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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 #include "llvm/CodeGen/Passes.h"
     15 #include "llvm/ADT/DenseMap.h"
     16 #include "llvm/ADT/SetVector.h"
     17 #include "llvm/ADT/SmallVector.h"
     18 #include "llvm/ADT/Statistic.h"
     19 #include "llvm/CodeGen/MachineFunction.h"
     20 #include "llvm/CodeGen/MachineFunctionPass.h"
     21 #include "llvm/CodeGen/MachineRegisterInfo.h"
     22 #include "llvm/Pass.h"
     23 #include "llvm/Support/Debug.h"
     24 #include "llvm/Support/ErrorHandling.h"
     25 #include "llvm/Support/raw_ostream.h"
     26 #include "llvm/Target/TargetInstrInfo.h"
     27 #include "llvm/Target/TargetRegisterInfo.h"
     28 using namespace llvm;
     29 
     30 #define DEBUG_TYPE "codegen-cp"
     31 
     32 STATISTIC(NumDeletes, "Number of dead copies deleted");
     33 
     34 namespace {
     35   class MachineCopyPropagation : public MachineFunctionPass {
     36     const TargetRegisterInfo *TRI;
     37     const TargetInstrInfo *TII;
     38     MachineRegisterInfo *MRI;
     39 
     40   public:
     41     static char ID; // Pass identification, replacement for typeid
     42     MachineCopyPropagation() : MachineFunctionPass(ID) {
     43      initializeMachineCopyPropagationPass(*PassRegistry::getPassRegistry());
     44     }
     45 
     46     bool runOnMachineFunction(MachineFunction &MF) override;
     47 
     48   private:
     49     typedef SmallVector<unsigned, 4> DestList;
     50     typedef DenseMap<unsigned, DestList> SourceMap;
     51 
     52     void SourceNoLongerAvailable(unsigned Reg,
     53                                  SourceMap &SrcMap,
     54                                  DenseMap<unsigned, MachineInstr*> &AvailCopyMap);
     55     bool CopyPropagateBlock(MachineBasicBlock &MBB);
     56     void removeCopy(MachineInstr *MI);
     57   };
     58 }
     59 char MachineCopyPropagation::ID = 0;
     60 char &llvm::MachineCopyPropagationID = MachineCopyPropagation::ID;
     61 
     62 INITIALIZE_PASS(MachineCopyPropagation, "machine-cp",
     63                 "Machine Copy Propagation Pass", false, false)
     64 
     65 void
     66 MachineCopyPropagation::SourceNoLongerAvailable(unsigned Reg,
     67                               SourceMap &SrcMap,
     68                               DenseMap<unsigned, MachineInstr*> &AvailCopyMap) {
     69   for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
     70     SourceMap::iterator SI = SrcMap.find(*AI);
     71     if (SI != SrcMap.end()) {
     72       const DestList& Defs = SI->second;
     73       for (DestList::const_iterator I = Defs.begin(), E = Defs.end();
     74            I != E; ++I) {
     75         unsigned MappedDef = *I;
     76         // Source of copy is no longer available for propagation.
     77         if (AvailCopyMap.erase(MappedDef)) {
     78           for (MCSubRegIterator SR(MappedDef, TRI); SR.isValid(); ++SR)
     79             AvailCopyMap.erase(*SR);
     80         }
     81       }
     82     }
     83   }
     84 }
     85 
     86 static bool NoInterveningSideEffect(const MachineInstr *CopyMI,
     87                                     const MachineInstr *MI) {
     88   const MachineBasicBlock *MBB = CopyMI->getParent();
     89   if (MI->getParent() != MBB)
     90     return false;
     91   MachineBasicBlock::const_iterator I = CopyMI;
     92   MachineBasicBlock::const_iterator E = MBB->end();
     93   MachineBasicBlock::const_iterator E2 = MI;
     94 
     95   ++I;
     96   while (I != E && I != E2) {
     97     if (I->hasUnmodeledSideEffects() || I->isCall() ||
     98         I->isTerminator())
     99       return false;
    100     ++I;
    101   }
    102   return true;
    103 }
    104 
    105 /// isNopCopy - Return true if the specified copy is really a nop. That is
    106 /// if the source of the copy is the same of the definition of the copy that
    107 /// supplied the source. If the source of the copy is a sub-register than it
    108 /// must check the sub-indices match. e.g.
    109 /// ecx = mov eax
    110 /// al  = mov cl
    111 /// But not
    112 /// ecx = mov eax
    113 /// al  = mov ch
    114 static bool isNopCopy(MachineInstr *CopyMI, unsigned Def, unsigned Src,
    115                       const TargetRegisterInfo *TRI) {
    116   unsigned SrcSrc = CopyMI->getOperand(1).getReg();
    117   if (Def == SrcSrc)
    118     return true;
    119   if (TRI->isSubRegister(SrcSrc, Def)) {
    120     unsigned SrcDef = CopyMI->getOperand(0).getReg();
    121     unsigned SubIdx = TRI->getSubRegIndex(SrcSrc, Def);
    122     if (!SubIdx)
    123       return false;
    124     return SubIdx == TRI->getSubRegIndex(SrcDef, Src);
    125   }
    126 
    127   return false;
    128 }
    129 
    130 // Remove MI from the function because it has been determined it is dead.
    131 // Turn it into a noop KILL instruction as opposed to removing it to
    132 // maintain imp-use/imp-def chains.
    133 void MachineCopyPropagation::removeCopy(MachineInstr *MI) {
    134   MI->setDesc(TII->get(TargetOpcode::KILL));
    135 }
    136 
    137 bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
    138   SmallSetVector<MachineInstr*, 8> MaybeDeadCopies;  // Candidates for deletion
    139   DenseMap<unsigned, MachineInstr*> AvailCopyMap;    // Def -> available copies map
    140   DenseMap<unsigned, MachineInstr*> CopyMap;         // Def -> copies map
    141   SourceMap SrcMap; // Src -> Def map
    142 
    143   DEBUG(dbgs() << "MCP: CopyPropagateBlock " << MBB.getName() << "\n");
    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           DEBUG(dbgs() << "MCP: copy is a NOP, removing: "; MI->dump());
    180 
    181           // Clear any kills of Def between CopyMI and MI. This extends the
    182           // live range.
    183           for (MachineBasicBlock::iterator I = CopyMI, E = MI; I != E; ++I)
    184             I->clearRegisterKills(Def, TRI);
    185 
    186           removeCopy(MI);
    187           Changed = true;
    188           ++NumDeletes;
    189           continue;
    190         }
    191       }
    192 
    193       // If Src is defined by a previous copy, it cannot be eliminated.
    194       for (MCRegAliasIterator AI(Src, TRI, true); AI.isValid(); ++AI) {
    195         CI = CopyMap.find(*AI);
    196         if (CI != CopyMap.end()) {
    197           DEBUG(dbgs() << "MCP: Copy is no longer dead: "; CI->second->dump());
    198           MaybeDeadCopies.remove(CI->second);
    199         }
    200       }
    201 
    202       DEBUG(dbgs() << "MCP: Copy is a deletion candidate: "; MI->dump());
    203 
    204       // Copy is now a candidate for deletion.
    205       MaybeDeadCopies.insert(MI);
    206 
    207       // If 'Src' is previously source of another copy, then this earlier copy's
    208       // source is no longer available. e.g.
    209       // %xmm9<def> = copy %xmm2
    210       // ...
    211       // %xmm2<def> = copy %xmm0
    212       // ...
    213       // %xmm2<def> = copy %xmm9
    214       SourceNoLongerAvailable(Def, SrcMap, AvailCopyMap);
    215 
    216       // Remember Def is defined by the copy.
    217       // ... Make sure to clear the def maps of aliases first.
    218       for (MCRegAliasIterator AI(Def, TRI, false); AI.isValid(); ++AI) {
    219         CopyMap.erase(*AI);
    220         AvailCopyMap.erase(*AI);
    221       }
    222       for (MCSubRegIterator SR(Def, TRI, /*IncludeSelf=*/true); SR.isValid();
    223            ++SR) {
    224         CopyMap[*SR] = MI;
    225         AvailCopyMap[*SR] = MI;
    226       }
    227 
    228       // Remember source that's copied to Def. Once it's clobbered, then
    229       // it's no longer available for copy propagation.
    230       if (std::find(SrcMap[Src].begin(), SrcMap[Src].end(), Def) ==
    231           SrcMap[Src].end()) {
    232         SrcMap[Src].push_back(Def);
    233       }
    234 
    235       continue;
    236     }
    237 
    238     // Not a copy.
    239     SmallVector<unsigned, 2> Defs;
    240     int RegMaskOpNum = -1;
    241     for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    242       MachineOperand &MO = MI->getOperand(i);
    243       if (MO.isRegMask())
    244         RegMaskOpNum = i;
    245       if (!MO.isReg())
    246         continue;
    247       unsigned Reg = MO.getReg();
    248       if (!Reg)
    249         continue;
    250 
    251       if (TargetRegisterInfo::isVirtualRegister(Reg))
    252         report_fatal_error("MachineCopyPropagation should be run after"
    253                            " register allocation!");
    254 
    255       if (MO.isDef()) {
    256         Defs.push_back(Reg);
    257         continue;
    258       }
    259 
    260       // If 'Reg' is defined by a copy, the copy is no longer a candidate
    261       // for elimination.
    262       for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
    263         DenseMap<unsigned, MachineInstr*>::iterator CI = CopyMap.find(*AI);
    264         if (CI != CopyMap.end()) {
    265           DEBUG(dbgs() << "MCP: Copy is used - not dead: "; CI->second->dump());
    266           MaybeDeadCopies.remove(CI->second);
    267         }
    268       }
    269     }
    270 
    271     // The instruction has a register mask operand which means that it clobbers
    272     // a large set of registers.  It is possible to use the register mask to
    273     // prune the available copies, but treat it like a basic block boundary for
    274     // now.
    275     if (RegMaskOpNum >= 0) {
    276       // Erase any MaybeDeadCopies whose destination register is clobbered.
    277       const MachineOperand &MaskMO = MI->getOperand(RegMaskOpNum);
    278       for (SmallSetVector<MachineInstr*, 8>::iterator
    279            DI = MaybeDeadCopies.begin(), DE = MaybeDeadCopies.end();
    280            DI != DE; ++DI) {
    281         unsigned Reg = (*DI)->getOperand(0).getReg();
    282         if (MRI->isReserved(Reg) || !MaskMO.clobbersPhysReg(Reg))
    283           continue;
    284         DEBUG(dbgs() << "MCP: Removing copy due to regmask clobbering: ";
    285               (*DI)->dump());
    286         removeCopy(*DI);
    287         Changed = true;
    288         ++NumDeletes;
    289       }
    290 
    291       // Clear all data structures as if we were beginning a new basic block.
    292       MaybeDeadCopies.clear();
    293       AvailCopyMap.clear();
    294       CopyMap.clear();
    295       SrcMap.clear();
    296       continue;
    297     }
    298 
    299     for (unsigned i = 0, e = Defs.size(); i != e; ++i) {
    300       unsigned Reg = Defs[i];
    301 
    302       // No longer defined by a copy.
    303       for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
    304         CopyMap.erase(*AI);
    305         AvailCopyMap.erase(*AI);
    306       }
    307 
    308       // If 'Reg' is previously source of a copy, it is no longer available for
    309       // copy propagation.
    310       SourceNoLongerAvailable(Reg, SrcMap, AvailCopyMap);
    311     }
    312   }
    313 
    314   // If MBB doesn't have successors, delete the copies whose defs are not used.
    315   // If MBB does have successors, then conservative assume the defs are live-out
    316   // since we don't want to trust live-in lists.
    317   if (MBB.succ_empty()) {
    318     for (SmallSetVector<MachineInstr*, 8>::iterator
    319            DI = MaybeDeadCopies.begin(), DE = MaybeDeadCopies.end();
    320          DI != DE; ++DI) {
    321       if (!MRI->isReserved((*DI)->getOperand(0).getReg())) {
    322         removeCopy(*DI);
    323         Changed = true;
    324         ++NumDeletes;
    325       }
    326     }
    327   }
    328 
    329   return Changed;
    330 }
    331 
    332 bool MachineCopyPropagation::runOnMachineFunction(MachineFunction &MF) {
    333   if (skipOptnoneFunction(*MF.getFunction()))
    334     return false;
    335 
    336   bool Changed = false;
    337 
    338   TRI = MF.getTarget().getRegisterInfo();
    339   TII = MF.getTarget().getInstrInfo();
    340   MRI = &MF.getRegInfo();
    341 
    342   for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
    343     Changed |= CopyPropagateBlock(*I);
    344 
    345   return Changed;
    346 }
    347