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_iterator I = std::next(CopyMI->getIterator()), 94 E = MBB->end(), E2 = MI->getIterator(); I != E && I != E2; ++I) { 95 if (I->hasUnmodeledSideEffects() || I->isCall() || 96 I->isTerminator()) 97 return false; 98 } 99 return true; 100 } 101 102 /// isNopCopy - Return true if the specified copy is really a nop. That is 103 /// if the source of the copy is the same of the definition of the copy that 104 /// supplied the source. If the source of the copy is a sub-register than it 105 /// must check the sub-indices match. e.g. 106 /// ecx = mov eax 107 /// al = mov cl 108 /// But not 109 /// ecx = mov eax 110 /// al = mov ch 111 static bool isNopCopy(const MachineInstr *CopyMI, unsigned Def, unsigned Src, 112 const TargetRegisterInfo *TRI) { 113 unsigned SrcSrc = CopyMI->getOperand(1).getReg(); 114 if (Def == SrcSrc) 115 return true; 116 if (TRI->isSubRegister(SrcSrc, Def)) { 117 unsigned SrcDef = CopyMI->getOperand(0).getReg(); 118 unsigned SubIdx = TRI->getSubRegIndex(SrcSrc, Def); 119 if (!SubIdx) 120 return false; 121 return SubIdx == TRI->getSubRegIndex(SrcDef, Src); 122 } 123 124 return false; 125 } 126 127 void MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) { 128 DEBUG(dbgs() << "MCP: CopyPropagateBlock " << MBB.getName() << "\n"); 129 130 for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E; ) { 131 MachineInstr *MI = &*I; 132 ++I; 133 134 if (MI->isCopy()) { 135 unsigned Def = MI->getOperand(0).getReg(); 136 unsigned Src = MI->getOperand(1).getReg(); 137 138 assert(!TargetRegisterInfo::isVirtualRegister(Def) && 139 !TargetRegisterInfo::isVirtualRegister(Src) && 140 "MachineCopyPropagation should be run after register allocation!"); 141 142 DenseMap<unsigned, MachineInstr*>::iterator CI = AvailCopyMap.find(Src); 143 if (CI != AvailCopyMap.end()) { 144 MachineInstr *CopyMI = CI->second; 145 if (!MRI->isReserved(Def) && 146 (!MRI->isReserved(Src) || NoInterveningSideEffect(CopyMI, MI)) && 147 isNopCopy(CopyMI, Def, Src, TRI)) { 148 // The two copies cancel out and the source of the first copy 149 // hasn't been overridden, eliminate the second one. e.g. 150 // %ECX<def> = COPY %EAX<kill> 151 // ... nothing clobbered EAX. 152 // %EAX<def> = COPY %ECX 153 // => 154 // %ECX<def> = COPY %EAX 155 // 156 // Also avoid eliminating a copy from reserved registers unless the 157 // definition is proven not clobbered. e.g. 158 // %RSP<def> = COPY %RAX 159 // CALL 160 // %RAX<def> = COPY %RSP 161 162 DEBUG(dbgs() << "MCP: copy is a NOP, removing: "; MI->dump()); 163 164 // Clear any kills of Def between CopyMI and MI. This extends the 165 // live range. 166 for (MachineInstr &MMI 167 : make_range(CopyMI->getIterator(), MI->getIterator())) 168 MMI.clearRegisterKills(Def, TRI); 169 170 MI->eraseFromParent(); 171 Changed = true; 172 ++NumDeletes; 173 continue; 174 } 175 } 176 177 // If Src is defined by a previous copy, the previous copy cannot be 178 // eliminated. 179 for (MCRegAliasIterator AI(Src, TRI, true); AI.isValid(); ++AI) { 180 Reg2MIMap::iterator CI = CopyMap.find(*AI); 181 if (CI != CopyMap.end()) { 182 DEBUG(dbgs() << "MCP: Copy is no longer dead: "; CI->second->dump()); 183 MaybeDeadCopies.remove(CI->second); 184 } 185 } 186 187 DEBUG(dbgs() << "MCP: Copy is a deletion candidate: "; MI->dump()); 188 189 // Copy is now a candidate for deletion. 190 if (!MRI->isReserved(Def)) 191 MaybeDeadCopies.insert(MI); 192 193 // If 'Def' is previously source of another copy, then this earlier copy's 194 // source is no longer available. e.g. 195 // %xmm9<def> = copy %xmm2 196 // ... 197 // %xmm2<def> = copy %xmm0 198 // ... 199 // %xmm2<def> = copy %xmm9 200 SourceNoLongerAvailable(Def); 201 202 // Remember Def is defined by the copy. 203 // ... Make sure to clear the def maps of aliases first. 204 for (MCRegAliasIterator AI(Def, TRI, false); AI.isValid(); ++AI) { 205 CopyMap.erase(*AI); 206 AvailCopyMap.erase(*AI); 207 } 208 for (MCSubRegIterator SR(Def, TRI, /*IncludeSelf=*/true); SR.isValid(); 209 ++SR) { 210 CopyMap[*SR] = MI; 211 AvailCopyMap[*SR] = MI; 212 } 213 214 // Remember source that's copied to Def. Once it's clobbered, then 215 // it's no longer available for copy propagation. 216 SmallVectorImpl<unsigned> &DestList = SrcMap[Src]; 217 if (std::find(DestList.begin(), DestList.end(), Def) == DestList.end()) 218 DestList.push_back(Def); 219 220 continue; 221 } 222 223 // Not a copy. 224 SmallVector<unsigned, 2> Defs; 225 const MachineOperand *RegMask = nullptr; 226 for (const MachineOperand &MO : MI->operands()) { 227 if (MO.isRegMask()) 228 RegMask = &MO; 229 if (!MO.isReg()) 230 continue; 231 unsigned Reg = MO.getReg(); 232 if (!Reg) 233 continue; 234 235 assert(!TargetRegisterInfo::isVirtualRegister(Reg) && 236 "MachineCopyPropagation should be run after register allocation!"); 237 238 if (MO.isDef()) { 239 Defs.push_back(Reg); 240 continue; 241 } 242 243 // If 'Reg' is defined by a copy, the copy is no longer a candidate 244 // for elimination. 245 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) { 246 Reg2MIMap::iterator CI = CopyMap.find(*AI); 247 if (CI != CopyMap.end()) { 248 DEBUG(dbgs() << "MCP: Copy is used - not dead: "; CI->second->dump()); 249 MaybeDeadCopies.remove(CI->second); 250 } 251 } 252 // Treat undef use like defs for copy propagation but not for 253 // dead copy. We would need to do a liveness check to be sure the copy 254 // is dead for undef uses. 255 // The backends are allowed to do whatever they want with undef value 256 // and we cannot be sure this register will not be rewritten to break 257 // some false dependencies for the hardware for instance. 258 if (MO.isUndef()) 259 Defs.push_back(Reg); 260 } 261 262 // The instruction has a register mask operand which means that it clobbers 263 // a large set of registers. It is possible to use the register mask to 264 // prune the available copies, but treat it like a basic block boundary for 265 // now. 266 if (RegMask) { 267 // Erase any MaybeDeadCopies whose destination register is clobbered. 268 for (MachineInstr *MaybeDead : MaybeDeadCopies) { 269 unsigned Reg = MaybeDead->getOperand(0).getReg(); 270 assert(!MRI->isReserved(Reg)); 271 if (!RegMask->clobbersPhysReg(Reg)) 272 continue; 273 DEBUG(dbgs() << "MCP: Removing copy due to regmask clobbering: "; 274 MaybeDead->dump()); 275 MaybeDead->eraseFromParent(); 276 Changed = true; 277 ++NumDeletes; 278 } 279 280 // Clear all data structures as if we were beginning a new basic block. 281 MaybeDeadCopies.clear(); 282 AvailCopyMap.clear(); 283 CopyMap.clear(); 284 SrcMap.clear(); 285 continue; 286 } 287 288 for (unsigned Reg : Defs) { 289 // No longer defined by a copy. 290 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) { 291 CopyMap.erase(*AI); 292 AvailCopyMap.erase(*AI); 293 } 294 295 // If 'Reg' is previously source of a copy, it is no longer available for 296 // copy propagation. 297 SourceNoLongerAvailable(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