lib/CodeGen/RegisterScavenging.cpp

   1 //===-- RegisterScavenging.cpp - Machine register scavenging --------------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This file implements the machine register scavenger. It can provide
  11 // information, such as unused registers, at any point in a machine basic block.
  12 // It also provides a mechanism to make registers available by evicting them to
  13 // spill slots.
  14 //
  15 //===----------------------------------------------------------------------===//
  16
  17 #define DEBUG_TYPE "reg-scavenging"
  18 #include "llvm/CodeGen/RegisterScavenging.h"
  19 #include "llvm/CodeGen/MachineFrameInfo.h"
  20 #include "llvm/CodeGen/MachineFunction.h"
  21 #include "llvm/CodeGen/MachineBasicBlock.h"
  22 #include "llvm/CodeGen/MachineInstr.h"
  23 #include "llvm/CodeGen/MachineRegisterInfo.h"
  24 #include "llvm/Support/Debug.h"
  25 #include "llvm/Support/ErrorHandling.h"
  26 #include "llvm/Support/raw_ostream.h"
  27 #include "llvm/Target/TargetRegisterInfo.h"
  28 #include "llvm/Target/TargetInstrInfo.h"
  29 #include "llvm/Target/TargetMachine.h"
  30 #include "llvm/ADT/DenseMap.h"
  31 #include "llvm/ADT/SmallPtrSet.h"
  32 #include "llvm/ADT/SmallVector.h"
  33 #include "llvm/ADT/STLExtras.h"
  34 using namespace llvm;
  35
  36 /// setUsed - Set the register and its sub-registers as being used.
  37 void RegScavenger::setUsed(unsigned Reg) {
  38   RegsAvailable.reset(Reg);
  39
  40   for (const unsigned *SubRegs = TRI->getSubRegisters(Reg);
  41        unsigned SubReg = *SubRegs; ++SubRegs)
  42     RegsAvailable.reset(SubReg);
  43 }
  44
  45 bool RegScavenger::isAliasUsed(unsigned Reg) const {
  46   if (isUsed(Reg))
  47     return true;
  48   for (const unsigned *R = TRI->getAliasSet(Reg); *R; ++R)
  49     if (isUsed(*R))
  50       return true;
  51   return false;
  52 }
  53
  54 void RegScavenger::initRegState() {
  55   ScavengedReg = 0;
  56   ScavengedRC = NULL;
  57   ScavengeRestore = NULL;
  58
  59   // All registers started out unused.
  60   RegsAvailable.set();
  61
  62   // Reserved registers are always used.
  63   RegsAvailable ^= ReservedRegs;
  64
  65   if (!MBB)
  66     return;
  67
  68   // Live-in registers are in use.
  69   for (MachineBasicBlock::livein_iterator I = MBB->livein_begin(),
  70          E = MBB->livein_end(); I != E; ++I)
  71     setUsed(*I);
  72
  73   // Pristine CSRs are also unavailable.
  74   BitVector PR = MBB->getParent()->getFrameInfo()->getPristineRegs(MBB);
  75   for (int I = PR.find_first(); I>0; I = PR.find_next(I))
  76     setUsed(I);
  77 }
  78
  79 void RegScavenger::enterBasicBlock(MachineBasicBlock *mbb) {
  80   MachineFunction &MF = *mbb->getParent();
  81   const TargetMachine &TM = MF.getTarget();
  82   TII = TM.getInstrInfo();
  83   TRI = TM.getRegisterInfo();
  84   MRI = &MF.getRegInfo();
  85
  86   assert((NumPhysRegs == 0 || NumPhysRegs == TRI->getNumRegs()) &&
  87          "Target changed?");
  88
  89   // Self-initialize.
  90   if (!MBB) {
  91     NumPhysRegs = TRI->getNumRegs();
  92     RegsAvailable.resize(NumPhysRegs);
  93
  94     // Create reserved registers bitvector.
  95     ReservedRegs = TRI->getReservedRegs(MF);
  96
  97     // Create callee-saved registers bitvector.
  98     CalleeSavedRegs.resize(NumPhysRegs);
  99     const unsigned *CSRegs = TRI->getCalleeSavedRegs();
 100     if (CSRegs != NULL)
 101       for (unsigned i = 0; CSRegs[i]; ++i)
 102         CalleeSavedRegs.set(CSRegs[i]);
 103   }
 104
 105   MBB = mbb;
 106   initRegState();
 107
 108   Tracking = false;
 109 }
 110
 111 void RegScavenger::addRegWithSubRegs(BitVector &BV, unsigned Reg) {
 112   BV.set(Reg);
 113   for (const unsigned *R = TRI->getSubRegisters(Reg); *R; R++)
 114     BV.set(*R);
 115 }
 116
 117 void RegScavenger::addRegWithAliases(BitVector &BV, unsigned Reg) {
 118   BV.set(Reg);
 119   for (const unsigned *R = TRI->getAliasSet(Reg); *R; R++)
 120     BV.set(*R);
 121 }
 122
 123 void RegScavenger::forward() {
 124   // Move ptr forward.
 125   if (!Tracking) {
 126     MBBI = MBB->begin();
 127     Tracking = true;
 128   } else {
 129     assert(MBBI != MBB->end() && "Already past the end of the basic block!");
 130     MBBI = llvm::next(MBBI);
 131   }
 132   assert(MBBI != MBB->end() && "Already at the end of the basic block!");
 133
 134   MachineInstr *MI = MBBI;
 135
 136   if (MI == ScavengeRestore) {
 137     ScavengedReg = 0;
 138     ScavengedRC = NULL;
 139     ScavengeRestore = NULL;
 140   }
 141
 142   if (MI->isDebugValue())
 143     return;
 144
 145   // Find out which registers are early clobbered, killed, defined, and marked
 146   // def-dead in this instruction.
 147   // FIXME: The scavenger is not predication aware. If the instruction is
 148   // predicated, conservatively assume "kill" markers do not actually kill the
 149   // register. Similarly ignores "dead" markers.
 150   bool isPred = TII->isPredicated(MI);
 151   BitVector EarlyClobberRegs(NumPhysRegs);
 152   BitVector KillRegs(NumPhysRegs);
 153   BitVector DefRegs(NumPhysRegs);
 154   BitVector DeadRegs(NumPhysRegs);
 155   for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
 156     const MachineOperand &MO = MI->getOperand(i);
 157     if (!MO.isReg())
 158       continue;
 159     unsigned Reg = MO.getReg();
 160     if (!Reg || isReserved(Reg))
 161       continue;
 162
 163     if (MO.isUse()) {
 164       // Ignore undef uses.
 165       if (MO.isUndef())
 166         continue;
 167       // Two-address operands implicitly kill.
 168       if (!isPred && (MO.isKill() || MI->isRegTiedToDefOperand(i)))
 169         addRegWithSubRegs(KillRegs, Reg);
 170     } else {
 171       assert(MO.isDef());
 172       if (!isPred && MO.isDead())
 173         addRegWithSubRegs(DeadRegs, Reg);
 174       else
 175         addRegWithSubRegs(DefRegs, Reg);
 176       if (MO.isEarlyClobber())
 177         addRegWithAliases(EarlyClobberRegs, Reg);
 178     }
 179   }
 180
 181   // Verify uses and defs.
 182   for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
 183     const MachineOperand &MO = MI->getOperand(i);
 184     if (!MO.isReg())
 185       continue;
 186     unsigned Reg = MO.getReg();
 187     if (!Reg || isReserved(Reg))
 188       continue;
 189     if (MO.isUse()) {
 190       if (MO.isUndef())
 191         continue;
 192       if (!isUsed(Reg)) {
 193         // Check if it's partial live: e.g.
 194         // D0 = insert_subreg D0<undef>, S0
 195         // ... D0
 196         // The problem is the insert_subreg could be eliminated. The use of
 197         // D0 is using a partially undef value. This is not *incorrect* since
 198         // S1 is can be freely clobbered.
 199         // Ideally we would like a way to model this, but leaving the
 200         // insert_subreg around causes both correctness and performance issues.
 201         bool SubUsed = false;
 202         for (const unsigned *SubRegs = TRI->getSubRegisters(Reg);
 203              unsigned SubReg = *SubRegs; ++SubRegs)
 204           if (isUsed(SubReg)) {
 205             SubUsed = true;
 206             break;
 207           }
 208         assert(SubUsed && "Using an undefined register!");
 209       }
 210       assert((!EarlyClobberRegs.test(Reg) || MI->isRegTiedToDefOperand(i)) &&
 211              "Using an early clobbered register!");
 212     } else {
 213       assert(MO.isDef());
 214 #if 0
 215       // FIXME: Enable this once we've figured out how to correctly transfer
 216       // implicit kills during codegen passes like the coalescer.
 217       assert((KillRegs.test(Reg) || isUnused(Reg) ||
 218               isLiveInButUnusedBefore(Reg, MI, MBB, TRI, MRI)) &&
 219              "Re-defining a live register!");
 220 #endif
 221     }
 222   }
 223
 224   // Commit the changes.
 225   setUnused(KillRegs);
 226   setUnused(DeadRegs);
 227   setUsed(DefRegs);
 228 }
 229
 230 void RegScavenger::getRegsUsed(BitVector &used, bool includeReserved) {
 231   if (includeReserved)
 232     used = ~RegsAvailable;
 233   else
 234     used = ~RegsAvailable & ~ReservedRegs;
 235 }
 236
 237 unsigned RegScavenger::FindUnusedReg(const TargetRegisterClass *RC) const {
 238   for (TargetRegisterClass::iterator I = RC->begin(), E = RC->end();
 239        I != E; ++I)
 240     if (!isAliasUsed(*I)) {
 241       DEBUG(dbgs() << "Scavenger found unused reg: " << TRI->getName(*I) <<
 242             "\n");
 243       return *I;
 244     }
 245   return 0;
 246 }
 247
 248 /// getRegsAvailable - Return all available registers in the register class
 249 /// in Mask.
 250 BitVector RegScavenger::getRegsAvailable(const TargetRegisterClass *RC) {
 251   BitVector Mask(TRI->getNumRegs());
 252   for (TargetRegisterClass::iterator I = RC->begin(), E = RC->end();
 253        I != E; ++I)
 254     if (!isAliasUsed(*I))
 255       Mask.set(*I);
 256   return Mask;
 257 }
 258
 259 /// findSurvivorReg - Return the candidate register that is unused for the
 260 /// longest after StargMII. UseMI is set to the instruction where the search
 261 /// stopped.
 262 ///
 263 /// No more than InstrLimit instructions are inspected.
 264 ///
 265 unsigned RegScavenger::findSurvivorReg(MachineBasicBlock::iterator StartMI,
 266                                        BitVector &Candidates,
 267                                        unsigned InstrLimit,
 268                                        MachineBasicBlock::iterator &UseMI) {
 269   int Survivor = Candidates.find_first();
 270   assert(Survivor > 0 && "No candidates for scavenging");
 271
 272   MachineBasicBlock::iterator ME = MBB->getFirstTerminator();
 273   assert(StartMI != ME && "MI already at terminator");
 274   MachineBasicBlock::iterator RestorePointMI = StartMI;
 275   MachineBasicBlock::iterator MI = StartMI;
 276
 277   bool inVirtLiveRange = false;
 278   for (++MI; InstrLimit > 0 && MI != ME; ++MI, --InstrLimit) {
 279     if (MI->isDebugValue()) {
 280       ++InstrLimit; // Don't count debug instructions
 281       continue;
 282     }
 283     bool isVirtKillInsn = false;
 284     bool isVirtDefInsn = false;
 285     // Remove any candidates touched by instruction.
 286     for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
 287       const MachineOperand &MO = MI->getOperand(i);
 288       if (!MO.isReg() || MO.isUndef() || !MO.getReg())
 289         continue;
 290       if (TargetRegisterInfo::isVirtualRegister(MO.getReg())) {
 291         if (MO.isDef())
 292           isVirtDefInsn = true;
 293         else if (MO.isKill())
 294           isVirtKillInsn = true;
 295         continue;
 296       }
 297       Candidates.reset(MO.getReg());
 298       for (const unsigned *R = TRI->getAliasSet(MO.getReg()); *R; R++)
 299         Candidates.reset(*R);
 300     }
 301     // If we're not in a virtual reg's live range, this is a valid
 302     // restore point.
 303     if (!inVirtLiveRange) RestorePointMI = MI;
 304
 305     // Update whether we're in the live range of a virtual register
 306     if (isVirtKillInsn) inVirtLiveRange = false;
 307     if (isVirtDefInsn) inVirtLiveRange = true;
 308
 309     // Was our survivor untouched by this instruction?
 310     if (Candidates.test(Survivor))
 311       continue;
 312
 313     // All candidates gone?
 314     if (Candidates.none())
 315       break;
 316
 317     Survivor = Candidates.find_first();
 318   }
 319   // If we ran off the end, that's where we want to restore.
 320   if (MI == ME) RestorePointMI = ME;
 321   assert (RestorePointMI != StartMI &&
 322           "No available scavenger restore location!");
 323
 324   // We ran out of candidates, so stop the search.
 325   UseMI = RestorePointMI;
 326   return Survivor;
 327 }
 328
 329 unsigned RegScavenger::scavengeRegister(const TargetRegisterClass *RC,
 330                                         MachineBasicBlock::iterator I,
 331                                         int SPAdj) {
 332   // Consider all allocatable registers in the register class initially
 333   BitVector Candidates =
 334     TRI->getAllocatableSet(*I->getParent()->getParent(), RC);
 335
 336   // Exclude all the registers being used by the instruction.
 337   for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
 338     MachineOperand &MO = I->getOperand(i);
 339     if (MO.isReg() && MO.getReg() != 0 &&
 340         !TargetRegisterInfo::isVirtualRegister(MO.getReg()))
 341       Candidates.reset(MO.getReg());
 342   }
 343
 344   // Try to find a register that's unused if there is one, as then we won't
 345   // have to spill. Search explicitly rather than masking out based on
 346   // RegsAvailable, as RegsAvailable does not take aliases into account.
 347   // That's what getRegsAvailable() is for.
 348   BitVector Available = getRegsAvailable(RC);
 349
 350   if ((Candidates & Available).any())
 351      Candidates &= Available;
 352
 353   // Find the register whose use is furthest away.
 354   MachineBasicBlock::iterator UseMI;
 355   unsigned SReg = findSurvivorReg(I, Candidates, 25, UseMI);
 356
 357   // If we found an unused register there is no reason to spill it.
 358   if (!isAliasUsed(SReg)) {
 359     DEBUG(dbgs() << "Scavenged register: " << TRI->getName(SReg) << "\n");
 360     return SReg;
 361   }
 362
 363   assert(ScavengedReg == 0 &&
 364          "Scavenger slot is live, unable to scavenge another register!");
 365
 366   // Avoid infinite regress
 367   ScavengedReg = SReg;
 368
 369   // If the target knows how to save/restore the register, let it do so;
 370   // otherwise, use the emergency stack spill slot.
 371   if (!TRI->saveScavengerRegister(*MBB, I, UseMI, RC, SReg)) {
 372     // Spill the scavenged register before I.
 373     assert(ScavengingFrameIndex >= 0 &&
 374            "Cannot scavenge register without an emergency spill slot!");
 375     TII->storeRegToStackSlot(*MBB, I, SReg, true, ScavengingFrameIndex, RC,TRI);
 376     MachineBasicBlock::iterator II = prior(I);
 377     TRI->eliminateFrameIndex(II, SPAdj, this);
 378
 379     // Restore the scavenged register before its use (or first terminator).
 380     TII->loadRegFromStackSlot(*MBB, UseMI, SReg, ScavengingFrameIndex, RC, TRI);
 381     II = prior(UseMI);
 382     TRI->eliminateFrameIndex(II, SPAdj, this);
 383   }
 384
 385   ScavengeRestore = prior(UseMI);
 386
 387   // Doing this here leads to infinite regress.
 388   // ScavengedReg = SReg;
 389   ScavengedRC = RC;
 390
 391   DEBUG(dbgs() << "Scavenged register (with spill): " << TRI->getName(SReg) <<
 392         "\n");
 393
 394   return SReg;
 395 }