lib/CodeGen/RegAllocBasic.cpp

//===-- RegAllocBasic.cpp - basic register allocator ----------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines the RABasic function pass, which provides a minimal
// implementation of the basic register allocator.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "regalloc"
#include "LiveIntervalUnion.h"
#include "RegAllocBase.h"
#include "RenderMachineFunction.h"
#include "Spiller.h"
#include "VirtRegMap.h"
#include "VirtRegRewriter.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Function.h"
#include "llvm/PassAnalysisSupport.h"
#include "llvm/CodeGen/CalcSpillWeights.h"
#include "llvm/CodeGen/LiveIntervalAnalysis.h"
#include "llvm/CodeGen/LiveStackAnalysis.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/RegAllocRegistry.h"
#include "llvm/CodeGen/RegisterCoalescer.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetOptions.h"
#include "llvm/Target/TargetRegisterInfo.h"
#ifndef NDEBUG
#include "llvm/ADT/SparseBitVector.h"
#endif
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"

#include <vector>
#include <queue>

using namespace llvm;

static RegisterRegAlloc basicRegAlloc("basic", "basic register allocator",
                                      createBasicRegisterAllocator);

// Temporary verification option until we can put verification inside
// MachineVerifier.
static cl::opt<bool>
VerifyRegAlloc("verify-regalloc",
               cl::desc("Verify live intervals before renaming"));

namespace {

class PhysicalRegisterDescription : public AbstractRegisterDescription {
  const TargetRegisterInfo *TRI;
public:
  PhysicalRegisterDescription(const TargetRegisterInfo *T): TRI(T) {}
  virtual const char *getName(unsigned Reg) const { return TRI->getName(Reg); }
};

/// RABasic provides a minimal implementation of the basic register allocation
/// algorithm. It prioritizes live virtual registers by spill weight and spills
/// whenever a register is unavailable. This is not practical in production but
/// provides a useful baseline both for measuring other allocators and comparing
/// the speed of the basic algorithm against other styles of allocators.
class RABasic : public MachineFunctionPass, public RegAllocBase
{
  // context
  MachineFunction *MF;
  const TargetMachine *TM;
  MachineRegisterInfo *MRI;

  BitVector ReservedRegs;

  // analyses
  LiveStacks *LS;
  RenderMachineFunction *RMF;

  // state
  std::auto_ptr<Spiller> SpillerInstance;

public:
  RABasic();

  /// Return the pass name.
  virtual const char* getPassName() const {
    return "Basic Register Allocator";
  }

  /// RABasic analysis usage.
  virtual void getAnalysisUsage(AnalysisUsage &AU) const;

  virtual void releaseMemory();

  virtual Spiller &spiller() { return *SpillerInstance; }

  virtual unsigned selectOrSplit(LiveInterval &VirtReg,
                                 SmallVectorImpl<LiveInterval*> &SplitVRegs);

  /// Perform register allocation.
  virtual bool runOnMachineFunction(MachineFunction &mf);

  static char ID;

private:
  void addMBBLiveIns();
};

char RABasic::ID = 0;

} // end anonymous namespace

RABasic::RABasic(): MachineFunctionPass(ID) {
  initializeLiveIntervalsPass(*PassRegistry::getPassRegistry());
  initializeSlotIndexesPass(*PassRegistry::getPassRegistry());
  initializeStrongPHIEliminationPass(*PassRegistry::getPassRegistry());
  initializeRegisterCoalescerAnalysisGroup(*PassRegistry::getPassRegistry());
  initializeCalculateSpillWeightsPass(*PassRegistry::getPassRegistry());
  initializeLiveStacksPass(*PassRegistry::getPassRegistry());
  initializeMachineDominatorTreePass(*PassRegistry::getPassRegistry());
  initializeMachineLoopInfoPass(*PassRegistry::getPassRegistry());
  initializeVirtRegMapPass(*PassRegistry::getPassRegistry());
  initializeRenderMachineFunctionPass(*PassRegistry::getPassRegistry());
}

void RABasic::getAnalysisUsage(AnalysisUsage &AU) const {
  AU.setPreservesCFG();
  AU.addRequired<AliasAnalysis>();
  AU.addPreserved<AliasAnalysis>();
  AU.addRequired<LiveIntervals>();
  AU.addPreserved<SlotIndexes>();
  if (StrongPHIElim)
    AU.addRequiredID(StrongPHIEliminationID);
  AU.addRequiredTransitive<RegisterCoalescer>();
  AU.addRequired<CalculateSpillWeights>();
  AU.addRequired<LiveStacks>();
  AU.addPreserved<LiveStacks>();
  AU.addRequiredID(MachineDominatorsID);
  AU.addPreservedID(MachineDominatorsID);
  AU.addRequired<MachineLoopInfo>();
  AU.addPreserved<MachineLoopInfo>();
  AU.addRequired<VirtRegMap>();
  AU.addPreserved<VirtRegMap>();
  DEBUG(AU.addRequired<RenderMachineFunction>());
  MachineFunctionPass::getAnalysisUsage(AU);
}

void RABasic::releaseMemory() {
  SpillerInstance.reset(0);
  RegAllocBase::releaseMemory();
}

#ifndef NDEBUG
// Verify each LiveIntervalUnion.
void RegAllocBase::verify() {
  LiveVirtRegBitSet VisitedVRegs;
  OwningArrayPtr<LiveVirtRegBitSet>
    unionVRegs(new LiveVirtRegBitSet[PhysReg2LiveUnion.numRegs()]);

  // Verify disjoint unions.
  for (unsigned PhysReg = 0; PhysReg < PhysReg2LiveUnion.numRegs(); ++PhysReg) {
    DEBUG(PhysicalRegisterDescription PRD(TRI);
          PhysReg2LiveUnion[PhysReg].dump(&PRD));
    LiveVirtRegBitSet &VRegs = unionVRegs[PhysReg];
    PhysReg2LiveUnion[PhysReg].verify(VRegs);
    // Union + intersection test could be done efficiently in one pass, but
    // don't add a method to SparseBitVector unless we really need it.
    assert(!VisitedVRegs.intersects(VRegs) && "vreg in multiple unions");
    VisitedVRegs |= VRegs;
  }

  // Verify vreg coverage.
  for (LiveIntervals::iterator liItr = LIS->begin(), liEnd = LIS->end();
       liItr != liEnd; ++liItr) {
    unsigned reg = liItr->first;
    if (TargetRegisterInfo::isPhysicalRegister(reg)) continue;
    if (!VRM->hasPhys(reg)) continue; // spilled?
    unsigned PhysReg = VRM->getPhys(reg);
    if (!unionVRegs[PhysReg].test(reg)) {
      dbgs() << "LiveVirtReg " << reg << " not in union " <<
        TRI->getName(PhysReg) << "\n";
      llvm_unreachable("unallocated live vreg");
    }
  }
  // FIXME: I'm not sure how to verify spilled intervals.
}
#endif //!NDEBUG

//===----------------------------------------------------------------------===//
//                         RegAllocBase Implementation
//===----------------------------------------------------------------------===//

// Instantiate a LiveIntervalUnion for each physical register.
void RegAllocBase::LiveUnionArray::init(unsigned NRegs) {
  Array.reset(new LiveIntervalUnion[NRegs]);
  NumRegs = NRegs;
  for (unsigned RegNum = 0; RegNum < NRegs; ++RegNum) {
    Array[RegNum].init(RegNum);
  }
}

void RegAllocBase::init(const TargetRegisterInfo &tri, VirtRegMap &vrm,
                        LiveIntervals &lis) {
  TRI = &tri;
  VRM = &vrm;
  LIS = &lis;
  PhysReg2LiveUnion.init(TRI->getNumRegs());
  // Cache an interferece query for each physical reg
  Queries.reset(new LiveIntervalUnion::Query[PhysReg2LiveUnion.numRegs()]);
}

void RegAllocBase::LiveUnionArray::clear() {
  NumRegs =  0;
  Array.reset(0);
}

void RegAllocBase::releaseMemory() {
  PhysReg2LiveUnion.clear();
}

namespace llvm {
/// This class defines a queue of live virtual registers prioritized by spill
/// weight. The heaviest vreg is popped first.
///
/// Currently, this is trivial wrapper that gives us an opaque type in the
/// header, but we may later give it a virtual interface for register allocators
/// to override the priority queue comparator.
class LiveVirtRegQueue {
  typedef std::priority_queue
    <LiveInterval*, std::vector<LiveInterval*>, LessSpillWeightPriority>
    PriorityQ;
  PriorityQ PQ;

public:
  // Is the queue empty?
  bool empty() { return PQ.empty(); }

  // Get the highest priority lvr (top + pop)
  LiveInterval *get() {
    LiveInterval *VirtReg = PQ.top();
    PQ.pop();
    return VirtReg;
  }
  // Add this lvr to the queue
  void push(LiveInterval *VirtReg) {
    PQ.push(VirtReg);
  }
};
} // end namespace llvm

// Visit all the live virtual registers. If they are already assigned to a
// physical register, unify them with the corresponding LiveIntervalUnion,
// otherwise push them on the priority queue for later assignment.
void RegAllocBase::seedLiveVirtRegs(LiveVirtRegQueue &VirtRegQ) {
  for (LiveIntervals::iterator I = LIS->begin(), E = LIS->end(); I != E; ++I) {
    unsigned RegNum = I->first;
    LiveInterval &VirtReg = *I->second;
    if (TargetRegisterInfo::isPhysicalRegister(RegNum)) {
      PhysReg2LiveUnion[RegNum].unify(VirtReg);
    }
    else {
      VirtRegQ.push(&VirtReg);
    }
  }
}

// Top-level driver to manage the queue of unassigned VirtRegs and call the
// selectOrSplit implementation.
void RegAllocBase::allocatePhysRegs() {

  // Push each vreg onto a queue or "precolor" by adding it to a physreg union.
  LiveVirtRegQueue VirtRegQ;
  seedLiveVirtRegs(VirtRegQ);

  // Continue assigning vregs one at a time to available physical registers.
  while (!VirtRegQ.empty()) {
    // Pop the highest priority vreg.
    LiveInterval *VirtReg = VirtRegQ.get();

    // selectOrSplit requests the allocator to return an available physical
    // register if possible and populate a list of new live intervals that
    // result from splitting.
    typedef SmallVector<LiveInterval*, 4> VirtRegVec;
    VirtRegVec SplitVRegs;
    unsigned AvailablePhysReg = selectOrSplit(*VirtReg, SplitVRegs);

    if (AvailablePhysReg) {
      DEBUG(dbgs() << "allocating: " << TRI->getName(AvailablePhysReg) <<
            " " << *VirtReg << '\n');
      assert(!VRM->hasPhys(VirtReg->reg) && "duplicate vreg in union");
      VRM->assignVirt2Phys(VirtReg->reg, AvailablePhysReg);
      PhysReg2LiveUnion[AvailablePhysReg].unify(*VirtReg);
    }
    for (VirtRegVec::iterator I = SplitVRegs.begin(), E = SplitVRegs.end();
         I != E; ++I) {
      LiveInterval* SplitVirtReg = *I;
      if (SplitVirtReg->empty()) continue;
      DEBUG(dbgs() << "queuing new interval: " << *SplitVirtReg << "\n");
      assert(TargetRegisterInfo::isVirtualRegister(SplitVirtReg->reg) &&
             "expect split value in virtual register");
      VirtRegQ.push(SplitVirtReg);
    }
  }
}

// Check if this live virtual register interferes with a physical register. If
// not, then check for interference on each register that aliases with the
// physical register. Return the interfering register.
unsigned RegAllocBase::checkPhysRegInterference(LiveInterval &VirtReg,
                                                unsigned PhysReg) {
  if (query(VirtReg, PhysReg).checkInterference())
    return PhysReg;
  for (const unsigned *AliasI = TRI->getAliasSet(PhysReg); *AliasI; ++AliasI) {
    if (query(VirtReg, *AliasI).checkInterference())
      return *AliasI;
  }
  return 0;
}

// Helper for spillInteferences() that spills all interfering vregs currently
// assigned to this physical register.
void RegAllocBase::spillReg(LiveInterval& VirtReg, unsigned PhysReg,
                            SmallVectorImpl<LiveInterval*> &SplitVRegs) {
  LiveIntervalUnion::Query &Q = query(VirtReg, PhysReg);
  assert(Q.seenAllInterferences() && "need collectInterferences()");
  const SmallVectorImpl<LiveInterval*> &PendingSpills = Q.interferingVRegs();

  for (SmallVectorImpl<LiveInterval*>::const_iterator I = PendingSpills.begin(),
         E = PendingSpills.end(); I != E; ++I) {
    LiveInterval &SpilledVReg = **I;
    DEBUG(dbgs() << "extracting from " <<
          TRI->getName(PhysReg) << " " << SpilledVReg << '\n');

    // Deallocate the interfering vreg by removing it from the union.
    // A LiveInterval instance may not be in a union during modification!
    PhysReg2LiveUnion[PhysReg].extract(SpilledVReg);

    // Clear the vreg assignment.
    VRM->clearVirt(SpilledVReg.reg);

    // Spill the extracted interval.
    spiller().spill(&SpilledVReg, SplitVRegs, PendingSpills);
  }
  // After extracting segments, the query's results are invalid. But keep the
  // contents valid until we're done accessing pendingSpills.
  Q.clear();
}

// Spill or split all live virtual registers currently unified under PhysReg
// that interfere with VirtReg. The newly spilled or split live intervals are
// returned by appending them to SplitVRegs.
bool
RegAllocBase::spillInterferences(LiveInterval &VirtReg, unsigned PhysReg,
                                 SmallVectorImpl<LiveInterval*> &SplitVRegs) {
  // Record each interference and determine if all are spillable before mutating
  // either the union or live intervals.

  // Collect interferences assigned to the requested physical register.
  LiveIntervalUnion::Query &QPreg = query(VirtReg, PhysReg);
  unsigned NumInterferences = QPreg.collectInterferingVRegs();
  if (QPreg.seenUnspillableVReg()) {
    return false;
  }
  // Collect interferences assigned to any alias of the physical register.
  for (const unsigned *asI = TRI->getAliasSet(PhysReg); *asI; ++asI) {
    LiveIntervalUnion::Query &QAlias = query(VirtReg, *asI);
    NumInterferences += QAlias.collectInterferingVRegs();
    if (QAlias.seenUnspillableVReg()) {
      return false;
    }
  }
  DEBUG(dbgs() << "spilling " << TRI->getName(PhysReg) <<
        " interferences with " << VirtReg << "\n");
  assert(NumInterferences > 0 && "expect interference");

  // Spill each interfering vreg allocated to PhysReg or an alias.
  spillReg(VirtReg, PhysReg, SplitVRegs);
  for (const unsigned *AliasI = TRI->getAliasSet(PhysReg); *AliasI; ++AliasI)
    spillReg(VirtReg, *AliasI, SplitVRegs);
  return true;
}

//===----------------------------------------------------------------------===//
//                         RABasic Implementation
//===----------------------------------------------------------------------===//

// Driver for the register assignment and splitting heuristics.
// Manages iteration over the LiveIntervalUnions.
//
// This is a minimal implementation of register assignment and splitting that
// spills whenever we run out of registers.
//
// selectOrSplit can only be called once per live virtual register. We then do a
// single interference test for each register the correct class until we find an
// available register. So, the number of interference tests in the worst case is
// |vregs| * |machineregs|. And since the number of interference tests is
// minimal, there is no value in caching them outside the scope of
// selectOrSplit().
unsigned RABasic::selectOrSplit(LiveInterval &VirtReg,
                                SmallVectorImpl<LiveInterval*> &SplitVRegs) {
  // Populate a list of physical register spill candidates.
  SmallVector<unsigned, 8> PhysRegSpillCands;

  // Check for an available register in this class.
  const TargetRegisterClass *TRC = MRI->getRegClass(VirtReg.reg);
  DEBUG(dbgs() << "RegClass: " << TRC->getName() << ' ');

  for (TargetRegisterClass::iterator I = TRC->allocation_order_begin(*MF),
         E = TRC->allocation_order_end(*MF);
       I != E; ++I) {

    unsigned PhysReg = *I;
    if (ReservedRegs.test(PhysReg)) continue;

    // Check interference and as a side effect, intialize queries for this
    // VirtReg and its aliases.
    unsigned interfReg = checkPhysRegInterference(VirtReg, PhysReg);
    if (interfReg == 0) {
      // Found an available register.
      return PhysReg;
    }
    LiveInterval *interferingVirtReg =
      Queries[interfReg].firstInterference().liveUnionPos()->VirtReg;

    // The current VirtReg must either spillable, or one of its interferences
    // must have less spill weight.
    if (interferingVirtReg->weight < VirtReg.weight ) {
      PhysRegSpillCands.push_back(PhysReg);
    }
  }
  // Try to spill another interfering reg with less spill weight.
  //
  // FIXME: RAGreedy will sort this list by spill weight.
  for (SmallVectorImpl<unsigned>::iterator PhysRegI = PhysRegSpillCands.begin(),
         PhysRegE = PhysRegSpillCands.end(); PhysRegI != PhysRegE; ++PhysRegI) {

    if (!spillInterferences(VirtReg, *PhysRegI, SplitVRegs)) continue;

    unsigned InterferingReg = checkPhysRegInterference(VirtReg, *PhysRegI);
    if (InterferingReg != 0) {
      const LiveSegment &seg =
        *Queries[InterferingReg].firstInterference().liveUnionPos();

      dbgs() << "spilling cannot free " << TRI->getName(*PhysRegI) <<
        " for " << VirtReg.reg << " with interference " << *seg.VirtReg << "\n";
      llvm_unreachable("Interference after spill.");
    }
    // Tell the caller to allocate to this newly freed physical register.
    return *PhysRegI;
  }
  // No other spill candidates were found, so spill the current VirtReg.
  DEBUG(dbgs() << "spilling: " << VirtReg << '\n');
  SmallVector<LiveInterval*, 1> pendingSpills;

  spiller().spill(&VirtReg, SplitVRegs, pendingSpills);

  // The live virtual register requesting allocation was spilled, so tell
  // the caller not to allocate anything during this round.
  return 0;
}

// Add newly allocated physical registers to the MBB live in sets.
void RABasic::addMBBLiveIns() {
  typedef SmallVector<MachineBasicBlock*, 8> MBBVec;
  MBBVec liveInMBBs;
  MachineBasicBlock &entryMBB = *MF->begin();

  for (unsigned PhysReg = 0; PhysReg < PhysReg2LiveUnion.numRegs(); ++PhysReg) {
    LiveIntervalUnion &LiveUnion = PhysReg2LiveUnion[PhysReg];

    for (LiveIntervalUnion::SegmentIter SI = LiveUnion.begin(),
           SegEnd = LiveUnion.end();
         SI != SegEnd; ++SI) {

      // Find the set of basic blocks which this range is live into...
      liveInMBBs.clear();
      if (!LIS->findLiveInMBBs(SI->Start, SI->End, liveInMBBs)) continue;

      // And add the physreg for this interval to their live-in sets.
      for (MBBVec::iterator I = liveInMBBs.begin(), E = liveInMBBs.end();
           I != E; ++I) {
        MachineBasicBlock *MBB = *I;
        if (MBB == &entryMBB) continue;
        if (MBB->isLiveIn(PhysReg)) continue;
        MBB->addLiveIn(PhysReg);
      }
    }
  }
}

bool RABasic::runOnMachineFunction(MachineFunction &mf) {
  DEBUG(dbgs() << "********** BASIC REGISTER ALLOCATION **********\n"
               << "********** Function: "
               << ((Value*)mf.getFunction())->getName() << '\n');

  MF = &mf;
  TM = &mf.getTarget();
  MRI = &mf.getRegInfo();

  DEBUG(RMF = &getAnalysis<RenderMachineFunction>());

  const TargetRegisterInfo *TRI = TM->getRegisterInfo();
  RegAllocBase::init(*TRI, getAnalysis<VirtRegMap>(),
                     getAnalysis<LiveIntervals>());

  ReservedRegs = TRI->getReservedRegs(*MF);

  SpillerInstance.reset(createSpiller(*this, *MF, *VRM));

  allocatePhysRegs();

  addMBBLiveIns();

  // Diagnostic output before rewriting
  DEBUG(dbgs() << "Post alloc VirtRegMap:\n" << *VRM << "\n");

  // optional HTML output
  DEBUG(RMF->renderMachineFunction("After basic register allocation.", VRM));

  // FIXME: Verification currently must run before VirtRegRewriter. We should
  // make the rewriter a separate pass and override verifyAnalysis instead. When
  // that happens, verification naturally falls under VerifyMachineCode.
#ifndef NDEBUG
  if (VerifyRegAlloc) {
    // Verify accuracy of LiveIntervals. The standard machine code verifier
    // ensures that each LiveIntervals covers all uses of the virtual reg.

    // FIXME: MachineVerifier is badly broken when using the standard
    // spiller. Always use -spiller=inline with -verify-regalloc. Even with the
    // inline spiller, some tests fail to verify because the coalescer does not
    // always generate verifiable code.
    MF->verify(this);

    // Verify that LiveIntervals are partitioned into unions and disjoint within
    // the unions.
    verify();
  }
#endif // !NDEBUG

  // Run rewriter
  std::auto_ptr<VirtRegRewriter> rewriter(createVirtRegRewriter());
  rewriter->runOnMachineFunction(*MF, *VRM, LIS);

  // The pass output is in VirtRegMap. Release all the transient data.
  releaseMemory();

  return true;
}

FunctionPass* llvm::createBasicRegisterAllocator()
{
  return new RABasic();
}