1 //===--------- PPCPreEmitPeephole.cpp - Late peephole optimizations -------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // A pre-emit peephole for catching opportunities introduced by late passes such 10 // as MachineBlockPlacement. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "PPC.h" 15 #include "PPCInstrInfo.h" 16 #include "PPCSubtarget.h" 17 #include "llvm/ADT/DenseMap.h" 18 #include "llvm/ADT/Statistic.h" 19 #include "llvm/CodeGen/LivePhysRegs.h" 20 #include "llvm/CodeGen/MachineBasicBlock.h" 21 #include "llvm/CodeGen/MachineFunctionPass.h" 22 #include "llvm/CodeGen/MachineInstrBuilder.h" 23 #include "llvm/CodeGen/MachineRegisterInfo.h" 24 #include "llvm/Support/CommandLine.h" 25 #include "llvm/ADT/Statistic.h" 26 #include "llvm/Support/Debug.h" 27 28 using namespace llvm; 29 30 #define DEBUG_TYPE "ppc-pre-emit-peephole" 31 32 STATISTIC(NumRRConvertedInPreEmit, 33 "Number of r+r instructions converted to r+i in pre-emit peephole"); 34 STATISTIC(NumRemovedInPreEmit, 35 "Number of instructions deleted in pre-emit peephole"); 36 STATISTIC(NumberOfSelfCopies, 37 "Number of self copy instructions eliminated"); 38 39 static cl::opt<bool> 40 RunPreEmitPeephole("ppc-late-peephole", cl::Hidden, cl::init(true), 41 cl::desc("Run pre-emit peephole optimizations.")); 42 43 namespace { 44 class PPCPreEmitPeephole : public MachineFunctionPass { 45 public: 46 static char ID; 47 PPCPreEmitPeephole() : MachineFunctionPass(ID) { 48 initializePPCPreEmitPeepholePass(*PassRegistry::getPassRegistry()); 49 } 50 51 void getAnalysisUsage(AnalysisUsage &AU) const override { 52 MachineFunctionPass::getAnalysisUsage(AU); 53 } 54 55 MachineFunctionProperties getRequiredProperties() const override { 56 return MachineFunctionProperties().set( 57 MachineFunctionProperties::Property::NoVRegs); 58 } 59 60 // This function removes any redundant load immediates. It has two level 61 // loops - The outer loop finds the load immediates BBI that could be used 62 // to replace following redundancy. The inner loop scans instructions that 63 // after BBI to find redundancy and update kill/dead flags accordingly. If 64 // AfterBBI is the same as BBI, it is redundant, otherwise any instructions 65 // that modify the def register of BBI would break the scanning. 66 // DeadOrKillToUnset is a pointer to the previous operand that had the 67 // kill/dead flag set. It keeps track of the def register of BBI, the use 68 // registers of AfterBBIs and the def registers of AfterBBIs. 69 bool removeRedundantLIs(MachineBasicBlock &MBB, 70 const TargetRegisterInfo *TRI) { 71 LLVM_DEBUG(dbgs() << "Remove redundant load immediates from MBB:\n"; 72 MBB.dump(); dbgs() << "\n"); 73 74 DenseSet<MachineInstr *> InstrsToErase; 75 for (auto BBI = MBB.instr_begin(); BBI != MBB.instr_end(); ++BBI) { 76 // Skip load immediate that is marked to be erased later because it 77 // cannot be used to replace any other instructions. 78 if (InstrsToErase.find(&*BBI) != InstrsToErase.end()) 79 continue; 80 // Skip non-load immediate. 81 unsigned Opc = BBI->getOpcode(); 82 if (Opc != PPC::LI && Opc != PPC::LI8 && Opc != PPC::LIS && 83 Opc != PPC::LIS8) 84 continue; 85 // Skip load immediate, where the operand is a relocation (e.g., $r3 = 86 // LI target-flags(ppc-lo) %const.0). 87 if (!BBI->getOperand(1).isImm()) 88 continue; 89 assert(BBI->getOperand(0).isReg() && 90 "Expected a register for the first operand"); 91 92 LLVM_DEBUG(dbgs() << "Scanning after load immediate: "; BBI->dump();); 93 94 unsigned Reg = BBI->getOperand(0).getReg(); 95 int64_t Imm = BBI->getOperand(1).getImm(); 96 MachineOperand *DeadOrKillToUnset = nullptr; 97 if (BBI->getOperand(0).isDead()) { 98 DeadOrKillToUnset = &BBI->getOperand(0); 99 LLVM_DEBUG(dbgs() << " Kill flag of " << *DeadOrKillToUnset 100 << " from load immediate " << *BBI 101 << " is a unsetting candidate\n"); 102 } 103 // This loop scans instructions after BBI to see if there is any 104 // redundant load immediate. 105 for (auto AfterBBI = std::next(BBI); AfterBBI != MBB.instr_end(); 106 ++AfterBBI) { 107 // Track the operand that kill Reg. We would unset the kill flag of 108 // the operand if there is a following redundant load immediate. 109 int KillIdx = AfterBBI->findRegisterUseOperandIdx(Reg, true, TRI); 110 if (KillIdx != -1) { 111 assert(!DeadOrKillToUnset && "Shouldn't kill same register twice"); 112 DeadOrKillToUnset = &AfterBBI->getOperand(KillIdx); 113 LLVM_DEBUG(dbgs() 114 << " Kill flag of " << *DeadOrKillToUnset << " from " 115 << *AfterBBI << " is a unsetting candidate\n"); 116 } 117 118 if (!AfterBBI->modifiesRegister(Reg, TRI)) 119 continue; 120 assert(DeadOrKillToUnset && 121 "Shouldn't overwrite a register before it is killed"); 122 // Finish scanning because Reg is overwritten by a non-load 123 // instruction. 124 if (AfterBBI->getOpcode() != Opc) 125 break; 126 assert(AfterBBI->getOperand(0).isReg() && 127 "Expected a register for the first operand"); 128 // Finish scanning because Reg is overwritten by a relocation or a 129 // different value. 130 if (!AfterBBI->getOperand(1).isImm() || 131 AfterBBI->getOperand(1).getImm() != Imm) 132 break; 133 134 // It loads same immediate value to the same Reg, which is redundant. 135 // We would unset kill flag in previous Reg usage to extend live range 136 // of Reg first, then remove the redundancy. 137 LLVM_DEBUG(dbgs() << " Unset dead/kill flag of " << *DeadOrKillToUnset 138 << " from " << *DeadOrKillToUnset->getParent()); 139 if (DeadOrKillToUnset->isDef()) 140 DeadOrKillToUnset->setIsDead(false); 141 else 142 DeadOrKillToUnset->setIsKill(false); 143 DeadOrKillToUnset = 144 AfterBBI->findRegisterDefOperand(Reg, true, true, TRI); 145 if (DeadOrKillToUnset) 146 LLVM_DEBUG(dbgs() 147 << " Dead flag of " << *DeadOrKillToUnset << " from " 148 << *AfterBBI << " is a unsetting candidate\n"); 149 InstrsToErase.insert(&*AfterBBI); 150 LLVM_DEBUG(dbgs() << " Remove redundant load immediate: "; 151 AfterBBI->dump()); 152 } 153 } 154 155 for (MachineInstr *MI : InstrsToErase) { 156 MI->eraseFromParent(); 157 } 158 NumRemovedInPreEmit += InstrsToErase.size(); 159 return !InstrsToErase.empty(); 160 } 161 162 bool runOnMachineFunction(MachineFunction &MF) override { 163 if (skipFunction(MF.getFunction()) || !RunPreEmitPeephole) 164 return false; 165 bool Changed = false; 166 const PPCInstrInfo *TII = MF.getSubtarget<PPCSubtarget>().getInstrInfo(); 167 const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo(); 168 SmallVector<MachineInstr *, 4> InstrsToErase; 169 for (MachineBasicBlock &MBB : MF) { 170 Changed |= removeRedundantLIs(MBB, TRI); 171 for (MachineInstr &MI : MBB) { 172 unsigned Opc = MI.getOpcode(); 173 // Detect self copies - these can result from running AADB. 174 if (PPCInstrInfo::isSameClassPhysRegCopy(Opc)) { 175 const MCInstrDesc &MCID = TII->get(Opc); 176 if (MCID.getNumOperands() == 3 && 177 MI.getOperand(0).getReg() == MI.getOperand(1).getReg() && 178 MI.getOperand(0).getReg() == MI.getOperand(2).getReg()) { 179 NumberOfSelfCopies++; 180 LLVM_DEBUG(dbgs() << "Deleting self-copy instruction: "); 181 LLVM_DEBUG(MI.dump()); 182 InstrsToErase.push_back(&MI); 183 continue; 184 } 185 else if (MCID.getNumOperands() == 2 && 186 MI.getOperand(0).getReg() == MI.getOperand(1).getReg()) { 187 NumberOfSelfCopies++; 188 LLVM_DEBUG(dbgs() << "Deleting self-copy instruction: "); 189 LLVM_DEBUG(MI.dump()); 190 InstrsToErase.push_back(&MI); 191 continue; 192 } 193 } 194 MachineInstr *DefMIToErase = nullptr; 195 if (TII->convertToImmediateForm(MI, &DefMIToErase)) { 196 Changed = true; 197 NumRRConvertedInPreEmit++; 198 LLVM_DEBUG(dbgs() << "Converted instruction to imm form: "); 199 LLVM_DEBUG(MI.dump()); 200 if (DefMIToErase) { 201 InstrsToErase.push_back(DefMIToErase); 202 } 203 } 204 } 205 206 // Eliminate conditional branch based on a constant CR bit by 207 // CRSET or CRUNSET. We eliminate the conditional branch or 208 // convert it into an unconditional branch. Also, if the CR bit 209 // is not used by other instructions, we eliminate CRSET as well. 210 auto I = MBB.getFirstInstrTerminator(); 211 if (I == MBB.instr_end()) 212 continue; 213 MachineInstr *Br = &*I; 214 if (Br->getOpcode() != PPC::BC && Br->getOpcode() != PPC::BCn) 215 continue; 216 MachineInstr *CRSetMI = nullptr; 217 unsigned CRBit = Br->getOperand(0).getReg(); 218 unsigned CRReg = getCRFromCRBit(CRBit); 219 bool SeenUse = false; 220 MachineBasicBlock::reverse_iterator It = Br, Er = MBB.rend(); 221 for (It++; It != Er; It++) { 222 if (It->modifiesRegister(CRBit, TRI)) { 223 if ((It->getOpcode() == PPC::CRUNSET || 224 It->getOpcode() == PPC::CRSET) && 225 It->getOperand(0).getReg() == CRBit) 226 CRSetMI = &*It; 227 break; 228 } 229 if (It->readsRegister(CRBit, TRI)) 230 SeenUse = true; 231 } 232 if (!CRSetMI) continue; 233 234 unsigned CRSetOp = CRSetMI->getOpcode(); 235 if ((Br->getOpcode() == PPC::BCn && CRSetOp == PPC::CRSET) || 236 (Br->getOpcode() == PPC::BC && CRSetOp == PPC::CRUNSET)) { 237 // Remove this branch since it cannot be taken. 238 InstrsToErase.push_back(Br); 239 MBB.removeSuccessor(Br->getOperand(1).getMBB()); 240 } 241 else { 242 // This conditional branch is always taken. So, remove all branches 243 // and insert an unconditional branch to the destination of this. 244 MachineBasicBlock::iterator It = Br, Er = MBB.end(); 245 for (; It != Er; It++) { 246 if (It->isDebugInstr()) continue; 247 assert(It->isTerminator() && "Non-terminator after a terminator"); 248 InstrsToErase.push_back(&*It); 249 } 250 if (!MBB.isLayoutSuccessor(Br->getOperand(1).getMBB())) { 251 ArrayRef<MachineOperand> NoCond; 252 TII->insertBranch(MBB, Br->getOperand(1).getMBB(), nullptr, 253 NoCond, Br->getDebugLoc()); 254 } 255 for (auto &Succ : MBB.successors()) 256 if (Succ != Br->getOperand(1).getMBB()) { 257 MBB.removeSuccessor(Succ); 258 break; 259 } 260 } 261 262 // If the CRBit is not used by another instruction, we can eliminate 263 // CRSET/CRUNSET instruction. 264 if (!SeenUse) { 265 // We need to check use of the CRBit in successors. 266 for (auto &SuccMBB : MBB.successors()) 267 if (SuccMBB->isLiveIn(CRBit) || SuccMBB->isLiveIn(CRReg)) { 268 SeenUse = true; 269 break; 270 } 271 if (!SeenUse) 272 InstrsToErase.push_back(CRSetMI); 273 } 274 } 275 for (MachineInstr *MI : InstrsToErase) { 276 LLVM_DEBUG(dbgs() << "PPC pre-emit peephole: erasing instruction: "); 277 LLVM_DEBUG(MI->dump()); 278 MI->eraseFromParent(); 279 NumRemovedInPreEmit++; 280 } 281 return Changed; 282 } 283 }; 284 } 285 286 INITIALIZE_PASS(PPCPreEmitPeephole, DEBUG_TYPE, "PowerPC Pre-Emit Peephole", 287 false, false) 288 char PPCPreEmitPeephole::ID = 0; 289 290 FunctionPass *llvm::createPPCPreEmitPeepholePass() { 291 return new PPCPreEmitPeephole(); 292 } 293