Addr != this) {
      if (P(M))
        MM.push_back(M);
      M = G.addr<NodeBase*>(M.Addr->getNext());
    }
    return MM;
  }

  template <typename T> struct Print;
  template <typename T>
  raw_ostream &operator<< (raw_ostream &OS, const Print<T> &P);

  template <typename T>
  struct Print {
    Print(const T &x, const DataFlowGraph &g) : Obj(x), G(g) {}

    const T &Obj;
    const DataFlowGraph &G;
  };

  template <typename T>
  struct PrintNode : Print<NodeAddr<T>> {
    PrintNode(const NodeAddr<T> &x, const DataFlowGraph &g)
      : Print<NodeAddr<T>>(x, g) {}
  };

} // end namespace rdf

} // end namespace llvm

#endif // LLVM_LIB_TARGET_HEXAGON_RDFGRAPH_H