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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/Pass.h"
     17 #include "llvm/CodeGen/MachineFunction.h"
     18 #include "llvm/CodeGen/MachineFunctionPass.h"
     19 #include "llvm/Target/TargetRegisterInfo.h"
     20 #include "llvm/Support/Debug.h"
     21 #include "llvm/Support/ErrorHandling.h"
     22 #include "llvm/Support/raw_ostream.h"
     23 #include "llvm/ADT/BitVector.h"
     24 #include "llvm/ADT/DenseMap.h"
     25 #include "llvm/ADT/SetVector.h"
     26 #include "llvm/ADT/SmallVector.h"
     27 #include "llvm/ADT/Statistic.h"
     28 using namespace llvm;
     29 
     30 STATISTIC(NumDeletes, "Number of dead copies deleted");
     31 
     32 namespace {
     33   class MachineCopyPropagation : public MachineFunctionPass {
     34     const TargetRegisterInfo *TRI;
     35     BitVector ReservedRegs;
     36 
     37   public:
     38     static char ID; // Pass identification, replacement for typeid
     39     MachineCopyPropagation() : MachineFunctionPass(ID) {
     40      initializeMachineCopyPropagationPass(*PassRegistry::getPassRegistry());
     41     }
     42 
     43     virtual bool runOnMachineFunction(MachineFunction &MF);
     44 
     45   private:
     46     typedef SmallVector<unsigned, 4> DestList;
     47     typedef DenseMap<unsigned, DestList> SourceMap;
     48 
     49     void SourceNoLongerAvailable(unsigned Reg,
     50                                  SourceMap &SrcMap,
     51                                  DenseMap<unsigned, MachineInstr*> &AvailCopyMap);
     52     bool CopyPropagateBlock(MachineBasicBlock &MBB);
     53   };
     54 }
     55 char MachineCopyPropagation::ID = 0;
     56 char &llvm::MachineCopyPropagationID = MachineCopyPropagation::ID;
     57 
     58 INITIALIZE_PASS(MachineCopyPropagation, "machine-cp",
     59                 "Machine Copy Propagation Pass", false, false)
     60 
     61 void
     62 MachineCopyPropagation::SourceNoLongerAvailable(unsigned Reg,
     63                               SourceMap &SrcMap,
     64                               DenseMap<unsigned, MachineInstr*> &AvailCopyMap) {
     65   SourceMap::iterator SI = SrcMap.find(Reg);
     66   if (SI != SrcMap.end()) {
     67     const DestList& Defs = SI->second;
     68     for (DestList::const_iterator I = Defs.begin(), E = Defs.end();
     69          I != E; ++I) {
     70       unsigned MappedDef = *I;
     71       // Source of copy is no longer available for propagation.
     72       if (AvailCopyMap.erase(MappedDef)) {
     73         for (const uint16_t *SR = TRI->getSubRegisters(MappedDef); *SR; ++SR)
     74           AvailCopyMap.erase(*SR);
     75       }
     76     }
     77   }
     78   for (const uint16_t *AS = TRI->getAliasSet(Reg); *AS; ++AS) {
     79     SI = SrcMap.find(*AS);
     80     if (SI != SrcMap.end()) {
     81       const DestList& Defs = SI->second;
     82       for (DestList::const_iterator I = Defs.begin(), E = Defs.end();
     83            I != E; ++I) {
     84         unsigned MappedDef = *I;
     85         if (AvailCopyMap.erase(MappedDef)) {
     86           for (const uint16_t *SR = TRI->getSubRegisters(MappedDef); *SR; ++SR)
     87             AvailCopyMap.erase(*SR);
     88         }
     89       }
     90     }
     91   }
     92 }
     93 
     94 static bool NoInterveningSideEffect(const MachineInstr *CopyMI,
     95                                     const MachineInstr *MI) {
     96   const MachineBasicBlock *MBB = CopyMI->getParent();
     97   if (MI->getParent() != MBB)
     98     return false;
     99   MachineBasicBlock::const_iterator I = CopyMI;
    100   MachineBasicBlock::const_iterator E = MBB->end();
    101   MachineBasicBlock::const_iterator E2 = MI;
    102 
    103   ++I;
    104   while (I != E && I != E2) {
    105     if (I->hasUnmodeledSideEffects() || I->isCall() ||
    106         I->isTerminator())
    107       return false;
    108     ++I;
    109   }
    110   return true;
    111 }
    112 
    113 /// isNopCopy - Return true if the specified copy is really a nop. That is
    114 /// if the source of the copy is the same of the definition of the copy that
    115 /// supplied the source. If the source of the copy is a sub-register than it
    116 /// must check the sub-indices match. e.g.
    117 /// ecx = mov eax
    118 /// al  = mov cl
    119 /// But not
    120 /// ecx = mov eax
    121 /// al  = mov ch
    122 static bool isNopCopy(MachineInstr *CopyMI, unsigned Def, unsigned Src,
    123                       const TargetRegisterInfo *TRI) {
    124   unsigned SrcSrc = CopyMI->getOperand(1).getReg();
    125   if (Def == SrcSrc)
    126     return true;
    127   if (TRI->isSubRegister(SrcSrc, Def)) {
    128     unsigned SrcDef = CopyMI->getOperand(0).getReg();
    129     unsigned SubIdx = TRI->getSubRegIndex(SrcSrc, Def);
    130     if (!SubIdx)
    131       return false;
    132     return SubIdx == TRI->getSubRegIndex(SrcDef, Src);
    133   }
    134 
    135   return false;
    136 }
    137 
    138 bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
    139   SmallSetVector<MachineInstr*, 8> MaybeDeadCopies;  // Candidates for deletion
    140   DenseMap<unsigned, MachineInstr*> AvailCopyMap;    // Def -> available copies map
    141   DenseMap<unsigned, MachineInstr*> CopyMap;         // Def -> copies map
    142   SourceMap SrcMap; // Src -> Def map
    143 
    144   bool Changed = false;
    145   for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E; ) {
    146     MachineInstr *MI = &*I;
    147     ++I;
    148 
    149     if (MI->isCopy()) {
    150       unsigned Def = MI->getOperand(0).getReg();
    151       unsigned Src = MI->getOperand(1).getReg();
    152 
    153       if (TargetRegisterInfo::isVirtualRegister(Def) ||
    154           TargetRegisterInfo::isVirtualRegister(Src))
    155         report_fatal_error("MachineCopyPropagation should be run after"
    156                            " register allocation!");
    157 
    158       DenseMap<unsigned, MachineInstr*>::iterator CI = AvailCopyMap.find(Src);
    159       if (CI != AvailCopyMap.end()) {
    160         MachineInstr *CopyMI = CI->second;
    161         if (!ReservedRegs.test(Def) &&
    162             (!ReservedRegs.test(Src) || NoInterveningSideEffect(CopyMI, MI)) &&
    163             isNopCopy(CopyMI, Def, Src, TRI)) {
    164           // The two copies cancel out and the source of the first copy
    165           // hasn't been overridden, eliminate the second one. e.g.
    166           //  %ECX<def> = COPY %EAX<kill>
    167           //  ... nothing clobbered EAX.
    168           //  %EAX<def> = COPY %ECX
    169           // =>
    170           //  %ECX<def> = COPY %EAX
    171           //
    172           // Also avoid eliminating a copy from reserved registers unless the
    173           // definition is proven not clobbered. e.g.
    174           // %RSP<def> = COPY %RAX
    175           // CALL
    176           // %RAX<def> = COPY %RSP
    177 
    178           // Clear any kills of Def between CopyMI and MI. This extends the
    179           // live range.
    180           for (MachineBasicBlock::iterator I = CopyMI, E = MI; I != E; ++I)
    181             I->clearRegisterKills(Def, TRI);
    182 
    183           MI->eraseFromParent();
    184           Changed = true;
    185           ++NumDeletes;
    186           continue;
    187         }
    188       }
    189 
    190       // If Src is defined by a previous copy, it cannot be eliminated.
    191       CI = CopyMap.find(Src);
    192       if (CI != CopyMap.end())
    193         MaybeDeadCopies.remove(CI->second);
    194       for (const uint16_t *AS = TRI->getAliasSet(Src); *AS; ++AS) {
    195         CI = CopyMap.find(*AS);
    196         if (CI != CopyMap.end())
    197           MaybeDeadCopies.remove(CI->second);
    198       }
    199 
    200       // Copy is now a candidate for deletion.
    201       MaybeDeadCopies.insert(MI);
    202 
    203       // If 'Src' is previously source of another copy, then this earlier copy's
    204       // source is no longer available. e.g.
    205       // %xmm9<def> = copy %xmm2
    206       // ...
    207       // %xmm2<def> = copy %xmm0
    208       // ...
    209       // %xmm2<def> = copy %xmm9
    210       SourceNoLongerAvailable(Def, SrcMap, AvailCopyMap);
    211 
    212       // Remember Def is defined by the copy.
    213       // ... Make sure to clear the def maps of aliases first.
    214       for (const uint16_t *AS = TRI->getAliasSet(Def); *AS; ++AS) {
    215         CopyMap.erase(*AS);
    216         AvailCopyMap.erase(*AS);
    217       }
    218       CopyMap[Def] = MI;
    219       AvailCopyMap[Def] = MI;
    220       for (const uint16_t *SR = TRI->getSubRegisters(Def); *SR; ++SR) {
    221         CopyMap[*SR] = MI;
    222         AvailCopyMap[*SR] = MI;
    223       }
    224 
    225       // Remember source that's copied to Def. Once it's clobbered, then
    226       // it's no longer available for copy propagation.
    227       if (std::find(SrcMap[Src].begin(), SrcMap[Src].end(), Def) ==
    228           SrcMap[Src].end()) {
    229         SrcMap[Src].push_back(Def);
    230       }
    231 
    232       continue;
    233     }
    234 
    235     // Not a copy.
    236     SmallVector<unsigned, 2> Defs;
    237     int RegMaskOpNum = -1;
    238     for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    239       MachineOperand &MO = MI->getOperand(i);
    240       if (MO.isRegMask())
    241         RegMaskOpNum = i;
    242       if (!MO.isReg())
    243         continue;
    244       unsigned Reg = MO.getReg();
    245       if (!Reg)
    246         continue;
    247 
    248       if (TargetRegisterInfo::isVirtualRegister(Reg))
    249         report_fatal_error("MachineCopyPropagation should be run after"
    250                            " register allocation!");
    251 
    252       if (MO.isDef()) {
    253         Defs.push_back(Reg);
    254         continue;
    255       }
    256 
    257       // If 'Reg' is defined by a copy, the copy is no longer a candidate
    258       // for elimination.
    259       DenseMap<unsigned, MachineInstr*>::iterator CI = CopyMap.find(Reg);
    260       if (CI != CopyMap.end())
    261         MaybeDeadCopies.remove(CI->second);
    262       for (const uint16_t *AS = TRI->getAliasSet(Reg); *AS; ++AS) {
    263         CI = CopyMap.find(*AS);
    264         if (CI != CopyMap.end())
    265           MaybeDeadCopies.remove(CI->second);
    266       }
    267     }
    268 
    269     // The instruction has a register mask operand which means that it clobbers
    270     // a large set of registers.  It is possible to use the register mask to
    271     // prune the available copies, but treat it like a basic block boundary for
    272     // now.
    273     if (RegMaskOpNum >= 0) {
    274       // Erase any MaybeDeadCopies whose destination register is clobbered.
    275       const MachineOperand &MaskMO = MI->getOperand(RegMaskOpNum);
    276       for (SmallSetVector<MachineInstr*, 8>::iterator
    277            DI = MaybeDeadCopies.begin(), DE = MaybeDeadCopies.end();
    278            DI != DE; ++DI) {
    279         unsigned Reg = (*DI)->getOperand(0).getReg();
    280         if (ReservedRegs.test(Reg) || !MaskMO.clobbersPhysReg(Reg))
    281           continue;
    282         (*DI)->eraseFromParent();
    283         Changed = true;
    284         ++NumDeletes;
    285       }
    286 
    287       // Clear all data structures as if we were beginning a new basic block.
    288       MaybeDeadCopies.clear();
    289       AvailCopyMap.clear();
    290       CopyMap.clear();
    291       SrcMap.clear();
    292       continue;
    293     }
    294 
    295     for (unsigned i = 0, e = Defs.size(); i != e; ++i) {
    296       unsigned Reg = Defs[i];
    297 
    298       // No longer defined by a copy.
    299       CopyMap.erase(Reg);
    300       AvailCopyMap.erase(Reg);
    301       for (const uint16_t *AS = TRI->getAliasSet(Reg); *AS; ++AS) {
    302         CopyMap.erase(*AS);
    303         AvailCopyMap.erase(*AS);
    304       }
    305 
    306       // If 'Reg' is previously source of a copy, it is no longer available for
    307       // copy propagation.
    308       SourceNoLongerAvailable(Reg, SrcMap, AvailCopyMap);
    309     }
    310   }
    311 
    312   // If MBB doesn't have successors, delete the copies whose defs are not used.
    313   // If MBB does have successors, then conservative assume the defs are live-out
    314   // since we don't want to trust live-in lists.
    315   if (MBB.succ_empty()) {
    316     for (SmallSetVector<MachineInstr*, 8>::iterator
    317            DI = MaybeDeadCopies.begin(), DE = MaybeDeadCopies.end();
    318          DI != DE; ++DI) {
    319       if (!ReservedRegs.test((*DI)->getOperand(0).getReg())) {
    320         (*DI)->eraseFromParent();
    321         Changed = true;
    322         ++NumDeletes;
    323       }
    324     }
    325   }
    326 
    327   return Changed;
    328 }
    329 
    330 bool MachineCopyPropagation::runOnMachineFunction(MachineFunction &MF) {
    331   bool Changed = false;
    332 
    333   TRI = MF.getTarget().getRegisterInfo();
    334   ReservedRegs = TRI->getReservedRegs(MF);
    335 
    336   for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
    337     Changed |= CopyPropagateBlock(*I);
    338 
    339   return Changed;
    340 }
    341