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 typedef SmallVector<unsigned, 4> RegList; 36 typedef DenseMap<unsigned, RegList> SourceMap; 37 typedef DenseMap<unsigned, MachineInstr*> Reg2MIMap; 38 39 class MachineCopyPropagation : public MachineFunctionPass { 40 const TargetRegisterInfo *TRI; 41 const TargetInstrInfo *TII; 42 const MachineRegisterInfo *MRI; 43 44 public: 45 static char ID; // Pass identification, replacement for typeid 46 MachineCopyPropagation() : MachineFunctionPass(ID) { 47 initializeMachineCopyPropagationPass(*PassRegistry::getPassRegistry()); 48 } 49 50 bool runOnMachineFunction(MachineFunction &MF) override; 51 52 private: 53 void ClobberRegister(unsigned Reg); 54 void CopyPropagateBlock(MachineBasicBlock &MBB); 55 56 /// Candidates for deletion. 57 SmallSetVector<MachineInstr*, 8> MaybeDeadCopies; 58 /// Def -> available copies map. 59 Reg2MIMap AvailCopyMap; 60 /// Def -> copies map. 61 Reg2MIMap CopyMap; 62 /// Src -> Def map 63 SourceMap SrcMap; 64 bool Changed; 65 }; 66 } 67 char MachineCopyPropagation::ID = 0; 68 char &llvm::MachineCopyPropagationID = MachineCopyPropagation::ID; 69 70 INITIALIZE_PASS(MachineCopyPropagation, "machine-cp", 71 "Machine Copy Propagation Pass", false, false) 72 73 /// Remove any entry in \p Map where the register is a subregister or equal to 74 /// a register contained in \p Regs. 75 static void removeRegsFromMap(Reg2MIMap &Map, const RegList &Regs, 76 const TargetRegisterInfo &TRI) { 77 for (unsigned Reg : Regs) { 78 // Source of copy is no longer available for propagation. 79 for (MCSubRegIterator SR(Reg, &TRI, true); SR.isValid(); ++SR) 80 Map.erase(*SR); 81 } 82 } 83 84 /// Remove any entry in \p Map that is marked clobbered in \p RegMask. 85 /// The map will typically have a lot fewer entries than the regmask clobbers, 86 /// so this is more efficient than iterating the clobbered registers and calling 87 /// ClobberRegister() on them. 88 static void removeClobberedRegsFromMap(Reg2MIMap &Map, 89 const MachineOperand &RegMask) { 90 for (Reg2MIMap::iterator I = Map.begin(), E = Map.end(), Next; I != E; 91 I = Next) { 92 Next = std::next(I); 93 unsigned Reg = I->first; 94 if (RegMask.clobbersPhysReg(Reg)) 95 Map.erase(I); 96 } 97 } 98 99 void MachineCopyPropagation::ClobberRegister(unsigned Reg) { 100 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) { 101 CopyMap.erase(*AI); 102 AvailCopyMap.erase(*AI); 103 104 SourceMap::iterator SI = SrcMap.find(*AI); 105 if (SI != SrcMap.end()) { 106 removeRegsFromMap(AvailCopyMap, SI->second, *TRI); 107 SrcMap.erase(SI); 108 } 109 } 110 } 111 112 /// isNopCopy - Return true if the specified copy is really a nop. That is 113 /// if the source of the copy is the same of the definition of the copy that 114 /// supplied the source. If the source of the copy is a sub-register than it 115 /// must check the sub-indices match. e.g. 116 /// ecx = mov eax 117 /// al = mov cl 118 /// But not 119 /// ecx = mov eax 120 /// al = mov ch 121 static bool isNopCopy(const MachineInstr *CopyMI, unsigned Def, unsigned Src, 122 const TargetRegisterInfo *TRI) { 123 unsigned SrcSrc = CopyMI->getOperand(1).getReg(); 124 if (Def == SrcSrc) 125 return true; 126 if (TRI->isSubRegister(SrcSrc, Def)) { 127 unsigned SrcDef = CopyMI->getOperand(0).getReg(); 128 unsigned SubIdx = TRI->getSubRegIndex(SrcSrc, Def); 129 if (!SubIdx) 130 return false; 131 return SubIdx == TRI->getSubRegIndex(SrcDef, Src); 132 } 133 134 return false; 135 } 136 137 void MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) { 138 DEBUG(dbgs() << "MCP: CopyPropagateBlock " << MBB.getName() << "\n"); 139 140 for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E; ) { 141 MachineInstr *MI = &*I; 142 ++I; 143 144 if (MI->isCopy()) { 145 unsigned Def = MI->getOperand(0).getReg(); 146 unsigned Src = MI->getOperand(1).getReg(); 147 148 assert(!TargetRegisterInfo::isVirtualRegister(Def) && 149 !TargetRegisterInfo::isVirtualRegister(Src) && 150 "MachineCopyPropagation should be run after register allocation!"); 151 152 DenseMap<unsigned, MachineInstr*>::iterator CI = AvailCopyMap.find(Src); 153 if (CI != AvailCopyMap.end()) { 154 MachineInstr *CopyMI = CI->second; 155 if (!MRI->isReserved(Def) && isNopCopy(CopyMI, Def, Src, TRI)) { 156 // The two copies cancel out and the source of the first copy 157 // hasn't been overridden, eliminate the second one. e.g. 158 // %ECX<def> = COPY %EAX<kill> 159 // ... nothing clobbered EAX. 160 // %EAX<def> = COPY %ECX 161 // => 162 // %ECX<def> = COPY %EAX 163 // 164 // Also avoid eliminating a copy from reserved registers unless the 165 // definition is proven not clobbered. e.g. 166 // %RSP<def> = COPY %RAX 167 // CALL 168 // %RAX<def> = COPY %RSP 169 170 DEBUG(dbgs() << "MCP: copy is a NOP, removing: "; MI->dump()); 171 172 // Clear any kills of Def between CopyMI and MI. This extends the 173 // live range. 174 for (MachineInstr &MMI 175 : make_range(CopyMI->getIterator(), MI->getIterator())) 176 MMI.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, the previous copy cannot be 186 // eliminated. 187 for (MCRegAliasIterator AI(Src, TRI, true); AI.isValid(); ++AI) { 188 Reg2MIMap::iterator CI = CopyMap.find(*AI); 189 if (CI != CopyMap.end()) { 190 DEBUG(dbgs() << "MCP: Copy is no longer dead: "; CI->second->dump()); 191 MaybeDeadCopies.remove(CI->second); 192 } 193 } 194 195 DEBUG(dbgs() << "MCP: Copy is a deletion candidate: "; MI->dump()); 196 197 // Copy is now a candidate for deletion. 198 if (!MRI->isReserved(Def)) 199 MaybeDeadCopies.insert(MI); 200 201 // If 'Def' 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 ClobberRegister(Def); 209 210 // Remember Def is defined by the copy. 211 for (MCSubRegIterator SR(Def, TRI, /*IncludeSelf=*/true); SR.isValid(); 212 ++SR) { 213 CopyMap[*SR] = MI; 214 AvailCopyMap[*SR] = MI; 215 } 216 217 // Remember source that's copied to Def. Once it's clobbered, then 218 // it's no longer available for copy propagation. 219 RegList &DestList = SrcMap[Src]; 220 if (std::find(DestList.begin(), DestList.end(), Def) == DestList.end()) 221 DestList.push_back(Def); 222 223 continue; 224 } 225 226 // Not a copy. 227 SmallVector<unsigned, 2> Defs; 228 const MachineOperand *RegMask = nullptr; 229 for (const MachineOperand &MO : MI->operands()) { 230 if (MO.isRegMask()) 231 RegMask = &MO; 232 if (!MO.isReg()) 233 continue; 234 unsigned Reg = MO.getReg(); 235 if (!Reg) 236 continue; 237 238 assert(!TargetRegisterInfo::isVirtualRegister(Reg) && 239 "MachineCopyPropagation should be run after register allocation!"); 240 241 if (MO.isDef()) { 242 Defs.push_back(Reg); 243 continue; 244 } 245 246 // If 'Reg' is defined by a copy, the copy is no longer a candidate 247 // for elimination. 248 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) { 249 Reg2MIMap::iterator CI = CopyMap.find(*AI); 250 if (CI != CopyMap.end()) { 251 DEBUG(dbgs() << "MCP: Copy is used - not dead: "; CI->second->dump()); 252 MaybeDeadCopies.remove(CI->second); 253 } 254 } 255 // Treat undef use like defs for copy propagation but not for 256 // dead copy. We would need to do a liveness check to be sure the copy 257 // is dead for undef uses. 258 // The backends are allowed to do whatever they want with undef value 259 // and we cannot be sure this register will not be rewritten to break 260 // some false dependencies for the hardware for instance. 261 if (MO.isUndef()) 262 Defs.push_back(Reg); 263 } 264 265 // The instruction has a register mask operand which means that it clobbers 266 // a large set of registers. Treat clobbered registers the same way as 267 // defined registers. 268 if (RegMask) { 269 // Erase any MaybeDeadCopies whose destination register is clobbered. 270 for (MachineInstr *MaybeDead : MaybeDeadCopies) { 271 unsigned Reg = MaybeDead->getOperand(0).getReg(); 272 assert(!MRI->isReserved(Reg)); 273 if (!RegMask->clobbersPhysReg(Reg)) 274 continue; 275 DEBUG(dbgs() << "MCP: Removing copy due to regmask clobbering: "; 276 MaybeDead->dump()); 277 MaybeDead->eraseFromParent(); 278 Changed = true; 279 ++NumDeletes; 280 } 281 MaybeDeadCopies.clear(); 282 283 removeClobberedRegsFromMap(AvailCopyMap, *RegMask); 284 removeClobberedRegsFromMap(CopyMap, *RegMask); 285 for (SourceMap::iterator I = SrcMap.begin(), E = SrcMap.end(), Next; 286 I != E; I = Next) { 287 Next = std::next(I); 288 if (RegMask->clobbersPhysReg(I->first)) { 289 removeRegsFromMap(AvailCopyMap, I->second, *TRI); 290 SrcMap.erase(I); 291 } 292 } 293 } 294 295 // Any previous copy definition or reading the Defs is no longer available. 296 for (unsigned Reg : Defs) { 297 ClobberRegister(Reg); 298 } 299 } 300 301 // If MBB doesn't have successors, delete the copies whose defs are not used. 302 // If MBB does have successors, then conservative assume the defs are live-out 303 // since we don't want to trust live-in lists. 304 if (MBB.succ_empty()) { 305 for (MachineInstr *MaybeDead : MaybeDeadCopies) { 306 assert(!MRI->isReserved(MaybeDead->getOperand(0).getReg())); 307 MaybeDead->eraseFromParent(); 308 Changed = true; 309 ++NumDeletes; 310 } 311 } 312 313 MaybeDeadCopies.clear(); 314 AvailCopyMap.clear(); 315 CopyMap.clear(); 316 SrcMap.clear(); 317 } 318 319 bool MachineCopyPropagation::runOnMachineFunction(MachineFunction &MF) { 320 if (skipOptnoneFunction(*MF.getFunction())) 321 return false; 322 323 Changed = false; 324 325 TRI = MF.getSubtarget().getRegisterInfo(); 326 TII = MF.getSubtarget().getInstrInfo(); 327 MRI = &MF.getRegInfo(); 328 329 for (MachineBasicBlock &MBB : MF) 330 CopyPropagateBlock(MBB); 331 332 return Changed; 333 } 334