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      1 //===- MachineCopyPropagation.cpp - Machine Copy Propagation Pass ---------===//
      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 is an extremely simple MachineInstr-level copy propagation pass.
     11 //
     12 //===----------------------------------------------------------------------===//
     13 
     14 #define DEBUG_TYPE "codegen-cp"
     15 #include "llvm/CodeGen/Passes.h"
     16 #include "llvm/ADT/DenseMap.h"
     17 #include "llvm/ADT/SetVector.h"
     18 #include "llvm/ADT/SmallVector.h"
     19 #include "llvm/ADT/Statistic.h"
     20 #include "llvm/CodeGen/MachineFunction.h"
     21 #include "llvm/CodeGen/MachineFunctionPass.h"
     22 #include "llvm/CodeGen/MachineRegisterInfo.h"
     23 #include "llvm/Pass.h"
     24 #include "llvm/Support/Debug.h"
     25 #include "llvm/Support/ErrorHandling.h"
     26 #include "llvm/Support/raw_ostream.h"
     27 #include "llvm/Target/TargetInstrInfo.h"
     28 #include "llvm/Target/TargetRegisterInfo.h"
     29 using namespace llvm;
     30 
     31 STATISTIC(NumDeletes, "Number of dead copies deleted");
     32 
     33 namespace {
     34   class MachineCopyPropagation : public MachineFunctionPass {
     35     const TargetRegisterInfo *TRI;
     36     const TargetInstrInfo *TII;
     37     MachineRegisterInfo *MRI;
     38 
     39   public:
     40     static char ID; // Pass identification, replacement for typeid
     41     MachineCopyPropagation() : MachineFunctionPass(ID) {
     42      initializeMachineCopyPropagationPass(*PassRegistry::getPassRegistry());
     43     }
     44 
     45     virtual bool runOnMachineFunction(MachineFunction &MF);
     46 
     47   private:
     48     typedef SmallVector<unsigned, 4> DestList;
     49     typedef DenseMap<unsigned, DestList> SourceMap;
     50 
     51     void SourceNoLongerAvailable(unsigned Reg,
     52                                  SourceMap &SrcMap,
     53                                  DenseMap<unsigned, MachineInstr*> &AvailCopyMap);
     54     bool CopyPropagateBlock(MachineBasicBlock &MBB);
     55     void removeCopy(MachineInstr *MI);
     56   };
     57 }
     58 char MachineCopyPropagation::ID = 0;
     59 char &llvm::MachineCopyPropagationID = MachineCopyPropagation::ID;
     60 
     61 INITIALIZE_PASS(MachineCopyPropagation, "machine-cp",
     62                 "Machine Copy Propagation Pass", false, false)
     63 
     64 void
     65 MachineCopyPropagation::SourceNoLongerAvailable(unsigned Reg,
     66                               SourceMap &SrcMap,
     67                               DenseMap<unsigned, MachineInstr*> &AvailCopyMap) {
     68   for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
     69     SourceMap::iterator SI = SrcMap.find(*AI);
     70     if (SI != SrcMap.end()) {
     71       const DestList& Defs = SI->second;
     72       for (DestList::const_iterator I = Defs.begin(), E = Defs.end();
     73            I != E; ++I) {
     74         unsigned MappedDef = *I;
     75         // Source of copy is no longer available for propagation.
     76         if (AvailCopyMap.erase(MappedDef)) {
     77           for (MCSubRegIterator SR(MappedDef, TRI); SR.isValid(); ++SR)
     78             AvailCopyMap.erase(*SR);
     79         }
     80       }
     81     }
     82   }
     83 }
     84 
     85 static bool NoInterveningSideEffect(const MachineInstr *CopyMI,
     86                                     const MachineInstr *MI) {
     87   const MachineBasicBlock *MBB = CopyMI->getParent();
     88   if (MI->getParent() != MBB)
     89     return false;
     90   MachineBasicBlock::const_iterator I = CopyMI;
     91   MachineBasicBlock::const_iterator E = MBB->end();
     92   MachineBasicBlock::const_iterator E2 = MI;
     93 
     94   ++I;
     95   while (I != E && I != E2) {
     96     if (I->hasUnmodeledSideEffects() || I->isCall() ||
     97         I->isTerminator())
     98       return false;
     99     ++I;
    100   }
    101   return true;
    102 }
    103 
    104 /// isNopCopy - Return true if the specified copy is really a nop. That is
    105 /// if the source of the copy is the same of the definition of the copy that
    106 /// supplied the source. If the source of the copy is a sub-register than it
    107 /// must check the sub-indices match. e.g.
    108 /// ecx = mov eax
    109 /// al  = mov cl
    110 /// But not
    111 /// ecx = mov eax
    112 /// al  = mov ch
    113 static bool isNopCopy(MachineInstr *CopyMI, unsigned Def, unsigned Src,
    114                       const TargetRegisterInfo *TRI) {
    115   unsigned SrcSrc = CopyMI->getOperand(1).getReg();
    116   if (Def == SrcSrc)
    117     return true;
    118   if (TRI->isSubRegister(SrcSrc, Def)) {
    119     unsigned SrcDef = CopyMI->getOperand(0).getReg();
    120     unsigned SubIdx = TRI->getSubRegIndex(SrcSrc, Def);
    121     if (!SubIdx)
    122       return false;
    123     return SubIdx == TRI->getSubRegIndex(SrcDef, Src);
    124   }
    125 
    126   return false;
    127 }
    128 
    129 // Remove MI from the function because it has been determined it is dead.
    130 // Turn it into a noop KILL instruction if it has super-register liveness
    131 // adjustments.
    132 void MachineCopyPropagation::removeCopy(MachineInstr *MI) {
    133   if (MI->getNumOperands() == 2)
    134     MI->eraseFromParent();
    135   else
    136     MI->setDesc(TII->get(TargetOpcode::KILL));
    137 }
    138 
    139 bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
    140   SmallSetVector<MachineInstr*, 8> MaybeDeadCopies;  // Candidates for deletion
    141   DenseMap<unsigned, MachineInstr*> AvailCopyMap;    // Def -> available copies map
    142   DenseMap<unsigned, MachineInstr*> CopyMap;         // Def -> copies map
    143   SourceMap SrcMap; // Src -> Def map
    144 
    145   bool Changed = false;
    146   for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E; ) {
    147     MachineInstr *MI = &*I;
    148     ++I;
    149 
    150     if (MI->isCopy()) {
    151       unsigned Def = MI->getOperand(0).getReg();
    152       unsigned Src = MI->getOperand(1).getReg();
    153 
    154       if (TargetRegisterInfo::isVirtualRegister(Def) ||
    155           TargetRegisterInfo::isVirtualRegister(Src))
    156         report_fatal_error("MachineCopyPropagation should be run after"
    157                            " register allocation!");
    158 
    159       DenseMap<unsigned, MachineInstr*>::iterator CI = AvailCopyMap.find(Src);
    160       if (CI != AvailCopyMap.end()) {
    161         MachineInstr *CopyMI = CI->second;
    162         if (!MRI->isReserved(Def) &&
    163             (!MRI->isReserved(Src) || NoInterveningSideEffect(CopyMI, MI)) &&
    164             isNopCopy(CopyMI, Def, Src, TRI)) {
    165           // The two copies cancel out and the source of the first copy
    166           // hasn't been overridden, eliminate the second one. e.g.
    167           //  %ECX<def> = COPY %EAX<kill>
    168           //  ... nothing clobbered EAX.
    169           //  %EAX<def> = COPY %ECX
    170           // =>
    171           //  %ECX<def> = COPY %EAX
    172           //
    173           // Also avoid eliminating a copy from reserved registers unless the
    174           // definition is proven not clobbered. e.g.
    175           // %RSP<def> = COPY %RAX
    176           // CALL
    177           // %RAX<def> = COPY %RSP
    178 
    179           // Clear any kills of Def between CopyMI and MI. This extends the
    180           // live range.
    181           for (MachineBasicBlock::iterator I = CopyMI, E = MI; I != E; ++I)
    182             I->clearRegisterKills(Def, TRI);
    183 
    184           removeCopy(MI);
    185           Changed = true;
    186           ++NumDeletes;
    187           continue;
    188         }
    189       }
    190 
    191       // If Src is defined by a previous copy, it cannot be eliminated.
    192       for (MCRegAliasIterator AI(Src, TRI, true); AI.isValid(); ++AI) {
    193         CI = CopyMap.find(*AI);
    194         if (CI != CopyMap.end())
    195           MaybeDeadCopies.remove(CI->second);
    196       }
    197 
    198       // Copy is now a candidate for deletion.
    199       MaybeDeadCopies.insert(MI);
    200 
    201       // If 'Src' is previously source of another copy, then this earlier copy's
    202       // source is no longer available. e.g.
    203       // %xmm9<def> = copy %xmm2
    204       // ...
    205       // %xmm2<def> = copy %xmm0
    206       // ...
    207       // %xmm2<def> = copy %xmm9
    208       SourceNoLongerAvailable(Def, SrcMap, AvailCopyMap);
    209 
    210       // Remember Def is defined by the copy.
    211       // ... Make sure to clear the def maps of aliases first.
    212       for (MCRegAliasIterator AI(Def, TRI, false); AI.isValid(); ++AI) {
    213         CopyMap.erase(*AI);
    214         AvailCopyMap.erase(*AI);
    215       }
    216       for (MCSubRegIterator SR(Def, TRI, /*IncludeSelf=*/true); SR.isValid();
    217            ++SR) {
    218         CopyMap[*SR] = MI;
    219         AvailCopyMap[*SR] = MI;
    220       }
    221 
    222       // Remember source that's copied to Def. Once it's clobbered, then
    223       // it's no longer available for copy propagation.
    224       if (std::find(SrcMap[Src].begin(), SrcMap[Src].end(), Def) ==
    225           SrcMap[Src].end()) {
    226         SrcMap[Src].push_back(Def);
    227       }
    228 
    229       continue;
    230     }
    231 
    232     // Not a copy.
    233     SmallVector<unsigned, 2> Defs;
    234     int RegMaskOpNum = -1;
    235     for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    236       MachineOperand &MO = MI->getOperand(i);
    237       if (MO.isRegMask())
    238         RegMaskOpNum = i;
    239       if (!MO.isReg())
    240         continue;
    241       unsigned Reg = MO.getReg();
    242       if (!Reg)
    243         continue;
    244 
    245       if (TargetRegisterInfo::isVirtualRegister(Reg))
    246         report_fatal_error("MachineCopyPropagation should be run after"
    247                            " register allocation!");
    248 
    249       if (MO.isDef()) {
    250         Defs.push_back(Reg);
    251         continue;
    252       }
    253 
    254       // If 'Reg' is defined by a copy, the copy is no longer a candidate
    255       // for elimination.
    256       for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
    257         DenseMap<unsigned, MachineInstr*>::iterator CI = CopyMap.find(*AI);
    258         if (CI != CopyMap.end())
    259           MaybeDeadCopies.remove(CI->second);
    260       }
    261     }
    262 
    263     // The instruction has a register mask operand which means that it clobbers
    264     // a large set of registers.  It is possible to use the register mask to
    265     // prune the available copies, but treat it like a basic block boundary for
    266     // now.
    267     if (RegMaskOpNum >= 0) {
    268       // Erase any MaybeDeadCopies whose destination register is clobbered.
    269       const MachineOperand &MaskMO = MI->getOperand(RegMaskOpNum);
    270       for (SmallSetVector<MachineInstr*, 8>::iterator
    271            DI = MaybeDeadCopies.begin(), DE = MaybeDeadCopies.end();
    272            DI != DE; ++DI) {
    273         unsigned Reg = (*DI)->getOperand(0).getReg();
    274         if (MRI->isReserved(Reg) || !MaskMO.clobbersPhysReg(Reg))
    275           continue;
    276         removeCopy(*DI);
    277         Changed = true;
    278         ++NumDeletes;
    279       }
    280 
    281       // Clear all data structures as if we were beginning a new basic block.
    282       MaybeDeadCopies.clear();
    283       AvailCopyMap.clear();
    284       CopyMap.clear();
    285       SrcMap.clear();
    286       continue;
    287     }
    288 
    289     for (unsigned i = 0, e = Defs.size(); i != e; ++i) {
    290       unsigned Reg = Defs[i];
    291 
    292       // No longer defined by a copy.
    293       for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
    294         CopyMap.erase(*AI);
    295         AvailCopyMap.erase(*AI);
    296       }
    297 
    298       // If 'Reg' is previously source of a copy, it is no longer available for
    299       // copy propagation.
    300       SourceNoLongerAvailable(Reg, SrcMap, AvailCopyMap);
    301     }
    302   }
    303 
    304   // If MBB doesn't have successors, delete the copies whose defs are not used.
    305   // If MBB does have successors, then conservative assume the defs are live-out
    306   // since we don't want to trust live-in lists.
    307   if (MBB.succ_empty()) {
    308     for (SmallSetVector<MachineInstr*, 8>::iterator
    309            DI = MaybeDeadCopies.begin(), DE = MaybeDeadCopies.end();
    310          DI != DE; ++DI) {
    311       if (!MRI->isReserved((*DI)->getOperand(0).getReg())) {
    312         removeCopy(*DI);
    313         Changed = true;
    314         ++NumDeletes;
    315       }
    316     }
    317   }
    318 
    319   return Changed;
    320 }
    321 
    322 bool MachineCopyPropagation::runOnMachineFunction(MachineFunction &MF) {
    323   bool Changed = false;
    324 
    325   TRI = MF.getTarget().getRegisterInfo();
    326   TII = MF.getTarget().getInstrInfo();
    327   MRI = &MF.getRegInfo();
    328 
    329   for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
    330     Changed |= CopyPropagateBlock(*I);
    331 
    332   return Changed;
    333 }
    334