1 //===-- PPCBranchSelector.cpp - Emit long conditional branches ------------===// 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 file contains a pass that scans a machine function to determine which 11 // conditional branches need more than 16 bits of displacement to reach their 12 // target basic block. It does this in two passes; a calculation of basic block 13 // positions pass, and a branch pseudo op to machine branch opcode pass. This 14 // pass should be run last, just before the assembly printer. 15 // 16 //===----------------------------------------------------------------------===// 17 18 #include "PPC.h" 19 #include "MCTargetDesc/PPCPredicates.h" 20 #include "PPCInstrBuilder.h" 21 #include "PPCInstrInfo.h" 22 #include "llvm/ADT/Statistic.h" 23 #include "llvm/CodeGen/MachineFunctionPass.h" 24 #include "llvm/Support/MathExtras.h" 25 #include "llvm/Target/TargetMachine.h" 26 #include "llvm/Target/TargetSubtargetInfo.h" 27 using namespace llvm; 28 29 #define DEBUG_TYPE "ppc-branch-select" 30 31 STATISTIC(NumExpanded, "Number of branches expanded to long format"); 32 33 namespace llvm { 34 void initializePPCBSelPass(PassRegistry&); 35 } 36 37 namespace { 38 struct PPCBSel : public MachineFunctionPass { 39 static char ID; 40 PPCBSel() : MachineFunctionPass(ID) { 41 initializePPCBSelPass(*PassRegistry::getPassRegistry()); 42 } 43 44 /// BlockSizes - The sizes of the basic blocks in the function. 45 std::vector<unsigned> BlockSizes; 46 47 bool runOnMachineFunction(MachineFunction &Fn) override; 48 49 const char *getPassName() const override { 50 return "PowerPC Branch Selector"; 51 } 52 }; 53 char PPCBSel::ID = 0; 54 } 55 56 INITIALIZE_PASS(PPCBSel, "ppc-branch-select", "PowerPC Branch Selector", 57 false, false) 58 59 /// createPPCBranchSelectionPass - returns an instance of the Branch Selection 60 /// Pass 61 /// 62 FunctionPass *llvm::createPPCBranchSelectionPass() { 63 return new PPCBSel(); 64 } 65 66 bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) { 67 const PPCInstrInfo *TII = 68 static_cast<const PPCInstrInfo *>(Fn.getSubtarget().getInstrInfo()); 69 // Give the blocks of the function a dense, in-order, numbering. 70 Fn.RenumberBlocks(); 71 BlockSizes.resize(Fn.getNumBlockIDs()); 72 73 auto GetAlignmentAdjustment = 74 [TII](MachineBasicBlock &MBB, unsigned Offset) -> unsigned { 75 unsigned Align = MBB.getAlignment(); 76 if (!Align) 77 return 0; 78 79 unsigned AlignAmt = 1 << Align; 80 unsigned ParentAlign = MBB.getParent()->getAlignment(); 81 82 if (Align <= ParentAlign) 83 return OffsetToAlignment(Offset, AlignAmt); 84 85 // The alignment of this MBB is larger than the function's alignment, so we 86 // can't tell whether or not it will insert nops. Assume that it will. 87 return AlignAmt + OffsetToAlignment(Offset, AlignAmt); 88 }; 89 90 // Measure each MBB and compute a size for the entire function. 91 unsigned FuncSize = 0; 92 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E; 93 ++MFI) { 94 MachineBasicBlock *MBB = &*MFI; 95 96 // The end of the previous block may have extra nops if this block has an 97 // alignment requirement. 98 if (MBB->getNumber() > 0) { 99 unsigned AlignExtra = GetAlignmentAdjustment(*MBB, FuncSize); 100 BlockSizes[MBB->getNumber()-1] += AlignExtra; 101 FuncSize += AlignExtra; 102 } 103 104 unsigned BlockSize = 0; 105 for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end(); 106 MBBI != EE; ++MBBI) 107 BlockSize += TII->GetInstSizeInBytes(MBBI); 108 109 BlockSizes[MBB->getNumber()] = BlockSize; 110 FuncSize += BlockSize; 111 } 112 113 // If the entire function is smaller than the displacement of a branch field, 114 // we know we don't need to shrink any branches in this function. This is a 115 // common case. 116 if (FuncSize < (1 << 15)) { 117 BlockSizes.clear(); 118 return false; 119 } 120 121 // For each conditional branch, if the offset to its destination is larger 122 // than the offset field allows, transform it into a long branch sequence 123 // like this: 124 // short branch: 125 // bCC MBB 126 // long branch: 127 // b!CC $PC+8 128 // b MBB 129 // 130 bool MadeChange = true; 131 bool EverMadeChange = false; 132 while (MadeChange) { 133 // Iteratively expand branches until we reach a fixed point. 134 MadeChange = false; 135 136 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E; 137 ++MFI) { 138 MachineBasicBlock &MBB = *MFI; 139 unsigned MBBStartOffset = 0; 140 for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); 141 I != E; ++I) { 142 MachineBasicBlock *Dest = nullptr; 143 if (I->getOpcode() == PPC::BCC && !I->getOperand(2).isImm()) 144 Dest = I->getOperand(2).getMBB(); 145 else if ((I->getOpcode() == PPC::BC || I->getOpcode() == PPC::BCn) && 146 !I->getOperand(1).isImm()) 147 Dest = I->getOperand(1).getMBB(); 148 else if ((I->getOpcode() == PPC::BDNZ8 || I->getOpcode() == PPC::BDNZ || 149 I->getOpcode() == PPC::BDZ8 || I->getOpcode() == PPC::BDZ) && 150 !I->getOperand(0).isImm()) 151 Dest = I->getOperand(0).getMBB(); 152 153 if (!Dest) { 154 MBBStartOffset += TII->GetInstSizeInBytes(I); 155 continue; 156 } 157 158 // Determine the offset from the current branch to the destination 159 // block. 160 int BranchSize; 161 if (Dest->getNumber() <= MBB.getNumber()) { 162 // If this is a backwards branch, the delta is the offset from the 163 // start of this block to this branch, plus the sizes of all blocks 164 // from this block to the dest. 165 BranchSize = MBBStartOffset; 166 167 for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i) 168 BranchSize += BlockSizes[i]; 169 } else { 170 // Otherwise, add the size of the blocks between this block and the 171 // dest to the number of bytes left in this block. 172 BranchSize = -MBBStartOffset; 173 174 for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i) 175 BranchSize += BlockSizes[i]; 176 } 177 178 // If this branch is in range, ignore it. 179 if (isInt<16>(BranchSize)) { 180 MBBStartOffset += 4; 181 continue; 182 } 183 184 // Otherwise, we have to expand it to a long branch. 185 MachineInstr *OldBranch = I; 186 DebugLoc dl = OldBranch->getDebugLoc(); 187 188 if (I->getOpcode() == PPC::BCC) { 189 // The BCC operands are: 190 // 0. PPC branch predicate 191 // 1. CR register 192 // 2. Target MBB 193 PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm(); 194 unsigned CRReg = I->getOperand(1).getReg(); 195 196 // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition. 197 BuildMI(MBB, I, dl, TII->get(PPC::BCC)) 198 .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2); 199 } else if (I->getOpcode() == PPC::BC) { 200 unsigned CRBit = I->getOperand(0).getReg(); 201 BuildMI(MBB, I, dl, TII->get(PPC::BCn)).addReg(CRBit).addImm(2); 202 } else if (I->getOpcode() == PPC::BCn) { 203 unsigned CRBit = I->getOperand(0).getReg(); 204 BuildMI(MBB, I, dl, TII->get(PPC::BC)).addReg(CRBit).addImm(2); 205 } else if (I->getOpcode() == PPC::BDNZ) { 206 BuildMI(MBB, I, dl, TII->get(PPC::BDZ)).addImm(2); 207 } else if (I->getOpcode() == PPC::BDNZ8) { 208 BuildMI(MBB, I, dl, TII->get(PPC::BDZ8)).addImm(2); 209 } else if (I->getOpcode() == PPC::BDZ) { 210 BuildMI(MBB, I, dl, TII->get(PPC::BDNZ)).addImm(2); 211 } else if (I->getOpcode() == PPC::BDZ8) { 212 BuildMI(MBB, I, dl, TII->get(PPC::BDNZ8)).addImm(2); 213 } else { 214 llvm_unreachable("Unhandled branch type!"); 215 } 216 217 // Uncond branch to the real destination. 218 I = BuildMI(MBB, I, dl, TII->get(PPC::B)).addMBB(Dest); 219 220 // Remove the old branch from the function. 221 OldBranch->eraseFromParent(); 222 223 // Remember that this instruction is 8-bytes, increase the size of the 224 // block by 4, remember to iterate. 225 BlockSizes[MBB.getNumber()] += 4; 226 MBBStartOffset += 8; 227 ++NumExpanded; 228 MadeChange = true; 229 } 230 } 231 EverMadeChange |= MadeChange; 232 } 233 234 BlockSizes.clear(); 235 return true; 236 } 237 238
