path: root/lib/CodeGen/LiveVariables.cpp

//===-- LiveVariables.cpp - Live Variable Analysis for Machine Code -------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the LiveVariable analysis pass.  For each machine
// instruction in the function, this pass calculates the set of registers that
// are immediately dead after the instruction (i.e., the instruction calculates
// the value, but it is never used) and the set of registers that are used by
// the instruction, but are never used after the instruction (i.e., they are
// killed).
//
// This class computes live variables using are sparse implementation based on
// the machine code SSA form.  This class computes live variable information for
// each virtual and _register allocatable_ physical register in a function.  It
// uses the dominance properties of SSA form to efficiently compute live
// variables for virtual registers, and assumes that physical registers are only
// live within a single basic block (allowing it to do a single local analysis
// to resolve physical register lifetimes in each basic block).  If a physical
// register is not register allocatable, it is not tracked.  This is useful for
// things like the stack pointer and condition codes.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/LiveVariables.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/Target/MRegisterInfo.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Config/alloca.h"
#include <algorithm>
#include <iostream>
using namespace llvm;

static RegisterPass<LiveVariables> X("livevars", "Live Variable Analysis");

void LiveVariables::VarInfo::dump() const {
  std::cerr << "Register Defined by: ";
  if (DefInst) 
    std::cerr << *DefInst;
  else
    std::cerr << "<null>\n";
  std::cerr << "  Alive in blocks: ";
  for (unsigned i = 0, e = AliveBlocks.size(); i != e; ++i)
    if (AliveBlocks[i]) std::cerr << i << ", ";
  std::cerr << "\n  Killed by:";
  if (Kills.empty())
    std::cerr << " No instructions.\n";
  else {
    for (unsigned i = 0, e = Kills.size(); i != e; ++i)
      std::cerr << "\n    #" << i << ": " << *Kills[i];
    std::cerr << "\n";
  }
}

LiveVariables::VarInfo &LiveVariables::getVarInfo(unsigned RegIdx) {
  assert(MRegisterInfo::isVirtualRegister(RegIdx) &&
         "getVarInfo: not a virtual register!");
  RegIdx -= MRegisterInfo::FirstVirtualRegister;
  if (RegIdx >= VirtRegInfo.size()) {
    if (RegIdx >= 2*VirtRegInfo.size())
      VirtRegInfo.resize(RegIdx*2);
    else
      VirtRegInfo.resize(2*VirtRegInfo.size());
  }
  return VirtRegInfo[RegIdx];
}

bool LiveVariables::KillsRegister(MachineInstr *MI, unsigned Reg) const {
  std::map<MachineInstr*, std::vector<unsigned> >::const_iterator I = 
  RegistersKilled.find(MI);
  if (I == RegistersKilled.end()) return false;
  
  // Do a binary search, as these lists can grow pretty big, particularly for
  // call instructions on targets with lots of call-clobbered registers.
  return std::binary_search(I->second.begin(), I->second.end(), Reg);
}

bool LiveVariables::RegisterDefIsDead(MachineInstr *MI, unsigned Reg) const {
  std::map<MachineInstr*, std::vector<unsigned> >::const_iterator I = 
  RegistersDead.find(MI);
  if (I == RegistersDead.end()) return false;
  
  // Do a binary search, as these lists can grow pretty big, particularly for
  // call instructions on targets with lots of call-clobbered registers.
  return std::binary_search(I->second.begin(), I->second.end(), Reg);
}


void LiveVariables::MarkVirtRegAliveInBlock(VarInfo &VRInfo,
                                            MachineBasicBlock *MBB) {
  unsigned BBNum = MBB->getNumber();

  // Check to see if this basic block is one of the killing blocks.  If so,
  // remove it...
  for (unsigned i = 0, e = VRInfo.Kills.size(); i != e; ++i)
    if (VRInfo.Kills[i]->getParent() == MBB) {
      VRInfo.Kills.erase(VRInfo.Kills.begin()+i);  // Erase entry
      break;
    }

  if (MBB == VRInfo.DefInst->getParent()) return;  // Terminate recursion

  if (VRInfo.AliveBlocks.size() <= BBNum)
    VRInfo.AliveBlocks.resize(BBNum+1);  // Make space...

  if (VRInfo.AliveBlocks[BBNum])
    return;  // We already know the block is live

  // Mark the variable known alive in this bb
  VRInfo.AliveBlocks[BBNum] = true;

  for (MachineBasicBlock::const_pred_iterator PI = MBB->pred_begin(),
         E = MBB->pred_end(); PI != E; ++PI)
    MarkVirtRegAliveInBlock(VRInfo, *PI);
}

void LiveVariables::HandleVirtRegUse(VarInfo &VRInfo, MachineBasicBlock *MBB,
                                     MachineInstr *MI) {
  assert(VRInfo.DefInst && "Register use before def!");

  // Check to see if this basic block is already a kill block...
  if (!VRInfo.Kills.empty() && VRInfo.Kills.back()->getParent() == MBB) {
    // Yes, this register is killed in this basic block already.  Increase the
    // live range by updating the kill instruction.
    VRInfo.Kills.back() = MI;
    return;
  }

#ifndef NDEBUG
  for (unsigned i = 0, e = VRInfo.Kills.size(); i != e; ++i)
    assert(VRInfo.Kills[i]->getParent() != MBB && "entry should be at end!");
#endif

  assert(MBB != VRInfo.DefInst->getParent() &&
         "Should have kill for defblock!");

  // Add a new kill entry for this basic block.
  VRInfo.Kills.push_back(MI);

  // Update all dominating blocks to mark them known live.
  for (MachineBasicBlock::const_pred_iterator PI = MBB->pred_begin(),
         E = MBB->pred_end(); PI != E; ++PI)
    MarkVirtRegAliveInBlock(VRInfo, *PI);
}

void LiveVariables::HandlePhysRegUse(unsigned Reg, MachineInstr *MI) {
  PhysRegInfo[Reg] = MI;
  PhysRegUsed[Reg] = true;

  for (const unsigned *AliasSet = RegInfo->getAliasSet(Reg);
       unsigned Alias = *AliasSet; ++AliasSet) {
    PhysRegInfo[Alias] = MI;
    PhysRegUsed[Alias] = true;
  }
}

void LiveVariables::HandlePhysRegDef(unsigned Reg, MachineInstr *MI) {
  // Does this kill a previous version of this register?
  if (MachineInstr *LastUse = PhysRegInfo[Reg]) {
    if (PhysRegUsed[Reg])
      RegistersKilled[LastUse].push_back(Reg);
    else
      RegistersDead[LastUse].push_back(Reg);
  }
  PhysRegInfo[Reg] = MI;
  PhysRegUsed[Reg] = false;

  for (const unsigned *AliasSet = RegInfo->getAliasSet(Reg);
       unsigned Alias = *AliasSet; ++AliasSet) {
    if (MachineInstr *LastUse = PhysRegInfo[Alias]) {
      if (PhysRegUsed[Alias])
        RegistersKilled[LastUse].push_back(Alias);
      else
        RegistersDead[LastUse].push_back(Alias);
    }
    PhysRegInfo[Alias] = MI;
    PhysRegUsed[Alias] = false;
  }
}

bool LiveVariables::runOnMachineFunction(MachineFunction &MF) {
  const TargetInstrInfo &TII = *MF.getTarget().getInstrInfo();
  RegInfo = MF.getTarget().getRegisterInfo();
  assert(RegInfo && "Target doesn't have register information?");

  AllocatablePhysicalRegisters = RegInfo->getAllocatableSet(MF);

  // PhysRegInfo - Keep track of which instruction was the last use of a
  // physical register.  This is a purely local property, because all physical
  // register references as presumed dead across basic blocks.
  //
  PhysRegInfo = (MachineInstr**)alloca(sizeof(MachineInstr*) *
                                       RegInfo->getNumRegs());
  PhysRegUsed = (bool*)alloca(sizeof(bool)*RegInfo->getNumRegs());
  std::fill(PhysRegInfo, PhysRegInfo+RegInfo->getNumRegs(), (MachineInstr*)0);

  //