LiveVariables.h

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//===-- llvm/CodeGen/LiveVariables.h - Live Variable Analysis ---*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the LiveVariables analysis pass.  For each machine
// instruction in the function, this pass calculates the set of registers that
// are immediately dead after the instruction (i.e., the instruction calculates
// the value, but it is never used) and the set of registers that are used by
// the instruction, but are never used after the instruction (i.e., they are
// killed).
//
// This class computes live variables using a sparse implementation based on
// the machine code SSA form.  This class computes live variable information for
// each virtual and _register allocatable_ physical register in a function.  It
// uses the dominance properties of SSA form to efficiently compute live
// variables for virtual registers, and assumes that physical registers are only
// live within a single basic block (allowing it to do a single local analysis
// to resolve physical register lifetimes in each basic block).  If a physical
// register is not register allocatable, it is not tracked.  This is useful for
// things like the stack pointer and condition codes.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_CODEGEN_LIVEVARIABLES_H
#define LLVM_CODEGEN_LIVEVARIABLES_H

#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallVector.h"

namespace llvm {

class MachineRegisterInfo;
class TargetRegisterInfo;

class LiveVariables : public MachineFunctionPass {
public:
  static char ID; // Pass identification, replacement for typeid
  LiveVariables() : MachineFunctionPass(&ID) {}

  /// VarInfo - This represents the regions where a virtual register is live in
  /// the program.  We represent this with three different pieces of
  /// information: the set of blocks in which the instruction is live
  /// throughout, the set of blocks in which the instruction is actually used,
  /// and the set of non-phi instructions that are the last users of the value.
  ///
  /// In the common case where a value is defined and killed in the same block,
  /// There is one killing instruction, and AliveBlocks is empty.
  ///
  /// Otherwise, the value is live out of the block.  If the value is live
  /// throughout any blocks, these blocks are listed in AliveBlocks.  Blocks
  /// where the liveness range ends are not included in AliveBlocks, instead
  /// being captured by the Kills set.  In these blocks, the value is live into
  /// the block (unless the value is defined and killed in the same block) and
  /// lives until the specified instruction.  Note that there cannot ever be a
  /// value whose Kills set contains two instructions from the same basic block.
  ///
  /// PHI nodes complicate things a bit.  If a PHI node is the last user of a
  /// value in one of its predecessor blocks, it is not listed in the kills set,
  /// but does include the predecessor block in the AliveBlocks set (unless that
  /// block also defines the value).  This leads to the (perfectly sensical)
  /// situation where a value is defined in a block, and the last use is a phi
  /// node in the successor.  In this case, AliveBlocks is empty (the value is
  /// not live across any  blocks) and Kills is empty (phi nodes are not
  /// included). This is sensical because the value must be live to the end of
  /// the block, but is not live in any successor blocks.
  struct VarInfo {
    /// AliveBlocks - Set of blocks in which this value is alive completely
    /// through.  This is a bit set which uses the basic block number as an
    /// index.
    ///
    BitVector AliveBlocks;

    /// UsedBlocks - Set of blocks in which this value is actually used. This
    /// is a bit set which uses the basic block number as an index.
    BitVector UsedBlocks;

    /// NumUses - Number of uses of this register across the entire function.
    ///
    unsigned NumUses;

    /// Kills - List of MachineInstruction's which are the last use of this
    /// virtual register (kill it) in their basic block.
    ///
    std::vector<MachineInstr*> Kills;

    VarInfo() : NumUses(0) {}

    /// removeKill - Delete a kill corresponding to the specified
    /// machine instruction. Returns true if there was a kill
    /// corresponding to this instruction, false otherwise.
    bool removeKill(MachineInstr *MI) {
      std::vector<MachineInstr*>::iterator
        I = std::find(Kills.begin(), Kills.end(), MI);
      if (I == Kills.end())
        return false;
      Kills.erase(I);
      return true;
    }
    
    void dump() const;
  };

private:
  /// VirtRegInfo - This list is a mapping from virtual register number to
  /// variable information.  FirstVirtualRegister is subtracted from the virtual
  /// register number before indexing into this list.
  ///
  std::vector<VarInfo> VirtRegInfo;

  /// ReservedRegisters - This vector keeps track of which registers
  /// are reserved register which are not allocatable by the target machine.
  /// We can not track liveness for values that are in this set.
  ///
  BitVector ReservedRegisters;

private:   // Intermediate data structures
  MachineFunction *MF;

  MachineRegisterInfo* MRI;

  const TargetRegisterInfo *TRI;

  // PhysRegInfo - Keep track of which instruction was the last def of a
  // physical register. This is a purely local property, because all physical
  // register references are presumed dead across basic blocks.
  MachineInstr **PhysRegDef;

  // PhysRegInfo - Keep track of which instruction was the last use of a
  // physical register. This is a purely local property, because all physical
  // register references are presumed dead across basic blocks.
  MachineInstr **PhysRegUse;

  SmallVector<unsigned, 4> *PHIVarInfo;

  // DistanceMap - Keep track the distance of a MI from the start of the
  // current basic block.
  DenseMap<MachineInstr*, unsigned> DistanceMap;

  /// HandlePhysRegKill - Add kills of Reg and its sub-registers to the
  /// uses. Pay special attention to the sub-register uses which may come below
  /// the last use of the whole register.
  bool HandlePhysRegKill(unsigned Reg, MachineInstr *MI);

  void HandlePhysRegUse(unsigned Reg, MachineInstr *MI);
  void HandlePhysRegDef(unsigned Reg, MachineInstr *MI);

  /// FindLastPartialDef - Return the last partial def of the specified register.
  /// Also returns the sub-register that's defined.
  MachineInstr *FindLastPartialDef(unsigned Reg, unsigned &PartDefReg);

  /// hasRegisterUseBelow - Return true if the specified register is used after
  /// the current instruction and before it's next definition.
  bool hasRegisterUseBelow(unsigned Reg, MachineBasicBlock::iterator I,
                           MachineBasicBlock *MBB);

  /// analyzePHINodes - Gather information about the PHI nodes in here. In
  /// particular, we want to map the variable information of a virtual
  /// register which is used in a PHI node. We map that to the BB the vreg
  /// is coming from.
  void analyzePHINodes(const MachineFunction& Fn);
public:

  virtual bool runOnMachineFunction(MachineFunction &MF);

  /// RegisterDefIsDead - Return true if the specified instruction defines the
  /// specified register, but that definition is dead.
  bool RegisterDefIsDead(MachineInstr *MI, unsigned Reg) const;

  //===--------------------------------------------------------------------===//
  //  API to update live variable information

  /// replaceKillInstruction - Update register kill info by replacing a kill
  /// instruction with a new one.
  void replaceKillInstruction(unsigned Reg, MachineInstr *OldMI,
                              MachineInstr *NewMI);

  /// addVirtualRegisterKilled - Add information about the fact that the
  /// specified register is killed after being used by the specified
  /// instruction. If AddIfNotFound is true, add a implicit operand if it's
  /// not found.
  void addVirtualRegisterKilled(unsigned IncomingReg, MachineInstr *MI,
                                bool AddIfNotFound = false) {
    if (MI->addRegisterKilled(IncomingReg, TRI, AddIfNotFound))
      getVarInfo(IncomingReg).Kills.push_back(MI); 
  }

  /// removeVirtualRegisterKilled - Remove the specified kill of the virtual
  /// register from the live variable information. Returns true if the
  /// variable was marked as killed by the specified instruction,
  /// false otherwise.
  bool removeVirtualRegisterKilled(unsigned reg, MachineInstr *MI) {
    if (!getVarInfo(reg).removeKill(MI))
      return false;

    bool Removed = false;
    for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
      MachineOperand &MO = MI->getOperand(i);
      if (MO.isReg() && MO.isKill() && MO.getReg() == reg) {
        MO.setIsKill(false);
        Removed = true;
        break;
      }
    }

    assert(Removed && "Register is not used by this instruction!");
    return true;
  }

  /// removeVirtualRegistersKilled - Remove all killed info for the specified
  /// instruction.
  void removeVirtualRegistersKilled(MachineInstr *MI);

  /// addVirtualRegisterDead - Add information about the fact that the specified
  /// register is dead after being used by the specified instruction. If
  /// AddIfNotFound is true, add a implicit operand if it's not found.
  void addVirtualRegisterDead(unsigned IncomingReg, MachineInstr *MI,
                              bool AddIfNotFound = false) {
    if (MI->addRegisterDead(IncomingReg, TRI, AddIfNotFound))
      getVarInfo(IncomingReg).Kills.push_back(MI);
  }

  /// removeVirtualRegisterDead - Remove the specified kill of the virtual
  /// register from the live variable information. Returns true if the
  /// variable was marked dead at the specified instruction, false
  /// otherwise.
  bool removeVirtualRegisterDead(unsigned reg, MachineInstr *MI) {
    if (!getVarInfo(reg).removeKill(MI))
      return false;

    bool Removed = false;
    for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
      MachineOperand &MO = MI->getOperand(i);
      if (MO.isReg() && MO.isDef() && MO.getReg() == reg) {
        MO.setIsDead(false);
        Removed = true;
        break;
      }
    }
    assert(Removed && "Register is not defined by this instruction!");
    return true;
  }
  
  void getAnalysisUsage(AnalysisUsage &AU) const;

  virtual void releaseMemory() {
    VirtRegInfo.clear();
  }

  /// getVarInfo - Return the VarInfo structure for the specified VIRTUAL
  /// register.
  VarInfo &getVarInfo(unsigned RegIdx);

  void MarkVirtRegAliveInBlock(VarInfo& VRInfo, MachineBasicBlock* DefBlock,
                               MachineBasicBlock *BB);
  void MarkVirtRegAliveInBlock(VarInfo& VRInfo, MachineBasicBlock* DefBlock,
                               MachineBasicBlock *BB,
                               std::vector<MachineBasicBlock*> &WorkList);
  void HandleVirtRegDef(unsigned reg, MachineInstr *MI);
  void HandleVirtRegUse(unsigned reg, MachineBasicBlock *MBB,
                        MachineInstr *MI);
};

} // End llvm namespace

#endif