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 "llvm/Support/Visibility.h" 00023 #include <map> 00024 using namespace llvm; 00025 00026 namespace { 00027 struct VISIBILITY_HIDDEN PPCBSel : public MachineFunctionPass { 00028 // OffsetMap - Mapping between BB and byte offset from start of function 00029 std::map<MachineBasicBlock*, unsigned> OffsetMap; 00030 00031 virtual bool runOnMachineFunction(MachineFunction &Fn); 00032 00033 virtual const char *getPassName() const { 00034 return "PowerPC Branch Selection"; 00035 } 00036 }; 00037 } 00038 00039 /// createPPCBranchSelectionPass - returns an instance of the Branch Selection 00040 /// Pass 00041 /// 00042 FunctionPass *llvm::createPPCBranchSelectionPass() { 00043 return new PPCBSel(); 00044 } 00045 00046 /// getNumBytesForInstruction - Return the number of bytes of code the specified 00047 /// instruction may be. This returns the maximum number of bytes. 00048 /// 00049 static unsigned getNumBytesForInstruction(MachineInstr *MI) { 00050 switch (MI->getOpcode()) { 00051 case PPC::COND_BRANCH: 00052 // while this will be 4 most of the time, if we emit 8 it is just a 00053 // minor pessimization that saves us from having to worry about 00054 // keeping the offsets up to date later when we emit long branch glue. 00055 return 8; 00056 case PPC::IMPLICIT_DEF_GPRC: // no asm emitted 00057 case PPC::IMPLICIT_DEF_G8RC: // no asm emitted 00058 case PPC::IMPLICIT_DEF_F4: // no asm emitted 00059 case PPC::IMPLICIT_DEF_F8: // no asm emitted 00060 return 0; 00061 case PPC::INLINEASM: // Inline Asm: Variable size. 00062 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) 00063 if (MI->getOperand(i).isExternalSymbol()) { 00064 const char *AsmStr = MI->getOperand(i).getSymbolName(); 00065 // Count the number of newline's in the asm string. 00066 unsigned NumInstrs = 0; 00067 for (; *AsmStr; ++AsmStr) 00068 NumInstrs += *AsmStr == '\n'; 00069 return NumInstrs*4; 00070 } 00071 assert(0 && "INLINEASM didn't have format string??"); 00072 default: 00073 return 4; // PowerPC instructions are all 4 bytes 00074 } 00075 } 00076 00077 00078 bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) { 00079 // Running total of instructions encountered since beginning of function 00080 unsigned ByteCount = 0; 00081 00082 // For each MBB, add its offset to the offset map, and count up its 00083 // instructions 00084 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E; 00085 ++MFI) { 00086 MachineBasicBlock *MBB = MFI; 00087 OffsetMap[MBB] = ByteCount; 00088 00089 for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end(); 00090 MBBI != EE; ++MBBI) 00091 ByteCount += getNumBytesForInstruction(MBBI); 00092 } 00093 00094 // We're about to run over the MBB's again, so reset the ByteCount 00095 ByteCount = 0; 00096 00097 // For each MBB, find the conditional branch pseudo instructions, and 00098 // calculate the difference between the target MBB and the current ICount 00099 // to decide whether or not to emit a short or long branch. 00100 // 00101 // short branch: 00102 // bCC .L_TARGET_MBB 00103 // 00104 // long branch: 00105 // bInverseCC $PC+8 00106 // b .L_TARGET_MBB 00107 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E; 00108 ++MFI) { 00109 MachineBasicBlock *MBB = MFI; 00110 00111 for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end(); 00112 MBBI != EE; ++MBBI) { 00113 // We may end up deleting the MachineInstr that MBBI points to, so 00114 // remember its opcode now so we can refer to it after calling erase() 00115 unsigned ByteSize = getNumBytesForInstruction(MBBI); 00116 if (MBBI->getOpcode() == PPC::COND_BRANCH) { 00117 MachineBasicBlock::iterator MBBJ = MBBI; 00118 ++MBBJ; 00119 00120 // condbranch operands: 00121 // 0. CR0 register 00122 // 1. bc opcode 00123 // 2. target MBB 00124 // 3. fallthrough MBB 00125 MachineBasicBlock *trueMBB = 00126 MBBI->getOperand(2).getMachineBasicBlock(); 00127 00128 int Displacement = OffsetMap[trueMBB] - ByteCount; 00129 unsigned Opcode = MBBI->getOperand(1).getImmedValue(); 00130 unsigned CRReg = MBBI->getOperand(0).getReg(); 00131 unsigned Inverted = PPCInstrInfo::invertPPCBranchOpcode(Opcode); 00132 00133 if (Displacement >= -32768 && Displacement <= 32767) { 00134 BuildMI(*MBB, MBBJ, Opcode, 2).addReg(CRReg).addMBB(trueMBB); 00135 } else { 00136 BuildMI(*MBB, MBBJ, Inverted, 2).addReg(CRReg).addImm(8); 00137 BuildMI(*MBB, MBBJ, PPC::B, 1).addMBB(trueMBB); 00138 } 00139 00140 // Erase the psuedo COND_BRANCH instruction, and then back up the 00141 // iterator so that when the for loop increments it, we end up in 00142 // the correct place rather than iterating off the end. 00143 MBB->erase(MBBI); 00144 MBBI = --MBBJ; 00145 } 00146 ByteCount += ByteSize; 00147 } 00148 } 00149 00150 OffsetMap.clear(); 00151 return true; 00152 } 00153