include/llvm/CodeGen/RegisterScavenging.h

   1 //===- RegisterScavenging.h - Machine register scavenging -------*- C++ -*-===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 /// \file
  10 /// This file declares the machine register scavenger class. It can provide
  11 /// information such as unused register at any point in a machine basic block.
  12 /// It also provides a mechanism to make registers available by evicting them
  13 /// to spill slots.
  14 //
  15 //===----------------------------------------------------------------------===//
  16
  17 #ifndef LLVM_CODEGEN_REGISTERSCAVENGING_H
  18 #define LLVM_CODEGEN_REGISTERSCAVENGING_H
  19
  20 #include "llvm/ADT/BitVector.h"
  21 #include "llvm/ADT/SmallVector.h"
  22 #include "llvm/CodeGen/LiveRegUnits.h"
  23 #include "llvm/CodeGen/MachineBasicBlock.h"
  24 #include "llvm/CodeGen/MachineRegisterInfo.h"
  25 #include "llvm/MC/LaneBitmask.h"
  26
  27 namespace llvm {
  28
  29 class MachineInstr;
  30 class TargetInstrInfo;
  31 class TargetRegisterClass;
  32 class TargetRegisterInfo;
  33
  34 class RegScavenger {
  35   const TargetRegisterInfo *TRI;
  36   const TargetInstrInfo *TII;
  37   MachineRegisterInfo* MRI;
  38   MachineBasicBlock *MBB = nullptr;
  39   MachineBasicBlock::iterator MBBI;
  40   unsigned NumRegUnits = 0;
  41
  42   /// True if RegScavenger is currently tracking the liveness of registers.
  43   bool Tracking = false;
  44
  45   /// Information on scavenged registers (held in a spill slot).
  46   struct ScavengedInfo {
  47     ScavengedInfo(int FI = -1) : FrameIndex(FI) {}
  48
  49     /// A spill slot used for scavenging a register post register allocation.
  50     int FrameIndex;
  51
  52     /// If non-zero, the specific register is currently being
  53     /// scavenged. That is, it is spilled to this scavenging stack slot.
  54     Register Reg;
  55
  56     /// The instruction that restores the scavenged register from stack.
  57     const MachineInstr *Restore = nullptr;
  58   };
  59
  60   /// A vector of information on scavenged registers.
  61   SmallVector<ScavengedInfo, 2> Scavenged;
  62
  63   LiveRegUnits LiveUnits;
  64
  65   // These BitVectors are only used internally to forward(). They are members
  66   // to avoid frequent reallocations.
  67   BitVector KillRegUnits, DefRegUnits;
  68   BitVector TmpRegUnits;
  69
  70 public:
  71   RegScavenger() = default;
  72
  73   /// Start tracking liveness from the begin of basic block \p MBB.
  74   void enterBasicBlock(MachineBasicBlock &MBB);
  75
  76   /// Start tracking liveness from the end of basic block \p MBB.
  77   /// Use backward() to move towards the beginning of the block. This is
  78   /// preferred to enterBasicBlock() and forward() because it does not depend
  79   /// on the presence of kill flags.
  80   void enterBasicBlockEnd(MachineBasicBlock &MBB);
  81
  82   /// Move the internal MBB iterator and update register states.
  83   void forward();
  84
  85   /// Move the internal MBB iterator and update register states until
  86   /// it has processed the specific iterator.
  87   void forward(MachineBasicBlock::iterator I) {
  88     if (!Tracking && MBB->begin() != I) forward();
  89     while (MBBI != I) forward();
  90   }
  91
  92   /// Invert the behavior of forward() on the current instruction (undo the
  93   /// changes to the available registers made by forward()).
  94   void unprocess();
  95
  96   /// Unprocess instructions until you reach the provided iterator.
  97   void unprocess(MachineBasicBlock::iterator I) {
  98     while (MBBI != I) unprocess();
  99   }
 100
 101   /// Update internal register state and move MBB iterator backwards.
 102   /// Contrary to unprocess() this method gives precise results even in the
 103   /// absence of kill flags.
 104   void backward();
 105
 106   /// Call backward() as long as the internal iterator does not point to \p I.
 107   void backward(MachineBasicBlock::iterator I) {
 108     while (MBBI != I)
 109       backward();
 110   }
 111
 112   /// Move the internal MBB iterator but do not update register states.
 113   void skipTo(MachineBasicBlock::iterator I) {
 114     if (I == MachineBasicBlock::iterator(nullptr))
 115       Tracking = false;
 116     MBBI = I;
 117   }
 118
 119   MachineBasicBlock::iterator getCurrentPosition() const { return MBBI; }
 120
 121   /// Return if a specific register is currently used.
 122   bool isRegUsed(Register Reg, bool includeReserved = true) const;
 123
 124   /// Return all available registers in the register class in Mask.
 125   BitVector getRegsAvailable(const TargetRegisterClass *RC);
 126
 127   /// Find an unused register of the specified register class.
 128   /// Return 0 if none is found.
 129   Register FindUnusedReg(const TargetRegisterClass *RC) const;
 130
 131   /// Add a scavenging frame index.
 132   void addScavengingFrameIndex(int FI) {
 133     Scavenged.push_back(ScavengedInfo(FI));
 134   }
 135
 136   /// Query whether a frame index is a scavenging frame index.
 137   bool isScavengingFrameIndex(int FI) const {
 138     for (SmallVectorImpl<ScavengedInfo>::const_iterator I = Scavenged.begin(),
 139          IE = Scavenged.end(); I != IE; ++I)
 140       if (I->FrameIndex == FI)
 141         return true;
 142
 143     return false;
 144   }
 145
 146   /// Get an array of scavenging frame indices.
 147   void getScavengingFrameIndices(SmallVectorImpl<int> &A) const {
 148     for (SmallVectorImpl<ScavengedInfo>::const_iterator I = Scavenged.begin(),
 149          IE = Scavenged.end(); I != IE; ++I)
 150       if (I->FrameIndex >= 0)
 151         A.push_back(I->FrameIndex);
 152   }
 153
 154   /// Make a register of the specific register class
 155   /// available and do the appropriate bookkeeping. SPAdj is the stack
 156   /// adjustment due to call frame, it's passed along to eliminateFrameIndex().
 157   /// Returns the scavenged register.
 158   /// This is deprecated as it depends on the quality of the kill flags being
 159   /// present; Use scavengeRegisterBackwards() instead!
 160   ///
 161   /// If \p AllowSpill is false, fail if a spill is required to make the
 162   /// register available, and return NoRegister.
 163   Register scavengeRegister(const TargetRegisterClass *RC,
 164                             MachineBasicBlock::iterator I, int SPAdj,
 165                             bool AllowSpill = true);
 166   Register scavengeRegister(const TargetRegisterClass *RegClass, int SPAdj,
 167                             bool AllowSpill = true) {
 168     return scavengeRegister(RegClass, MBBI, SPAdj, AllowSpill);
 169   }
 170
 171   /// Make a register of the specific register class available from the current
 172   /// position backwards to the place before \p To. If \p RestoreAfter is true
 173   /// this includes the instruction following the current position.
 174   /// SPAdj is the stack adjustment due to call frame, it's passed along to
 175   /// eliminateFrameIndex().
 176   /// Returns the scavenged register.
 177   ///
 178   /// If \p AllowSpill is false, fail if a spill is required to make the
 179   /// register available, and return NoRegister.
 180   Register scavengeRegisterBackwards(const TargetRegisterClass &RC,
 181                                      MachineBasicBlock::iterator To,
 182                                      bool RestoreAfter, int SPAdj,
 183                                      bool AllowSpill = true);
 184
 185   /// Tell the scavenger a register is used.
 186   void setRegUsed(Register Reg, LaneBitmask LaneMask = LaneBitmask::getAll());
 187
 188 private:
 189   /// Returns true if a register is reserved. It is never "unused".
 190   bool isReserved(Register Reg) const { return MRI->isReserved(Reg); }
 191
 192   /// setUsed / setUnused - Mark the state of one or a number of register units.
 193   ///
 194   void setUsed(const BitVector &RegUnits) {
 195     LiveUnits.addUnits(RegUnits);
 196   }
 197   void setUnused(const BitVector &RegUnits) {
 198     LiveUnits.removeUnits(RegUnits);
 199   }
 200
 201   /// Processes the current instruction and fill the KillRegUnits and
 202   /// DefRegUnits bit vectors.
 203   void determineKillsAndDefs();
 204
 205   /// Add all Reg Units that Reg contains to BV.
 206   void addRegUnits(BitVector &BV, Register Reg);
 207
 208   /// Remove all Reg Units that \p Reg contains from \p BV.
 209   void removeRegUnits(BitVector &BV, Register Reg);
 210
 211   /// Return the candidate register that is unused for the longest after
 212   /// StartMI. UseMI is set to the instruction where the search stopped.
 213   ///
 214   /// No more than InstrLimit instructions are inspected.
 215   Register findSurvivorReg(MachineBasicBlock::iterator StartMI,
 216                            BitVector &Candidates,
 217                            unsigned InstrLimit,
 218                            MachineBasicBlock::iterator &UseMI);
 219
 220   /// Initialize RegisterScavenger.
 221   void init(MachineBasicBlock &MBB);
 222
 223   /// Mark live-in registers of basic block as used.
 224   void setLiveInsUsed(const MachineBasicBlock &MBB);
 225
 226   /// Spill a register after position \p After and reload it before position
 227   /// \p UseMI.
 228   ScavengedInfo &spill(Register Reg, const TargetRegisterClass &RC, int SPAdj,
 229                        MachineBasicBlock::iterator Before,
 230                        MachineBasicBlock::iterator &UseMI);
 231 };
 232
 233 /// Replaces all frame index virtual registers with physical registers. Uses the
 234 /// register scavenger to find an appropriate register to use.
 235 void scavengeFrameVirtualRegs(MachineFunction &MF, RegScavenger &RS);
 236
 237 } // end namespace llvm
 238
 239 #endif // LLVM_CODEGEN_REGISTERSCAVENGING_H