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 52 void SourceNoLongerAvailable(unsigned Reg, SourceMap &SrcMap, 53 DenseMap<unsigned, MachineInstr*> &AvailCopyMap); 54 bool CopyPropagateBlock(MachineBasicBlock &MBB); 55 }; 56 } 57 char MachineCopyPropagation::ID = 0; 58 char &llvm::MachineCopyPropagationID = MachineCopyPropagation::ID; 59 60 INITIALIZE_PASS(MachineCopyPropagation, "machine-cp", 61 "Machine Copy Propagation Pass", false, false) 62 63 void 64 MachineCopyPropagation::SourceNoLongerAvailable(unsigned Reg, SourceMap &SrcMap, 65 DenseMap<unsigned, MachineInstr*> &AvailCopyMap) { 66 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) { 67 SourceMap::iterator SI = SrcMap.find(*AI); 68 if (SI != SrcMap.end()) { 69 const DestList& Defs = SI->second; 70 for (unsigned MappedDef : Defs) { 71 // Source of copy is no longer available for propagation. 72 for (MCSubRegIterator SR(MappedDef, TRI, true); SR.isValid(); ++SR) 73 AvailCopyMap.erase(*SR); 74 } 75 } 76 } 77 } 78 79 static bool NoInterveningSideEffect(const MachineInstr *CopyMI, 80 const MachineInstr *MI) { 81 const MachineBasicBlock *MBB = CopyMI->getParent(); 82 if (MI->getParent() != MBB) 83 return false; 84 85 for (MachineBasicBlock::const_iterator I = std::next(CopyMI->getIterator()), 86 E = MBB->end(), E2 = MI->getIterator(); I != E && I != E2; ++I) { 87 if (I->hasUnmodeledSideEffects() || I->isCall() || 88 I->isTerminator()) 89 return false; 90 } 91 return true; 92 } 93 94 /// isNopCopy - Return true if the specified copy is really a nop. That is 95 /// if the source of the copy is the same of the definition of the copy that 96 /// supplied the source. If the source of the copy is a sub-register than it 97 /// must check the sub-indices match. e.g. 98 /// ecx = mov eax 99 /// al = mov cl 100 /// But not 101 /// ecx = mov eax 102 /// al = mov ch 103 static bool isNopCopy(const MachineInstr *CopyMI, unsigned Def, unsigned Src, 104 const TargetRegisterInfo *TRI) { 105 unsigned SrcSrc = CopyMI->getOperand(1).getReg(); 106 if (Def == SrcSrc) 107 return true; 108 if (TRI->isSubRegister(SrcSrc, Def)) { 109 unsigned SrcDef = CopyMI->getOperand(0).getReg(); 110 unsigned SubIdx = TRI->getSubRegIndex(SrcSrc, Def); 111 if (!SubIdx) 112 return false; 113 return SubIdx == TRI->getSubRegIndex(SrcDef, Src); 114 } 115 116 return false; 117 } 118 119 bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) { 120 SmallSetVector<MachineInstr*, 8> MaybeDeadCopies; // Candidates for deletion 121 DenseMap<unsigned, MachineInstr*> AvailCopyMap; // Def -> available copies map 122 DenseMap<unsigned, MachineInstr*> CopyMap; // Def -> copies map 123 SourceMap SrcMap; // Src -> Def map 124 125 DEBUG(dbgs() << "MCP: CopyPropagateBlock " << MBB.getName() << "\n"); 126 127 bool Changed = false; 128 for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E; ) { 129 MachineInstr *MI = &*I; 130 ++I; 131 132 if (MI->isCopy()) { 133 unsigned Def = MI->getOperand(0).getReg(); 134 unsigned Src = MI->getOperand(1).getReg(); 135 136 assert(!TargetRegisterInfo::isVirtualRegister(Def) && 137 !TargetRegisterInfo::isVirtualRegister(Src) && 138 "MachineCopyPropagation should be run after register allocation!"); 139 140 DenseMap<unsigned, MachineInstr*>::iterator CI = AvailCopyMap.find(Src); 141 if (CI != AvailCopyMap.end()) { 142 MachineInstr *CopyMI = CI->second; 143 if (!MRI->isReserved(Def) && 144 (!MRI->isReserved(Src) || NoInterveningSideEffect(CopyMI, MI)) && 145 isNopCopy(CopyMI, Def, Src, TRI)) { 146 // The two copies cancel out and the source of the first copy 147 // hasn't been overridden, eliminate the second one. e.g. 148 // %ECX<def> = COPY %EAX<kill> 149 // ... nothing clobbered EAX. 150 // %EAX<def> = COPY %ECX 151 // => 152 // %ECX<def> = COPY %EAX 153 // 154 // Also avoid eliminating a copy from reserved registers unless the 155 // definition is proven not clobbered. e.g. 156 // %RSP<def> = COPY %RAX 157 // CALL 158 // %RAX<def> = COPY %RSP 159 160 DEBUG(dbgs() << "MCP: copy is a NOP, removing: "; MI->dump()); 161 162 // Clear any kills of Def between CopyMI and MI. This extends the 163 // live range. 164 for (MachineInstr &MMI 165 : make_range(CopyMI->getIterator(), MI->getIterator())) 166 MMI.clearRegisterKills(Def, TRI); 167 168 MI->eraseFromParent(); 169 Changed = true; 170 ++NumDeletes; 171 continue; 172 } 173 } 174 175 // If Src is defined by a previous copy, the previous copy cannot be 176 // eliminated. 177 for (MCRegAliasIterator AI(Src, TRI, true); AI.isValid(); ++AI) { 178 CI = CopyMap.find(*AI); 179 if (CI != CopyMap.end()) { 180 DEBUG(dbgs() << "MCP: Copy is no longer dead: "; CI->second->dump()); 181 MaybeDeadCopies.remove(CI->second); 182 } 183 } 184 185 DEBUG(dbgs() << "MCP: Copy is a deletion candidate: "; MI->dump()); 186 187 // Copy is now a candidate for deletion. 188 if (!MRI->isReserved(Def)) 189 MaybeDeadCopies.insert(MI); 190 191 // If 'Def' is previously source of another copy, then this earlier copy's 192 // source is no longer available. e.g. 193 // %xmm9<def> = copy %xmm2 194 // ... 195 // %xmm2<def> = copy %xmm0 196 // ... 197 // %xmm2<def> = copy %xmm9 198 SourceNoLongerAvailable(Def, SrcMap, AvailCopyMap); 199 200 // Remember Def is defined by the copy. 201 // ... Make sure to clear the def maps of aliases first. 202 for (MCRegAliasIterator AI(Def, TRI, false); AI.isValid(); ++AI) { 203 CopyMap.erase(*AI); 204 AvailCopyMap.erase(*AI); 205 } 206 for (MCSubRegIterator SR(Def, TRI, /*IncludeSelf=*/true); SR.isValid(); 207 ++SR) { 208 CopyMap[*SR] = MI; 209 AvailCopyMap[*SR] = MI; 210 } 211 212 // Remember source that's copied to Def. Once it's clobbered, then 213 // it's no longer available for copy propagation. 214 SmallVectorImpl<unsigned> &DestList = SrcMap[Src]; 215 if (std::find(DestList.begin(), DestList.end(), Def) == DestList.end()) 216 DestList.push_back(Def); 217 218 continue; 219 } 220 221 // Not a copy. 222 SmallVector<unsigned, 2> Defs; 223 const MachineOperand *RegMask = nullptr; 224 for (const MachineOperand &MO : MI->operands()) { 225 if (MO.isRegMask()) 226 RegMask = &MO; 227 if (!MO.isReg()) 228 continue; 229 unsigned Reg = MO.getReg(); 230 if (!Reg) 231 continue; 232 233 assert(!TargetRegisterInfo::isVirtualRegister(Reg) && 234 "MachineCopyPropagation should be run after register allocation!"); 235 236 if (MO.isDef()) { 237 Defs.push_back(Reg); 238 continue; 239 } 240 241 // If 'Reg' is defined by a copy, the copy is no longer a candidate 242 // for elimination. 243 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) { 244 DenseMap<unsigned, MachineInstr*>::iterator CI = CopyMap.find(*AI); 245 if (CI != CopyMap.end()) { 246 DEBUG(dbgs() << "MCP: Copy is used - not dead: "; CI->second->dump()); 247 MaybeDeadCopies.remove(CI->second); 248 } 249 } 250 // Treat undef use like defs for copy propagation but not for 251 // dead copy. We would need to do a liveness check to be sure the copy 252 // is dead for undef uses. 253 // The backends are allowed to do whatever they want with undef value 254 // and we cannot be sure this register will not be rewritten to break 255 // some false dependencies for the hardware for instance. 256 if (MO.isUndef()) 257 Defs.push_back(Reg); 258 } 259 260 // The instruction has a register mask operand which means that it clobbers 261 // a large set of registers. It is possible to use the register mask to 262 // prune the available copies, but treat it like a basic block boundary for 263 // now. 264 if (RegMask) { 265 // Erase any MaybeDeadCopies whose destination register is clobbered. 266 for (MachineInstr *MaybeDead : MaybeDeadCopies) { 267 unsigned Reg = MaybeDead->getOperand(0).getReg(); 268 assert(!MRI->isReserved(Reg)); 269 if (!RegMask->clobbersPhysReg(Reg)) 270 continue; 271 DEBUG(dbgs() << "MCP: Removing copy due to regmask clobbering: "; 272 MaybeDead->dump()); 273 MaybeDead->eraseFromParent(); 274 Changed = true; 275 ++NumDeletes; 276 } 277 278 // Clear all data structures as if we were beginning a new basic block. 279 MaybeDeadCopies.clear(); 280 AvailCopyMap.clear(); 281 CopyMap.clear(); 282 SrcMap.clear(); 283 continue; 284 } 285 286 for (unsigned Reg : Defs) { 287 // No longer defined by a copy. 288 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) { 289 CopyMap.erase(*AI); 290 AvailCopyMap.erase(*AI); 291 } 292 293 // If 'Reg' is previously source of a copy, it is no longer available for 294 // copy propagation. 295 SourceNoLongerAvailable(Reg, SrcMap, AvailCopyMap); 296 } 297 } 298 299 // If MBB doesn't have successors, delete the copies whose defs are not used. 300 // If MBB does have successors, then conservative assume the defs are live-out 301 // since we don't want to trust live-in lists. 302 if (MBB.succ_empty()) { 303 for (MachineInstr *MaybeDead : MaybeDeadCopies) { 304 assert(!MRI->isReserved(MaybeDead->getOperand(0).getReg())); 305 MaybeDead->eraseFromParent(); 306 Changed = true; 307 ++NumDeletes; 308 } 309 } 310 311 return Changed; 312 } 313 314 bool MachineCopyPropagation::runOnMachineFunction(MachineFunction &MF) { 315 if (skipOptnoneFunction(*MF.getFunction())) 316 return false; 317 318 bool Changed = false; 319 320 TRI = MF.getSubtarget().getRegisterInfo(); 321 TII = MF.getSubtarget().getInstrInfo(); 322 MRI = &MF.getRegInfo(); 323 324 for (MachineBasicBlock &MBB : MF) 325 Changed |= CopyPropagateBlock(MBB); 326 327 return Changed; 328 } 329