LLVM API Documentation
00001 //===-- PPCBranchSelector.cpp - Emit long conditional branches-----*- C++ -*-=// 00002 // 00003 // The LLVM Compiler Infrastructure 00004 // 00005 // This file was developed by Nate Baegeman and is distributed under the 00006 // University of Illinois Open Source License. See LICENSE.TXT for details. 00007 // 00008 //===----------------------------------------------------------------------===// 00009 // 00010 // This file contains a pass that scans a machine function to determine which 00011 // conditional branches need more than 16 bits of displacement to reach their 00012 // target basic block. It does this in two passes; a calculation of basic block 00013 // positions pass, and a branch psuedo op to machine branch opcode pass. This 00014 // pass should be run last, just before the assembly printer. 00015 // 00016 //===----------------------------------------------------------------------===// 00017 00018 #include "PPC.h" 00019 #include "PPCInstrBuilder.h" 00020 #include "PPCInstrInfo.h" 00021 #include "llvm/CodeGen/MachineFunctionPass.h" 00022 #include <map> 00023 using namespace llvm; 00024 00025 namespace { 00026 struct PPCBSel : public MachineFunctionPass { 00027 // OffsetMap - Mapping between BB and byte offset from start of function 00028 std::map<MachineBasicBlock*, unsigned> OffsetMap; 00029 00030 virtual bool runOnMachineFunction(MachineFunction &Fn); 00031 00032 virtual const char *getPassName() const { 00033 return "PowerPC Branch Selection"; 00034 } 00035 }; 00036 } 00037 00038 /// createPPCBranchSelectionPass - returns an instance of the Branch Selection 00039 /// Pass 00040 /// 00041 FunctionPass *llvm::createPPCBranchSelectionPass() { 00042 return new PPCBSel(); 00043 } 00044 00045 /// getNumBytesForInstruction - Return the number of bytes of code the specified 00046 /// instruction may be. This returns the maximum number of bytes. 00047 /// 00048 static unsigned getNumBytesForInstruction(MachineInstr *MI) { 00049 switch (MI->getOpcode()) { 00050 case PPC::COND_BRANCH: 00051 // while this will be 4 most of the time, if we emit 8 it is just a 00052 // minor pessimization that saves us from having to worry about 00053 // keeping the offsets up to date later when we emit long branch glue. 00054 return 8; 00055 case PPC::IMPLICIT_DEF_GPR: // no asm emitted 00056 case PPC::IMPLICIT_DEF_F4: // no asm emitted 00057 case PPC::IMPLICIT_DEF_F8: // no asm emitted 00058 return 0; 00059 case PPC::INLINEASM: // Inline Asm: Variable size. 00060 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) 00061 if (MI->getOperand(i).isExternalSymbol()) { 00062 const char *AsmStr = MI->getOperand(i).getSymbolName(); 00063 // Count the number of newline's in the asm string. 00064 unsigned NumInstrs = 0; 00065 for (; *AsmStr; ++AsmStr) 00066 NumInstrs += *AsmStr == '\n'; 00067 return NumInstrs*4; 00068 } 00069 assert(0 && "INLINEASM didn't have format string??"); 00070 default: 00071 return 4; // PowerPC instructions are all 4 bytes 00072 } 00073 } 00074 00075 00076 bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) { 00077 // Running total of instructions encountered since beginning of function 00078 unsigned ByteCount = 0; 00079 00080 // For each MBB, add its offset to the offset map, and count up its 00081 // instructions 00082 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E; 00083 ++MFI) { 00084 MachineBasicBlock *MBB = MFI; 00085 OffsetMap[MBB] = ByteCount; 00086 00087 for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end(); 00088 MBBI != EE; ++MBBI) 00089 ByteCount += getNumBytesForInstruction(MBBI); 00090 } 00091 00092 // We're about to run over the MBB's again, so reset the ByteCount 00093 ByteCount = 0; 00094 00095 // For each MBB, find the conditional branch pseudo instructions, and 00096 // calculate the difference between the target MBB and the current ICount 00097 // to decide whether or not to emit a short or long branch. 00098 // 00099 // short branch: 00100 // bCC .L_TARGET_MBB 00101 // 00102 // long branch: 00103 // bInverseCC $PC+8 00104 // b .L_TARGET_MBB 00105 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E; 00106 ++MFI) { 00107 MachineBasicBlock *MBB = MFI; 00108 00109 for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end(); 00110 MBBI != EE; ++MBBI) { 00111 // We may end up deleting the MachineInstr that MBBI points to, so 00112 // remember its opcode now so we can refer to it after calling erase() 00113 unsigned ByteSize = getNumBytesForInstruction(MBBI); 00114 if (MBBI->getOpcode() == PPC::COND_BRANCH) { 00115 MachineBasicBlock::iterator MBBJ = MBBI; 00116 ++MBBJ; 00117 00118 // condbranch operands: 00119 // 0. CR0 register 00120 // 1. bc opcode 00121 // 2. target MBB 00122 // 3. fallthrough MBB 00123 MachineBasicBlock *trueMBB = 00124 MBBI->getOperand(2).getMachineBasicBlock(); 00125 00126 int Displacement = OffsetMap[trueMBB] - ByteCount; 00127 unsigned Opcode = MBBI->getOperand(1).getImmedValue(); 00128 unsigned CRReg = MBBI->getOperand(0).getReg(); 00129 unsigned Inverted = PPCInstrInfo::invertPPCBranchOpcode(Opcode); 00130 00131 if (Displacement >= -32768 && Displacement <= 32767) { 00132 BuildMI(*MBB, MBBJ, Opcode, 2).addReg(CRReg).addMBB(trueMBB); 00133 } else { 00134 BuildMI(*MBB, MBBJ, Inverted, 2).addReg(CRReg).addSImm(8); 00135 BuildMI(*MBB, MBBJ, PPC::B, 1).addMBB(trueMBB); 00136 } 00137 00138 // Erase the psuedo COND_BRANCH instruction, and then back up the 00139 // iterator so that when the for loop increments it, we end up in 00140 // the correct place rather than iterating off the end. 00141 MBB->erase(MBBI); 00142 MBBI = --MBBJ; 00143 } 00144 ByteCount += ByteSize; 00145 } 00146 } 00147 00148 OffsetMap.clear(); 00149 return true; 00150 } 00151