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/raw_ostream.h" 25 #include "llvm/Target/TargetInstrInfo.h" 26 #include "llvm/Target/TargetRegisterInfo.h" 27 #include "llvm/Target/TargetSubtargetInfo.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 }; 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 AvailCopyMap.erase(MappedDef); 77 for (MCSubRegIterator SR(MappedDef, TRI); SR.isValid(); ++SR) 78 AvailCopyMap.erase(*SR); 79 } 80 } 81 } 82 } 83 84 static bool NoInterveningSideEffect(const MachineInstr *CopyMI, 85 const MachineInstr *MI) { 86 const MachineBasicBlock *MBB = CopyMI->getParent(); 87 if (MI->getParent() != MBB) 88 return false; 89 MachineBasicBlock::const_iterator I = CopyMI; 90 MachineBasicBlock::const_iterator E = MBB->end(); 91 MachineBasicBlock::const_iterator E2 = MI; 92 93 ++I; 94 while (I != E && I != E2) { 95 if (I->hasUnmodeledSideEffects() || I->isCall() || 96 I->isTerminator()) 97 return false; 98 ++I; 99 } 100 return true; 101 } 102 103 /// isNopCopy - Return true if the specified copy is really a nop. That is 104 /// if the source of the copy is the same of the definition of the copy that 105 /// supplied the source. If the source of the copy is a sub-register than it 106 /// must check the sub-indices match. e.g. 107 /// ecx = mov eax 108 /// al = mov cl 109 /// But not 110 /// ecx = mov eax 111 /// al = mov ch 112 static bool isNopCopy(MachineInstr *CopyMI, unsigned Def, unsigned Src, 113 const TargetRegisterInfo *TRI) { 114 unsigned SrcSrc = CopyMI->getOperand(1).getReg(); 115 if (Def == SrcSrc) 116 return true; 117 if (TRI->isSubRegister(SrcSrc, Def)) { 118 unsigned SrcDef = CopyMI->getOperand(0).getReg(); 119 unsigned SubIdx = TRI->getSubRegIndex(SrcSrc, Def); 120 if (!SubIdx) 121 return false; 122 return SubIdx == TRI->getSubRegIndex(SrcDef, Src); 123 } 124 125 return false; 126 } 127 128 bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) { 129 SmallSetVector<MachineInstr*, 8> MaybeDeadCopies; // Candidates for deletion 130 DenseMap<unsigned, MachineInstr*> AvailCopyMap; // Def -> available copies map 131 DenseMap<unsigned, MachineInstr*> CopyMap; // Def -> copies map 132 SourceMap SrcMap; // Src -> Def map 133 134 DEBUG(dbgs() << "MCP: CopyPropagateBlock " << MBB.getName() << "\n"); 135 136 bool Changed = false; 137 for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E; ) { 138 MachineInstr *MI = &*I; 139 ++I; 140 141 if (MI->isCopy()) { 142 unsigned Def = MI->getOperand(0).getReg(); 143 unsigned Src = MI->getOperand(1).getReg(); 144 145 assert(!TargetRegisterInfo::isVirtualRegister(Def) && 146 !TargetRegisterInfo::isVirtualRegister(Src) && 147 "MachineCopyPropagation should be run after register allocation!"); 148 149 DenseMap<unsigned, MachineInstr*>::iterator CI = AvailCopyMap.find(Src); 150 if (CI != AvailCopyMap.end()) { 151 MachineInstr *CopyMI = CI->second; 152 if (!MRI->isReserved(Def) && 153 (!MRI->isReserved(Src) || NoInterveningSideEffect(CopyMI, MI)) && 154 isNopCopy(CopyMI, Def, Src, TRI)) { 155 // The two copies cancel out and the source of the first copy 156 // hasn't been overridden, eliminate the second one. e.g. 157 // %ECX<def> = COPY %EAX<kill> 158 // ... nothing clobbered EAX. 159 // %EAX<def> = COPY %ECX 160 // => 161 // %ECX<def> = COPY %EAX 162 // 163 // Also avoid eliminating a copy from reserved registers unless the 164 // definition is proven not clobbered. e.g. 165 // %RSP<def> = COPY %RAX 166 // CALL 167 // %RAX<def> = COPY %RSP 168 169 DEBUG(dbgs() << "MCP: copy is a NOP, removing: "; MI->dump()); 170 171 // Clear any kills of Def between CopyMI and MI. This extends the 172 // live range. 173 for (MachineBasicBlock::iterator I = CopyMI, E = MI; I != E; ++I) 174 I->clearRegisterKills(Def, TRI); 175 176 MI->eraseFromParent(); 177 Changed = true; 178 ++NumDeletes; 179 continue; 180 } 181 } 182 183 // If Src is defined by a previous copy, the previous copy cannot be 184 // eliminated. 185 for (MCRegAliasIterator AI(Src, TRI, true); AI.isValid(); ++AI) { 186 CI = CopyMap.find(*AI); 187 if (CI != CopyMap.end()) { 188 DEBUG(dbgs() << "MCP: Copy is no longer dead: "; CI->second->dump()); 189 MaybeDeadCopies.remove(CI->second); 190 } 191 } 192 193 DEBUG(dbgs() << "MCP: Copy is a deletion candidate: "; MI->dump()); 194 195 // Copy is now a candidate for deletion. 196 MaybeDeadCopies.insert(MI); 197 198 // If 'Def' is previously source of another copy, then this earlier copy's 199 // source is no longer available. e.g. 200 // %xmm9<def> = copy %xmm2 201 // ... 202 // %xmm2<def> = copy %xmm0 203 // ... 204 // %xmm2<def> = copy %xmm9 205 SourceNoLongerAvailable(Def, SrcMap, AvailCopyMap); 206 207 // Remember Def is defined by the copy. 208 // ... Make sure to clear the def maps of aliases first. 209 for (MCRegAliasIterator AI(Def, TRI, false); AI.isValid(); ++AI) { 210 CopyMap.erase(*AI); 211 AvailCopyMap.erase(*AI); 212 } 213 for (MCSubRegIterator SR(Def, TRI, /*IncludeSelf=*/true); SR.isValid(); 214 ++SR) { 215 CopyMap[*SR] = MI; 216 AvailCopyMap[*SR] = MI; 217 } 218 219 // Remember source that's copied to Def. Once it's clobbered, then 220 // it's no longer available for copy propagation. 221 if (std::find(SrcMap[Src].begin(), SrcMap[Src].end(), Def) == 222 SrcMap[Src].end()) { 223 SrcMap[Src].push_back(Def); 224 } 225 226 continue; 227 } 228 229 // Not a copy. 230 SmallVector<unsigned, 2> Defs; 231 int RegMaskOpNum = -1; 232 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 233 MachineOperand &MO = MI->getOperand(i); 234 if (MO.isRegMask()) 235 RegMaskOpNum = i; 236 if (!MO.isReg()) 237 continue; 238 unsigned Reg = MO.getReg(); 239 if (!Reg) 240 continue; 241 242 assert(!TargetRegisterInfo::isVirtualRegister(Reg) && 243 "MachineCopyPropagation should be run after register allocation!"); 244 245 if (MO.isDef()) { 246 Defs.push_back(Reg); 247 continue; 248 } 249 250 // If 'Reg' is defined by a copy, the copy is no longer a candidate 251 // for elimination. 252 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) { 253 DenseMap<unsigned, MachineInstr*>::iterator CI = CopyMap.find(*AI); 254 if (CI != CopyMap.end()) { 255 DEBUG(dbgs() << "MCP: Copy is used - not dead: "; CI->second->dump()); 256 MaybeDeadCopies.remove(CI->second); 257 } 258 } 259 // Treat undef use like defs for copy propagation but not for 260 // dead copy. We would need to do a liveness check to be sure the copy 261 // is dead for undef uses. 262 // The backends are allowed to do whatever they want with undef value 263 // and we cannot be sure this register will not be rewritten to break 264 // some false dependencies for the hardware for instance. 265 if (MO.isUndef()) 266 Defs.push_back(Reg); 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 (MRI->isReserved(Reg) || !MaskMO.clobbersPhysReg(Reg)) 281 continue; 282 DEBUG(dbgs() << "MCP: Removing copy due to regmask clobbering: "; 283 (*DI)->dump()); 284 (*DI)->eraseFromParent(); 285 Changed = true; 286 ++NumDeletes; 287 } 288 289 // Clear all data structures as if we were beginning a new basic block. 290 MaybeDeadCopies.clear(); 291 AvailCopyMap.clear(); 292 CopyMap.clear(); 293 SrcMap.clear(); 294 continue; 295 } 296 297 for (unsigned i = 0, e = Defs.size(); i != e; ++i) { 298 unsigned Reg = Defs[i]; 299 300 // No longer defined by a copy. 301 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) { 302 CopyMap.erase(*AI); 303 AvailCopyMap.erase(*AI); 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 (!MRI->isReserved((*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 if (skipOptnoneFunction(*MF.getFunction())) 332 return false; 333 334 bool Changed = false; 335 336 TRI = MF.getSubtarget().getRegisterInfo(); 337 TII = MF.getSubtarget().getInstrInfo(); 338 MRI = &MF.getRegInfo(); 339 340 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) 341 Changed |= CopyPropagateBlock(*I); 342 343 return Changed; 344 } 345