lib/Target/PowerPC/PPCBranchSelector.cpp

//===-- 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.
//
//===----------------------------------------------------------------------===//

#include "PPC.h"
#include "PPCInstrBuilder.h"
#include "PPCInstrInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetAsmInfo.h"
#include <map>
using namespace llvm;

namespace {
  struct VISIBILITY_HIDDEN PPCBSel : public MachineFunctionPass {
    /// OffsetMap - Mapping between BB and byte offset from start of function.
    /// TODO: replace this with a vector, using the MBB idx as the key.
    std::map<MachineBasicBlock*, unsigned> OffsetMap;

    virtual bool runOnMachineFunction(MachineFunction &Fn);

    virtual const char *getPassName() const {
      return "PowerPC Branch Selection";
    }
  };
}

/// createPPCBranchSelectionPass - returns an instance of the Branch Selection
/// Pass
///
FunctionPass *llvm::createPPCBranchSelectionPass() {
  return new PPCBSel();
}

/// getNumBytesForInstruction - Return the number of bytes of code the specified
/// instruction may be.  This returns the maximum number of bytes.
///
static unsigned getNumBytesForInstruction(MachineInstr *MI) {
  switch (MI->getOpcode()) {
  case PPC::COND_BRANCH:
    // while this will be 4 most of the time, if we emit 8 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 8;
  case PPC::IMPLICIT_DEF_GPRC: // no asm emitted
  case PPC::IMPLICIT_DEF_G8RC: // no asm emitted
  case PPC::IMPLICIT_DEF_F4:   // no asm emitted
  case PPC::IMPLICIT_DEF_F8:   // no asm emitted
    return 0;
  case PPC::INLINEASM: {       // Inline Asm: Variable size.
    MachineFunction *MF = MI->getParent()->getParent();
    const char *AsmStr = MI->getOperand(0).getSymbolName();
    return MF->getTarget().getTargetAsmInfo()->getInlineAsmLength(AsmStr);
  }
  default:
    return 4; // PowerPC instructions are all 4 bytes
  }
}


bool PPCBSel::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 += getNumBytesForInstruction(MBBI);
  }
  
  // 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
  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 ByteSize = getNumBytesForInstruction(MBBI);
      if (MBBI->getOpcode() == 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();
        
        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 {
          // Long branch, skip next branch instruction (i.e. $PC+8).
          BuildMI(*MBB, MBBJ, Inverted, 2).addReg(CRReg).addImm(2);
          BuildMI(*MBB, MBBJ, PPC::B, 1).addMBB(trueMBB);
        }
        
        // 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 += ByteSize;
    }
  }
  
  OffsetMap.clear();
  return true;
}