//===-- PPCBranchSelector.cpp - Emit long conditional branches-----*- C++ -*-=// // // The LLVM Compiler Infrastructure // // This file was developed by Nate Baegeman and is distributed under the // University of Illinois Open Source License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // This file contains a pass that scans a machine function to determine which // conditional branches need more than 16 bits of displacement to reach their // target basic block. It does this in two passes; a calculation of basic block // positions pass, and a branch psuedo op to machine branch opcode pass. This // pass should be run last, just before the assembly printer. // //===----------------------------------------------------------------------===// #define DEBUG_TYPE "bsel" #include "PPC.h" #include "PPCInstrBuilder.h" #include "PPCInstrInfo.h" #include "llvm/CodeGen/MachineFunctionPass.h" #include "llvm/Support/Debug.h" #include <map> using namespace llvm; namespace { struct BSel : public MachineFunctionPass { // OffsetMap - Mapping between BB and byte offset from start of function std::map<MachineBasicBlock*, unsigned> OffsetMap; /// bytesForOpcode - A convenience function for totalling up the number of /// bytes in a basic block. /// static unsigned bytesForOpcode(unsigned opcode) { switch (opcode) { case PPC::COND_BRANCH: // while this will be 4 most of the time, if we emit 12 it is just a // minor pessimization that saves us from having to worry about // keeping the offsets up to date later when we emit long branch glue. return 12; case PPC::IMPLICIT_DEF_GPR: // no asm emitted case PPC::IMPLICIT_DEF_F4: // no asm emitted case PPC::IMPLICIT_DEF_F8: // no asm emitted return 0; default: break; } return 4; // PowerPC instructions are all 4 bytes } virtual bool runOnMachineFunction(MachineFunction &Fn) { // Running total of instructions encountered since beginning of function unsigned ByteCount = 0; // For each MBB, add its offset to the offset map, and count up its // instructions for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E; ++MFI) { MachineBasicBlock *MBB = MFI; OffsetMap[MBB] = ByteCount; for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end(); MBBI != EE; ++MBBI) ByteCount += bytesForOpcode(MBBI->getOpcode()); } // We're about to run over the MBB's again, so reset the ByteCount ByteCount = 0; // For each MBB, find the conditional branch pseudo instructions, and // calculate the difference between the target MBB and the current ICount // to decide whether or not to emit a short or long branch. // // short branch: // bCC .L_TARGET_MBB // // long branch: // bInverseCC $PC+8 // b .L_TARGET_MBB // b .L_FALLTHROUGH_MBB for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E; ++MFI) { MachineBasicBlock *MBB = MFI; for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end(); MBBI != EE; ++MBBI) { // We may end up deleting the MachineInstr that MBBI points to, so // remember its opcode now so we can refer to it after calling erase() unsigned OpcodeToReplace = MBBI->getOpcode(); if (OpcodeToReplace == PPC::COND_BRANCH) { MachineBasicBlock::iterator MBBJ = MBBI; ++MBBJ; // condbranch operands: // 0. CR0 register // 1. bc opcode // 2. target MBB // 3. fallthrough MBB MachineBasicBlock *trueMBB = MBBI->getOperand(2).getMachineBasicBlock(); MachineBasicBlock *falseMBB = MBBI->getOperand(3).getMachineBasicBlock(); int Displacement = OffsetMap[trueMBB] - ByteCount; unsigned Opcode = MBBI->getOperand(1).getImmedValue(); unsigned CRReg = MBBI->getOperand(0).getReg(); unsigned Inverted = PPCInstrInfo::invertPPCBranchOpcode(Opcode); if (Displacement >= -32768 && Displacement <= 32767) { BuildMI(*MBB, MBBJ, Opcode, 2).addReg(CRReg).addMBB(trueMBB); } else { BuildMI(*MBB, MBBJ, Inverted, 2).addReg(CRReg).addSImm(8); BuildMI(*MBB, MBBJ, PPC::B, 1).addMBB(trueMBB); BuildMI(*MBB, MBBJ, PPC::B, 1).addMBB(falseMBB); } // Erase the psuedo COND_BRANCH instruction, and then back up the // iterator so that when the for loop increments it, we end up in // the correct place rather than iterating off the end. MBB->erase(MBBI); MBBI = --MBBJ; } ByteCount += bytesForOpcode(OpcodeToReplace); } } OffsetMap.clear(); return true; } virtual const char *getPassName() const { return "PowerPC Branch Selection"; } }; } /// createPPCBranchSelectionPass - returns an instance of the Branch Selection /// Pass /// FunctionPass *llvm::createPPCBranchSelectionPass() { return new BSel(); }