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