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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 #include "llvm/Target/TargetSubtargetInfo.h"
     29 using namespace llvm;
     30 
     31 #define DEBUG_TYPE "codegen-cp"
     32 
     33 STATISTIC(NumDeletes, "Number of dead copies deleted");
     34 
     35 namespace {
     36   class MachineCopyPropagation : public MachineFunctionPass {
     37     const TargetRegisterInfo *TRI;
     38     const TargetInstrInfo *TII;
     39     MachineRegisterInfo *MRI;
     40 
     41   public:
     42     static char ID; // Pass identification, replacement for typeid
     43     MachineCopyPropagation() : MachineFunctionPass(ID) {
     44      initializeMachineCopyPropagationPass(*PassRegistry::getPassRegistry());
     45     }
     46 
     47     bool runOnMachineFunction(MachineFunction &MF) override;
     48 
     49   private:
     50     typedef SmallVector<unsigned, 4> DestList;
     51     typedef DenseMap<unsigned, DestList> SourceMap;
     52 
     53     void SourceNoLongerAvailable(unsigned Reg,
     54                                  SourceMap &SrcMap,
     55                                  DenseMap<unsigned, MachineInstr*> &AvailCopyMap);
     56     bool CopyPropagateBlock(MachineBasicBlock &MBB);
     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         AvailCopyMap.erase(MappedDef);
     78         for (MCSubRegIterator SR(MappedDef, TRI); SR.isValid(); ++SR)
     79           AvailCopyMap.erase(*SR);
     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 bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
    130   SmallSetVector<MachineInstr*, 8> MaybeDeadCopies;  // Candidates for deletion
    131   DenseMap<unsigned, MachineInstr*> AvailCopyMap;    // Def -> available copies map
    132   DenseMap<unsigned, MachineInstr*> CopyMap;         // Def -> copies map
    133   SourceMap SrcMap; // Src -> Def map
    134 
    135   DEBUG(dbgs() << "MCP: CopyPropagateBlock " << MBB.getName() << "\n");
    136 
    137   bool Changed = false;
    138   for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E; ) {
    139     MachineInstr *MI = &*I;
    140     ++I;
    141 
    142     if (MI->isCopy()) {
    143       unsigned Def = MI->getOperand(0).getReg();
    144       unsigned Src = MI->getOperand(1).getReg();
    145 
    146       if (TargetRegisterInfo::isVirtualRegister(Def) ||
    147           TargetRegisterInfo::isVirtualRegister(Src))
    148         report_fatal_error("MachineCopyPropagation should be run after"
    149                            " register allocation!");
    150 
    151       DenseMap<unsigned, MachineInstr*>::iterator CI = AvailCopyMap.find(Src);
    152       if (CI != AvailCopyMap.end()) {
    153         MachineInstr *CopyMI = CI->second;
    154         if (!MRI->isReserved(Def) &&
    155             (!MRI->isReserved(Src) || NoInterveningSideEffect(CopyMI, MI)) &&
    156             isNopCopy(CopyMI, Def, Src, TRI)) {
    157           // The two copies cancel out and the source of the first copy
    158           // hasn't been overridden, eliminate the second one. e.g.
    159           //  %ECX<def> = COPY %EAX<kill>
    160           //  ... nothing clobbered EAX.
    161           //  %EAX<def> = COPY %ECX
    162           // =>
    163           //  %ECX<def> = COPY %EAX
    164           //
    165           // Also avoid eliminating a copy from reserved registers unless the
    166           // definition is proven not clobbered. e.g.
    167           // %RSP<def> = COPY %RAX
    168           // CALL
    169           // %RAX<def> = COPY %RSP
    170 
    171           DEBUG(dbgs() << "MCP: copy is a NOP, removing: "; MI->dump());
    172 
    173           // Clear any kills of Def between CopyMI and MI. This extends the
    174           // live range.
    175           for (MachineBasicBlock::iterator I = CopyMI, E = MI; I != E; ++I)
    176             I->clearRegisterKills(Def, TRI);
    177 
    178           MI->eraseFromParent();
    179           Changed = true;
    180           ++NumDeletes;
    181           continue;
    182         }
    183       }
    184 
    185       // If Src is defined by a previous copy, it cannot be eliminated.
    186       for (MCRegAliasIterator AI(Src, TRI, true); AI.isValid(); ++AI) {
    187         CI = CopyMap.find(*AI);
    188         if (CI != CopyMap.end()) {
    189           DEBUG(dbgs() << "MCP: Copy is no longer dead: "; CI->second->dump());
    190           MaybeDeadCopies.remove(CI->second);
    191         }
    192       }
    193 
    194       DEBUG(dbgs() << "MCP: Copy is a deletion candidate: "; MI->dump());
    195 
    196       // Copy is now a candidate for deletion.
    197       MaybeDeadCopies.insert(MI);
    198 
    199       // If 'Src' is previously source of another copy, then this earlier copy's
    200       // source is no longer available. e.g.
    201       // %xmm9<def> = copy %xmm2
    202       // ...
    203       // %xmm2<def> = copy %xmm0
    204       // ...
    205       // %xmm2<def> = copy %xmm9
    206       SourceNoLongerAvailable(Def, SrcMap, AvailCopyMap);
    207 
    208       // Remember Def is defined by the copy.
    209       // ... Make sure to clear the def maps of aliases first.
    210       for (MCRegAliasIterator AI(Def, TRI, false); AI.isValid(); ++AI) {
    211         CopyMap.erase(*AI);
    212         AvailCopyMap.erase(*AI);
    213       }
    214       for (MCSubRegIterator SR(Def, TRI, /*IncludeSelf=*/true); SR.isValid();
    215            ++SR) {
    216         CopyMap[*SR] = MI;
    217         AvailCopyMap[*SR] = MI;
    218       }
    219 
    220       // Remember source that's copied to Def. Once it's clobbered, then
    221       // it's no longer available for copy propagation.
    222       if (std::find(SrcMap[Src].begin(), SrcMap[Src].end(), Def) ==
    223           SrcMap[Src].end()) {
    224         SrcMap[Src].push_back(Def);
    225       }
    226 
    227       continue;
    228     }
    229 
    230     // Not a copy.
    231     SmallVector<unsigned, 2> Defs;
    232     int RegMaskOpNum = -1;
    233     for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    234       MachineOperand &MO = MI->getOperand(i);
    235       if (MO.isRegMask())
    236         RegMaskOpNum = i;
    237       if (!MO.isReg())
    238         continue;
    239       unsigned Reg = MO.getReg();
    240       if (!Reg)
    241         continue;
    242 
    243       if (TargetRegisterInfo::isVirtualRegister(Reg))
    244         report_fatal_error("MachineCopyPropagation should be run after"
    245                            " register allocation!");
    246 
    247       if (MO.isDef()) {
    248         Defs.push_back(Reg);
    249         continue;
    250       }
    251 
    252       // If 'Reg' is defined by a copy, the copy is no longer a candidate
    253       // for elimination.
    254       for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
    255         DenseMap<unsigned, MachineInstr*>::iterator CI = CopyMap.find(*AI);
    256         if (CI != CopyMap.end()) {
    257           DEBUG(dbgs() << "MCP: Copy is used - not dead: "; CI->second->dump());
    258           MaybeDeadCopies.remove(CI->second);
    259         }
    260       }
    261       // Treat undef use like defs for copy propagation but not for
    262       // dead copy. We would need to do a liveness check to be sure the copy
    263       // is dead for undef uses.
    264       // The backends are allowed to do whatever they want with undef value
    265       // and we cannot be sure this register will not be rewritten to break
    266       // some false dependencies for the hardware for instance.
    267       if (MO.isUndef())
    268         Defs.push_back(Reg);
    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         (*DI)->eraseFromParent();
    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         (*DI)->eraseFromParent();
    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.getSubtarget().getRegisterInfo();
    339   TII = MF.getSubtarget().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