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 const 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 typedef DenseMap<unsigned, MachineInstr*> Reg2MIMap; 52 53 void SourceNoLongerAvailable(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 void MachineCopyPropagation::SourceNoLongerAvailable(unsigned Reg) { 74 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) { 75 SourceMap::iterator SI = SrcMap.find(*AI); 76 if (SI != SrcMap.end()) { 77 const DestList& Defs = SI->second; 78 for (unsigned MappedDef : Defs) { 79 // Source of copy is no longer available for propagation. 80 for (MCSubRegIterator SR(MappedDef, TRI, true); SR.isValid(); ++SR) 81 AvailCopyMap.erase(*SR); 82 } 83 } 84 } 85 } 86 87 static bool NoInterveningSideEffect(const MachineInstr *CopyMI, 88 const MachineInstr *MI) { 89 const MachineBasicBlock *MBB = CopyMI->getParent(); 90 if (MI->getParent() != MBB) 91 return false; 92 93 for (MachineBasicBlock::const_instr_iterator 94 I = std::next(CopyMI->getInstrIterator()), 95 E = MBB->instr_end(), E2 = MI->getInstrIterator(); 96 I != E && I != E2; ++I) { 97 if (I->hasUnmodeledSideEffects() || I->isCall() || 98 I->isTerminator()) 99 return false; 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(const 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 void MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) { 130 DEBUG(dbgs() << "MCP: CopyPropagateBlock " << MBB.getName() << "\n"); 131 132 for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E; ) { 133 MachineInstr *MI = &*I; 134 ++I; 135 136 if (MI->isCopy()) { 137 unsigned Def = MI->getOperand(0).getReg(); 138 unsigned Src = MI->getOperand(1).getReg(); 139 140 assert(!TargetRegisterInfo::isVirtualRegister(Def) && 141 !TargetRegisterInfo::isVirtualRegister(Src) && 142 "MachineCopyPropagation should be run after register allocation!"); 143 144 DenseMap<unsigned, MachineInstr*>::iterator CI = AvailCopyMap.find(Src); 145 if (CI != AvailCopyMap.end()) { 146 MachineInstr *CopyMI = CI->second; 147 if (!MRI->isReserved(Def) && 148 (!MRI->isReserved(Src) || NoInterveningSideEffect(CopyMI, MI)) && 149 isNopCopy(CopyMI, Def, Src, TRI)) { 150 // The two copies cancel out and the source of the first copy 151 // hasn't been overridden, eliminate the second one. e.g. 152 // %ECX<def> = COPY %EAX<kill> 153 // ... nothing clobbered EAX. 154 // %EAX<def> = COPY %ECX 155 // => 156 // %ECX<def> = COPY %EAX 157 // 158 // Also avoid eliminating a copy from reserved registers unless the 159 // definition is proven not clobbered. e.g. 160 // %RSP<def> = COPY %RAX 161 // CALL 162 // %RAX<def> = COPY %RSP 163 164 DEBUG(dbgs() << "MCP: copy is a NOP, removing: "; MI->dump()); 165 166 // Clear any kills of Def between CopyMI and MI. This extends the 167 // live range. 168 for (MachineInstr &MMI : 169 make_range(CopyMI->getInstrIterator(), MI->getInstrIterator())) 170 MMI.clearRegisterKills(Def, TRI); 171 172 MI->eraseFromParent(); 173 Changed = true; 174 ++NumDeletes; 175 continue; 176 } 177 } 178 179 // If Src is defined by a previous copy, the previous copy cannot be 180 // eliminated. 181 for (MCRegAliasIterator AI(Src, TRI, true); AI.isValid(); ++AI) { 182 Reg2MIMap::iterator CI = CopyMap.find(*AI); 183 if (CI != CopyMap.end()) { 184 DEBUG(dbgs() << "MCP: Copy is no longer dead: "; CI->second->dump()); 185 MaybeDeadCopies.remove(CI->second); 186 } 187 } 188 189 DEBUG(dbgs() << "MCP: Copy is a deletion candidate: "; MI->dump()); 190 191 // Copy is now a candidate for deletion. 192 if (!MRI->isReserved(Def)) 193 MaybeDeadCopies.insert(MI); 194 195 // If 'Def' is previously source of another copy, then this earlier copy's 196 // source is no longer available. e.g. 197 // %xmm9<def> = copy %xmm2 198 // ... 199 // %xmm2<def> = copy %xmm0 200 // ... 201 // %xmm2<def> = copy %xmm9 202 SourceNoLongerAvailable(Def); 203 204 // Remember Def is defined by the copy. 205 // ... Make sure to clear the def maps of aliases first. 206 for (MCRegAliasIterator AI(Def, TRI, false); AI.isValid(); ++AI) { 207 CopyMap.erase(*AI); 208 AvailCopyMap.erase(*AI); 209 } 210 for (MCSubRegIterator SR(Def, TRI, /*IncludeSelf=*/true); SR.isValid(); 211 ++SR) { 212 CopyMap[*SR] = MI; 213 AvailCopyMap[*SR] = MI; 214 } 215 216 // Remember source that's copied to Def. Once it's clobbered, then 217 // it's no longer available for copy propagation. 218 SmallVectorImpl<unsigned> &DestList = SrcMap[Src]; 219 if (std::find(DestList.begin(), DestList.end(), Def) == DestList.end()) 220 DestList.push_back(Def); 221 222 continue; 223 } 224 225 // Not a copy. 226 SmallVector<unsigned, 2> Defs; 227 const MachineOperand *RegMask = nullptr; 228 for (const MachineOperand &MO : MI->operands()) { 229 if (MO.isRegMask()) 230 RegMask = &MO; 231 if (!MO.isReg()) 232 continue; 233 unsigned Reg = MO.getReg(); 234 if (!Reg) 235 continue; 236 237 assert(!TargetRegisterInfo::isVirtualRegister(Reg) && 238 "MachineCopyPropagation should be run after register allocation!"); 239 240 if (MO.isDef()) { 241 Defs.push_back(Reg); 242 continue; 243 } 244 245 // If 'Reg' is defined by a copy, the copy is no longer a candidate 246 // for elimination. 247 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) { 248 Reg2MIMap::iterator CI = CopyMap.find(*AI); 249 if (CI != CopyMap.end()) { 250 DEBUG(dbgs() << "MCP: Copy is used - not dead: "; CI->second->dump()); 251 MaybeDeadCopies.remove(CI->second); 252 } 253 } 254 // Treat undef use like defs for copy propagation but not for 255 // dead copy. We would need to do a liveness check to be sure the copy 256 // is dead for undef uses. 257 // The backends are allowed to do whatever they want with undef value 258 // and we cannot be sure this register will not be rewritten to break 259 // some false dependencies for the hardware for instance. 260 if (MO.isUndef()) 261 Defs.push_back(Reg); 262 } 263 264 // The instruction has a register mask operand which means that it clobbers 265 // a large set of registers. It is possible to use the register mask to 266 // prune the available copies, but treat it like a basic block boundary for 267 // now. 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 } 281 282 // Clear all data structures as if we were beginning a new basic block. 283 MaybeDeadCopies.clear(); 284 AvailCopyMap.clear(); 285 CopyMap.clear(); 286 SrcMap.clear(); 287 continue; 288 } 289 290 for (unsigned Reg : Defs) { 291 // No longer defined by a copy. 292 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) { 293 CopyMap.erase(*AI); 294 AvailCopyMap.erase(*AI); 295 } 296 297 // If 'Reg' is previously source of a copy, it is no longer available for 298 // copy propagation. 299 SourceNoLongerAvailable(Reg); 300 } 301 } 302 303 // If MBB doesn't have successors, delete the copies whose defs are not used. 304 // If MBB does have successors, then conservative assume the defs are live-out 305 // since we don't want to trust live-in lists. 306 if (MBB.succ_empty()) { 307 for (MachineInstr *MaybeDead : MaybeDeadCopies) { 308 assert(!MRI->isReserved(MaybeDead->getOperand(0).getReg())); 309 MaybeDead->eraseFromParent(); 310 Changed = true; 311 ++NumDeletes; 312 } 313 } 314 315 MaybeDeadCopies.clear(); 316 AvailCopyMap.clear(); 317 CopyMap.clear(); 318 SrcMap.clear(); 319 } 320 321 bool MachineCopyPropagation::runOnMachineFunction(MachineFunction &MF) { 322 if (skipOptnoneFunction(*MF.getFunction())) 323 return false; 324 325 Changed = false; 326 327 TRI = MF.getSubtarget().getRegisterInfo(); 328 TII = MF.getSubtarget().getInstrInfo(); 329 MRI = &MF.getRegInfo(); 330 331 for (MachineBasicBlock &MBB : MF) 332 CopyPropagateBlock(MBB); 333 334 return Changed; 335 } 336