1 //===------------- PPCEarlyReturn.cpp - Form Early Returns ----------------===// 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 pass that form early (predicated) returns. If-conversion handles some of 10 // this, but this pass picks up some remaining cases. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "MCTargetDesc/PPCPredicates.h" 15 #include "PPC.h" 16 #include "PPCInstrBuilder.h" 17 #include "PPCInstrInfo.h" 18 #include "PPCMachineFunctionInfo.h" 19 #include "PPCTargetMachine.h" 20 #include "llvm/ADT/STLExtras.h" 21 #include "llvm/ADT/Statistic.h" 22 #include "llvm/CodeGen/MachineFrameInfo.h" 23 #include "llvm/CodeGen/MachineFunctionPass.h" 24 #include "llvm/CodeGen/MachineInstrBuilder.h" 25 #include "llvm/CodeGen/MachineMemOperand.h" 26 #include "llvm/CodeGen/MachineRegisterInfo.h" 27 #include "llvm/MC/MCAsmInfo.h" 28 #include "llvm/Support/Debug.h" 29 #include "llvm/Support/ErrorHandling.h" 30 #include "llvm/Support/TargetRegistry.h" 31 #include "llvm/Support/raw_ostream.h" 32 33 using namespace llvm; 34 35 #define DEBUG_TYPE "ppc-early-ret" 36 STATISTIC(NumBCLR, "Number of early conditional returns"); 37 STATISTIC(NumBLR, "Number of early returns"); 38 39 namespace { 40 // PPCEarlyReturn pass - For simple functions without epilogue code, move 41 // returns up, and create conditional returns, to avoid unnecessary 42 // branch-to-blr sequences. 43 struct PPCEarlyReturn : public MachineFunctionPass { 44 static char ID; 45 PPCEarlyReturn() : MachineFunctionPass(ID) { 46 initializePPCEarlyReturnPass(*PassRegistry::getPassRegistry()); 47 } 48 49 const TargetInstrInfo *TII; 50 51 protected: 52 bool processBlock(MachineBasicBlock &ReturnMBB) { 53 bool Changed = false; 54 55 MachineBasicBlock::iterator I = ReturnMBB.begin(); 56 I = ReturnMBB.SkipPHIsLabelsAndDebug(I); 57 58 // The block must be essentially empty except for the blr. 59 if (I == ReturnMBB.end() || 60 (I->getOpcode() != PPC::BLR && I->getOpcode() != PPC::BLR8) || 61 I != ReturnMBB.getLastNonDebugInstr()) 62 return Changed; 63 64 SmallVector<MachineBasicBlock*, 8> PredToRemove; 65 for (MachineBasicBlock::pred_iterator PI = ReturnMBB.pred_begin(), 66 PIE = ReturnMBB.pred_end(); PI != PIE; ++PI) { 67 bool OtherReference = false, BlockChanged = false; 68 69 if ((*PI)->empty()) 70 continue; 71 72 for (MachineBasicBlock::iterator J = (*PI)->getLastNonDebugInstr();;) { 73 if (J == (*PI)->end()) 74 break; 75 76 if (J->getOpcode() == PPC::B) { 77 if (J->getOperand(0).getMBB() == &ReturnMBB) { 78 // This is an unconditional branch to the return. Replace the 79 // branch with a blr. 80 BuildMI(**PI, J, J->getDebugLoc(), TII->get(I->getOpcode())) 81 .copyImplicitOps(*I); 82 MachineBasicBlock::iterator K = J--; 83 K->eraseFromParent(); 84 BlockChanged = true; 85 ++NumBLR; 86 continue; 87 } 88 } else if (J->getOpcode() == PPC::BCC) { 89 if (J->getOperand(2).getMBB() == &ReturnMBB) { 90 // This is a conditional branch to the return. Replace the branch 91 // with a bclr. 92 BuildMI(**PI, J, J->getDebugLoc(), TII->get(PPC::BCCLR)) 93 .addImm(J->getOperand(0).getImm()) 94 .addReg(J->getOperand(1).getReg()) 95 .copyImplicitOps(*I); 96 MachineBasicBlock::iterator K = J--; 97 K->eraseFromParent(); 98 BlockChanged = true; 99 ++NumBCLR; 100 continue; 101 } 102 } else if (J->getOpcode() == PPC::BC || J->getOpcode() == PPC::BCn) { 103 if (J->getOperand(1).getMBB() == &ReturnMBB) { 104 // This is a conditional branch to the return. Replace the branch 105 // with a bclr. 106 BuildMI( 107 **PI, J, J->getDebugLoc(), 108 TII->get(J->getOpcode() == PPC::BC ? PPC::BCLR : PPC::BCLRn)) 109 .addReg(J->getOperand(0).getReg()) 110 .copyImplicitOps(*I); 111 MachineBasicBlock::iterator K = J--; 112 K->eraseFromParent(); 113 BlockChanged = true; 114 ++NumBCLR; 115 continue; 116 } 117 } else if (J->isBranch()) { 118 if (J->isIndirectBranch()) { 119 if (ReturnMBB.hasAddressTaken()) 120 OtherReference = true; 121 } else 122 for (unsigned i = 0; i < J->getNumOperands(); ++i) 123 if (J->getOperand(i).isMBB() && 124 J->getOperand(i).getMBB() == &ReturnMBB) 125 OtherReference = true; 126 } else if (!J->isTerminator() && !J->isDebugInstr()) 127 break; 128 129 if (J == (*PI)->begin()) 130 break; 131 132 --J; 133 } 134 135 if ((*PI)->canFallThrough() && (*PI)->isLayoutSuccessor(&ReturnMBB)) 136 OtherReference = true; 137 138 // Predecessors are stored in a vector and can't be removed here. 139 if (!OtherReference && BlockChanged) { 140 PredToRemove.push_back(*PI); 141 } 142 143 if (BlockChanged) 144 Changed = true; 145 } 146 147 for (unsigned i = 0, ie = PredToRemove.size(); i != ie; ++i) 148 PredToRemove[i]->removeSuccessor(&ReturnMBB, true); 149 150 if (Changed && !ReturnMBB.hasAddressTaken()) { 151 // We now might be able to merge this blr-only block into its 152 // by-layout predecessor. 153 if (ReturnMBB.pred_size() == 1) { 154 MachineBasicBlock &PrevMBB = **ReturnMBB.pred_begin(); 155 if (PrevMBB.isLayoutSuccessor(&ReturnMBB) && PrevMBB.canFallThrough()) { 156 // Move the blr into the preceding block. 157 PrevMBB.splice(PrevMBB.end(), &ReturnMBB, I); 158 PrevMBB.removeSuccessor(&ReturnMBB, true); 159 } 160 } 161 162 if (ReturnMBB.pred_empty()) 163 ReturnMBB.eraseFromParent(); 164 } 165 166 return Changed; 167 } 168 169 public: 170 bool runOnMachineFunction(MachineFunction &MF) override { 171 if (skipFunction(MF.getFunction())) 172 return false; 173 174 TII = MF.getSubtarget().getInstrInfo(); 175 176 bool Changed = false; 177 178 // If the function does not have at least two blocks, then there is 179 // nothing to do. 180 if (MF.size() < 2) 181 return Changed; 182 183 // We can't use a range-based for loop due to clobbering the iterator. 184 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E;) { 185 MachineBasicBlock &B = *I++; 186 Changed |= processBlock(B); 187 } 188 189 return Changed; 190 } 191 192 MachineFunctionProperties getRequiredProperties() const override { 193 return MachineFunctionProperties().set( 194 MachineFunctionProperties::Property::NoVRegs); 195 } 196 197 void getAnalysisUsage(AnalysisUsage &AU) const override { 198 MachineFunctionPass::getAnalysisUsage(AU); 199 } 200 }; 201 } 202 203 INITIALIZE_PASS(PPCEarlyReturn, DEBUG_TYPE, 204 "PowerPC Early-Return Creation", false, false) 205 206 char PPCEarlyReturn::ID = 0; 207 FunctionPass* 208 llvm::createPPCEarlyReturnPass() { return new PPCEarlyReturn(); } 209