lib/CodeGen/LiveRangeEdit.cpp

   1 //===-- LiveRangeEdit.cpp - Basic tools for editing a register live range -===//
   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 // The LiveRangeEdit class represents changes done to a virtual register when it
  10 // is spilled or split.
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "llvm/CodeGen/LiveRangeEdit.h"
  14 #include "llvm/ADT/Statistic.h"
  15 #include "llvm/CodeGen/CalcSpillWeights.h"
  16 #include "llvm/CodeGen/LiveIntervals.h"
  17 #include "llvm/CodeGen/MachineRegisterInfo.h"
  18 #include "llvm/CodeGen/TargetInstrInfo.h"
  19 #include "llvm/CodeGen/VirtRegMap.h"
  20 #include "llvm/Support/Debug.h"
  21 #include "llvm/Support/raw_ostream.h"
  22
  23 using namespace llvm;
  24
  25 #define DEBUG_TYPE "regalloc"
  26
  27 STATISTIC(NumDCEDeleted,     "Number of instructions deleted by DCE");
  28 STATISTIC(NumDCEFoldedLoads, "Number of single use loads folded after DCE");
  29 STATISTIC(NumFracRanges,     "Number of live ranges fractured by DCE");
  30
  31 void LiveRangeEdit::Delegate::anchor() { }
  32
  33 LiveInterval &LiveRangeEdit::createEmptyIntervalFrom(unsigned OldReg,
  34                                                      bool createSubRanges) {
  35   Register VReg = MRI.createVirtualRegister(MRI.getRegClass(OldReg));
  36   if (VRM)
  37     VRM->setIsSplitFromReg(VReg, VRM->getOriginal(OldReg));
  38
  39   LiveInterval &LI = LIS.createEmptyInterval(VReg);
  40   if (Parent && !Parent->isSpillable())
  41     LI.markNotSpillable();
  42   if (createSubRanges) {
  43     // Create empty subranges if the OldReg's interval has them. Do not create
  44     // the main range here---it will be constructed later after the subranges
  45     // have been finalized.
  46     LiveInterval &OldLI = LIS.getInterval(OldReg);
  47     VNInfo::Allocator &Alloc = LIS.getVNInfoAllocator();
  48     for (LiveInterval::SubRange &S : OldLI.subranges())
  49       LI.createSubRange(Alloc, S.LaneMask);
  50   }
  51   return LI;
  52 }
  53
  54 unsigned LiveRangeEdit::createFrom(unsigned OldReg) {
  55   Register VReg = MRI.createVirtualRegister(MRI.getRegClass(OldReg));
  56   if (VRM) {
  57     VRM->setIsSplitFromReg(VReg, VRM->getOriginal(OldReg));
  58   }
  59   // FIXME: Getting the interval here actually computes it.
  60   // In theory, this may not be what we want, but in practice
  61   // the createEmptyIntervalFrom API is used when this is not
  62   // the case. Generally speaking we just want to annotate the
  63   // LiveInterval when it gets created but we cannot do that at
  64   // the moment.
  65   if (Parent && !Parent->isSpillable())
  66     LIS.getInterval(VReg).markNotSpillable();
  67   return VReg;
  68 }
  69
  70 bool LiveRangeEdit::checkRematerializable(VNInfo *VNI,
  71                                           const MachineInstr *DefMI,
  72                                           AliasAnalysis *aa) {
  73   assert(DefMI && "Missing instruction");
  74   ScannedRemattable = true;
  75   if (!TII.isTriviallyReMaterializable(*DefMI, aa))
  76     return false;
  77   Remattable.insert(VNI);
  78   return true;
  79 }
  80
  81 void LiveRangeEdit::scanRemattable(AliasAnalysis *aa) {
  82   for (VNInfo *VNI : getParent().valnos) {
  83     if (VNI->isUnused())
  84       continue;
  85     unsigned Original = VRM->getOriginal(getReg());
  86     LiveInterval &OrigLI = LIS.getInterval(Original);
  87     VNInfo *OrigVNI = OrigLI.getVNInfoAt(VNI->def);
  88     if (!OrigVNI)
  89       continue;
  90     MachineInstr *DefMI = LIS.getInstructionFromIndex(OrigVNI->def);
  91     if (!DefMI)
  92       continue;
  93     checkRematerializable(OrigVNI, DefMI, aa);
  94   }
  95   ScannedRemattable = true;
  96 }
  97
  98 bool LiveRangeEdit::anyRematerializable(AliasAnalysis *aa) {
  99   if (!ScannedRemattable)
 100     scanRemattable(aa);
 101   return !Remattable.empty();
 102 }
 103
 104 /// allUsesAvailableAt - Return true if all registers used by OrigMI at
 105 /// OrigIdx are also available with the same value at UseIdx.
 106 bool LiveRangeEdit::allUsesAvailableAt(const MachineInstr *OrigMI,
 107                                        SlotIndex OrigIdx,
 108                                        SlotIndex UseIdx) const {
 109   OrigIdx = OrigIdx.getRegSlot(true);
 110   UseIdx = UseIdx.getRegSlot(true);
 111   for (unsigned i = 0, e = OrigMI->getNumOperands(); i != e; ++i) {
 112     const MachineOperand &MO = OrigMI->getOperand(i);
 113     if (!MO.isReg() || !MO.getReg() || !MO.readsReg())
 114       continue;
 115
 116     // We can't remat physreg uses, unless it is a constant.
 117     if (Register::isPhysicalRegister(MO.getReg())) {
 118       if (MRI.isConstantPhysReg(MO.getReg()))
 119         continue;
 120       return false;
 121     }
 122
 123     LiveInterval &li = LIS.getInterval(MO.getReg());
 124     const VNInfo *OVNI = li.getVNInfoAt(OrigIdx);
 125     if (!OVNI)
 126       continue;
 127
 128     // Don't allow rematerialization immediately after the original def.
 129     // It would be incorrect if OrigMI redefines the register.
 130     // See PR14098.
 131     if (SlotIndex::isSameInstr(OrigIdx, UseIdx))
 132       return false;
 133
 134     if (OVNI != li.getVNInfoAt(UseIdx))
 135       return false;
 136   }
 137   return true;
 138 }
 139
 140 bool LiveRangeEdit::canRematerializeAt(Remat &RM, VNInfo *OrigVNI,
 141                                        SlotIndex UseIdx, bool cheapAsAMove) {
 142   assert(ScannedRemattable && "Call anyRematerializable first");
 143
 144   // Use scanRemattable info.
 145   if (!Remattable.count(OrigVNI))
 146     return false;
 147
 148   // No defining instruction provided.
 149   SlotIndex DefIdx;
 150   assert(RM.OrigMI && "No defining instruction for remattable value");
 151   DefIdx = LIS.getInstructionIndex(*RM.OrigMI);
 152
 153   // If only cheap remats were requested, bail out early.
 154   if (cheapAsAMove && !TII.isAsCheapAsAMove(*RM.OrigMI))
 155     return false;
 156
 157   // Verify that all used registers are available with the same values.
 158   if (!allUsesAvailableAt(RM.OrigMI, DefIdx, UseIdx))
 159     return false;
 160
 161   return true;
 162 }
 163
 164 SlotIndex LiveRangeEdit::rematerializeAt(MachineBasicBlock &MBB,
 165                                          MachineBasicBlock::iterator MI,
 166                                          unsigned DestReg,
 167                                          const Remat &RM,
 168                                          const TargetRegisterInfo &tri,
 169                                          bool Late) {
 170   assert(RM.OrigMI && "Invalid remat");
 171   TII.reMaterialize(MBB, MI, DestReg, 0, *RM.OrigMI, tri);
 172   // DestReg of the cloned instruction cannot be Dead. Set isDead of DestReg
 173   // to false anyway in case the isDead flag of RM.OrigMI's dest register
 174   // is true.
 175   (*--MI).getOperand(0).setIsDead(false);
 176   Rematted.insert(RM.ParentVNI);
 177   return LIS.getSlotIndexes()->insertMachineInstrInMaps(*MI, Late).getRegSlot();
 178 }
 179
 180 void LiveRangeEdit::eraseVirtReg(unsigned Reg) {
 181   if (TheDelegate && TheDelegate->LRE_CanEraseVirtReg(Reg))
 182     LIS.removeInterval(Reg);
 183 }
 184
 185 bool LiveRangeEdit::foldAsLoad(LiveInterval *LI,
 186                                SmallVectorImpl<MachineInstr*> &Dead) {
 187   MachineInstr *DefMI = nullptr, *UseMI = nullptr;
 188
 189   // Check that there is a single def and a single use.
 190   for (MachineOperand &MO : MRI.reg_nodbg_operands(LI->reg)) {
 191     MachineInstr *MI = MO.getParent();
 192     if (MO.isDef()) {
 193       if (DefMI && DefMI != MI)
 194         return false;
 195       if (!MI->canFoldAsLoad())
 196         return false;
 197       DefMI = MI;
 198     } else if (!MO.isUndef()) {
 199       if (UseMI && UseMI != MI)
 200         return false;
 201       // FIXME: Targets don't know how to fold subreg uses.
 202       if (MO.getSubReg())
 203         return false;
 204       UseMI = MI;
 205     }
 206   }
 207   if (!DefMI || !UseMI)
 208     return false;
 209
 210   // Since we're moving the DefMI load, make sure we're not extending any live
 211   // ranges.
 212   if (!allUsesAvailableAt(DefMI, LIS.getInstructionIndex(*DefMI),
 213                           LIS.getInstructionIndex(*UseMI)))
 214     return false;
 215
 216   // We also need to make sure it is safe to move the load.
 217   // Assume there are stores between DefMI and UseMI.
 218   bool SawStore = true;
 219   if (!DefMI->isSafeToMove(nullptr, SawStore))
 220     return false;
 221
 222   LLVM_DEBUG(dbgs() << "Try to fold single def: " << *DefMI
 223                     << "       into single use: " << *UseMI);
 224
 225   SmallVector<unsigned, 8> Ops;
 226   if (UseMI->readsWritesVirtualRegister(LI->reg, &Ops).second)
 227     return false;
 228
 229   MachineInstr *FoldMI = TII.foldMemoryOperand(*UseMI, Ops, *DefMI, &LIS);
 230   if (!FoldMI)
 231     return false;
 232   LLVM_DEBUG(dbgs() << "                folded: " << *FoldMI);
 233   LIS.ReplaceMachineInstrInMaps(*UseMI, *FoldMI);
 234   if (UseMI->isCall())
 235     UseMI->getMF()->moveCallSiteInfo(UseMI, FoldMI);
 236   UseMI->eraseFromParent();
 237   DefMI->addRegisterDead(LI->reg, nullptr);
 238   Dead.push_back(DefMI);
 239   ++NumDCEFoldedLoads;
 240   return true;
 241 }
 242
 243 bool LiveRangeEdit::useIsKill(const LiveInterval &LI,
 244                               const MachineOperand &MO) const {
 245   const MachineInstr &MI = *MO.getParent();
 246   SlotIndex Idx = LIS.getInstructionIndex(MI).getRegSlot();
 247   if (LI.Query(Idx).isKill())
 248     return true;
 249   const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
 250   unsigned SubReg = MO.getSubReg();
 251   LaneBitmask LaneMask = TRI.getSubRegIndexLaneMask(SubReg);
 252   for (const LiveInterval::SubRange &S : LI.subranges()) {
 253     if ((S.LaneMask & LaneMask).any() && S.Query(Idx).isKill())
 254       return true;
 255   }
 256   return false;
 257 }
 258
 259 /// Find all live intervals that need to shrink, then remove the instruction.
 260 void LiveRangeEdit::eliminateDeadDef(MachineInstr *MI, ToShrinkSet &ToShrink,
 261                                      AliasAnalysis *AA) {
 262   assert(MI->allDefsAreDead() && "Def isn't really dead");
 263   SlotIndex Idx = LIS.getInstructionIndex(*MI).getRegSlot();
 264
 265   // Never delete a bundled instruction.
 266   if (MI->isBundled()) {
 267     return;
 268   }
 269   // Never delete inline asm.
 270   if (MI->isInlineAsm()) {
 271     LLVM_DEBUG(dbgs() << "Won't delete: " << Idx << '\t' << *MI);
 272     return;
 273   }
 274
 275   // Use the same criteria as DeadMachineInstructionElim.
 276   bool SawStore = false;
 277   if (!MI->isSafeToMove(nullptr, SawStore)) {
 278     LLVM_DEBUG(dbgs() << "Can't delete: " << Idx << '\t' << *MI);
 279     return;
 280   }
 281
 282   LLVM_DEBUG(dbgs() << "Deleting dead def " << Idx << '\t' << *MI);
 283
 284   // Collect virtual registers to be erased after MI is gone.
 285   SmallVector<unsigned, 8> RegsToErase;
 286   bool ReadsPhysRegs = false;
 287   bool isOrigDef = false;
 288   unsigned Dest;
 289   // Only optimize rematerialize case when the instruction has one def, since
 290   // otherwise we could leave some dead defs in the code.  This case is
 291   // extremely rare.
 292   if (VRM && MI->getOperand(0).isReg() && MI->getOperand(0).isDef() &&
 293       MI->getDesc().getNumDefs() == 1) {
 294     Dest = MI->getOperand(0).getReg();
 295     unsigned Original = VRM->getOriginal(Dest);
 296     LiveInterval &OrigLI = LIS.getInterval(Original);
 297     VNInfo *OrigVNI = OrigLI.getVNInfoAt(Idx);
 298     // The original live-range may have been shrunk to
 299     // an empty live-range. It happens when it is dead, but
 300     // we still keep it around to be able to rematerialize
 301     // other values that depend on it.
 302     if (OrigVNI)
 303       isOrigDef = SlotIndex::isSameInstr(OrigVNI->def, Idx);
 304   }
 305
 306   // Check for live intervals that may shrink
 307   for (MachineInstr::mop_iterator MOI = MI->operands_begin(),
 308          MOE = MI->operands_end(); MOI != MOE; ++MOI) {
 309     if (!MOI->isReg())
 310       continue;
 311     Register Reg = MOI->getReg();
 312     if (!Register::isVirtualRegister(Reg)) {
 313       // Check if MI reads any unreserved physregs.
 314       if (Reg && MOI->readsReg() && !MRI.isReserved(Reg))
 315         ReadsPhysRegs = true;
 316       else if (MOI->isDef())
 317         LIS.removePhysRegDefAt(Reg, Idx);
 318       continue;
 319     }
 320     LiveInterval &LI = LIS.getInterval(Reg);
 321
 322     // Shrink read registers, unless it is likely to be expensive and
 323     // unlikely to change anything. We typically don't want to shrink the
 324     // PIC base register that has lots of uses everywhere.
 325     // Always shrink COPY uses that probably come from live range splitting.
 326     if ((MI->readsVirtualRegister(Reg) && (MI->isCopy() || MOI->isDef())) ||
 327         (MOI->readsReg() && (MRI.hasOneNonDBGUse(Reg) || useIsKill(LI, *MOI))))
 328       ToShrink.insert(&LI);
 329
 330     // Remove defined value.
 331     if (MOI->isDef()) {
 332       if (TheDelegate && LI.getVNInfoAt(Idx) != nullptr)
 333         TheDelegate->LRE_WillShrinkVirtReg(LI.reg);
 334       LIS.removeVRegDefAt(LI, Idx);
 335       if (LI.empty())
 336         RegsToErase.push_back(Reg);
 337     }
 338   }
 339
 340   // Currently, we don't support DCE of physreg live ranges. If MI reads
 341   // any unreserved physregs, don't erase the instruction, but turn it into
 342   // a KILL instead. This way, the physreg live ranges don't end up
 343   // dangling.
 344   // FIXME: It would be better to have something like shrinkToUses() for
 345   // physregs. That could potentially enable more DCE and it would free up
 346   // the physreg. It would not happen often, though.
 347   if (ReadsPhysRegs) {
 348     MI->setDesc(TII.get(TargetOpcode::KILL));
 349     // Remove all operands that aren't physregs.
 350     for (unsigned i = MI->getNumOperands(); i; --i) {
 351       const MachineOperand &MO = MI->getOperand(i-1);
 352       if (MO.isReg() && Register::isPhysicalRegister(MO.getReg()))
 353         continue;
 354       MI->RemoveOperand(i-1);
 355     }
 356     LLVM_DEBUG(dbgs() << "Converted physregs to:\t" << *MI);
 357   } else {
 358     // If the dest of MI is an original reg and MI is reMaterializable,
 359     // don't delete the inst. Replace the dest with a new reg, and keep
 360     // the inst for remat of other siblings. The inst is saved in
 361     // LiveRangeEdit::DeadRemats and will be deleted after all the
 362     // allocations of the func are done.
 363     if (isOrigDef && DeadRemats && TII.isTriviallyReMaterializable(*MI, AA)) {
 364       LiveInterval &NewLI = createEmptyIntervalFrom(Dest, false);
 365       VNInfo *VNI = NewLI.getNextValue(Idx, LIS.getVNInfoAllocator());
 366       NewLI.addSegment(LiveInterval::Segment(Idx, Idx.getDeadSlot(), VNI));
 367       pop_back();
 368       DeadRemats->insert(MI);
 369       const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
 370       MI->substituteRegister(Dest, NewLI.reg, 0, TRI);
 371       MI->getOperand(0).setIsDead(true);
 372     } else {
 373       if (TheDelegate)
 374         TheDelegate->LRE_WillEraseInstruction(MI);
 375       LIS.RemoveMachineInstrFromMaps(*MI);
 376       MI->eraseFromParent();
 377       ++NumDCEDeleted;
 378     }
 379   }
 380
 381   // Erase any virtregs that are now empty and unused. There may be <undef>
 382   // uses around. Keep the empty live range in that case.
 383   for (unsigned i = 0, e = RegsToErase.size(); i != e; ++i) {
 384     unsigned Reg = RegsToErase[i];
 385     if (LIS.hasInterval(Reg) && MRI.reg_nodbg_empty(Reg)) {
 386       ToShrink.remove(&LIS.getInterval(Reg));
 387       eraseVirtReg(Reg);
 388     }
 389   }
 390 }
 391
 392 void LiveRangeEdit::eliminateDeadDefs(SmallVectorImpl<MachineInstr *> &Dead,
 393                                       ArrayRef<unsigned> RegsBeingSpilled,
 394                                       AliasAnalysis *AA) {
 395   ToShrinkSet ToShrink;
 396
 397   for (;;) {
 398     // Erase all dead defs.
 399     while (!Dead.empty())
 400       eliminateDeadDef(Dead.pop_back_val(), ToShrink, AA);
 401
 402     if (ToShrink.empty())
 403       break;
 404
 405     // Shrink just one live interval. Then delete new dead defs.
 406     LiveInterval *LI = ToShrink.back();
 407     ToShrink.pop_back();
 408     if (foldAsLoad(LI, Dead))
 409       continue;
 410     unsigned VReg = LI->reg;
 411     if (TheDelegate)
 412       TheDelegate->LRE_WillShrinkVirtReg(VReg);
 413     if (!LIS.shrinkToUses(LI, &Dead))
 414       continue;
 415
 416     // Don't create new intervals for a register being spilled.
 417     // The new intervals would have to be spilled anyway so its not worth it.
 418     // Also they currently aren't spilled so creating them and not spilling
 419     // them results in incorrect code.
 420     bool BeingSpilled = false;
 421     for (unsigned i = 0, e = RegsBeingSpilled.size(); i != e; ++i) {
 422       if (VReg == RegsBeingSpilled[i]) {
 423         BeingSpilled = true;
 424         break;
 425       }
 426     }
 427
 428     if (BeingSpilled) continue;
 429
 430     // LI may have been separated, create new intervals.
 431     LI->RenumberValues();
 432     SmallVector<LiveInterval*, 8> SplitLIs;
 433     LIS.splitSeparateComponents(*LI, SplitLIs);
 434     if (!SplitLIs.empty())
 435       ++NumFracRanges;
 436
 437     unsigned Original = VRM ? VRM->getOriginal(VReg) : 0;
 438     for (const LiveInterval *SplitLI : SplitLIs) {
 439       // If LI is an original interval that hasn't been split yet, make the new
 440       // intervals their own originals instead of referring to LI. The original
 441       // interval must contain all the split products, and LI doesn't.
 442       if (Original != VReg && Original != 0)
 443         VRM->setIsSplitFromReg(SplitLI->reg, Original);
 444       if (TheDelegate)
 445         TheDelegate->LRE_DidCloneVirtReg(SplitLI->reg, VReg);
 446     }
 447   }
 448 }
 449
 450 // Keep track of new virtual registers created via
 451 // MachineRegisterInfo::createVirtualRegister.
 452 void
 453 LiveRangeEdit::MRI_NoteNewVirtualRegister(unsigned VReg)
 454 {
 455   if (VRM)
 456     VRM->grow();
 457
 458   NewRegs.push_back(VReg);
 459 }
 460
 461 void
 462 LiveRangeEdit::calculateRegClassAndHint(MachineFunction &MF,
 463                                         const MachineLoopInfo &Loops,
 464                                         const MachineBlockFrequencyInfo &MBFI) {
 465   VirtRegAuxInfo VRAI(MF, LIS, VRM, Loops, MBFI);
 466   for (unsigned I = 0, Size = size(); I < Size; ++I) {
 467     LiveInterval &LI = LIS.getInterval(get(I));
 468     if (MRI.recomputeRegClass(LI.reg))
 469       LLVM_DEBUG({
 470         const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
 471         dbgs() << "Inflated " << printReg(LI.reg) << " to "
 472                << TRI->getRegClassName(MRI.getRegClass(LI.reg)) << '\n';
 473       });
 474     VRAI.calculateSpillWeightAndHint(LI);
 475   }
 476 }