llvm-project/llvm/lib/CodeGen/LiveRangeCalc.cpp

//===- LiveRangeCalc.cpp - Calculate live ranges --------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// Implementation of the LiveRangeCalc class.
//
//===----------------------------------------------------------------------===//

#include "LiveRangeCalc.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/CodeGen/LiveInterval.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/SlotIndexes.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/MC/LaneBitmask.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <iterator>
#include <tuple>
#include <utility>

using namespace llvm;

#define DEBUG_TYPE "regalloc"

// Reserve an address that indicates a value that is known to be "undef".
static VNInfo UndefVNI(0xbad, SlotIndex());

void LiveRangeCalc::resetLiveOutMap() {
  unsigned NumBlocks = MF->getNumBlockIDs();
  Seen.clear();
  Seen.resize(NumBlocks);
  EntryInfos.clear();
  Map.resize(NumBlocks);
}

void LiveRangeCalc::reset(const MachineFunction *mf,
                          SlotIndexes *SI,
                          MachineDominatorTree *MDT,
                          VNInfo::Allocator *VNIA) {
  MF = mf;
  MRI = &MF->getRegInfo();
  Indexes = SI;
  DomTree = MDT;
  Alloc = VNIA;
  resetLiveOutMap();
  LiveIn.clear();
}

static void createDeadDef(SlotIndexes &Indexes, VNInfo::Allocator &Alloc,
                          LiveRange &LR, const MachineOperand &MO) {
  const MachineInstr &MI = *MO.getParent();
  SlotIndex DefIdx =
      Indexes.getInstructionIndex(MI).getRegSlot(MO.isEarlyClobber());

  // Create the def in LR. This may find an existing def.
  LR.createDeadDef(DefIdx, Alloc);
}

void LiveRangeCalc::calculate(LiveInterval &LI, bool TrackSubRegs) {
  assert(MRI && Indexes && "call reset() first");

  // Step 1: Create minimal live segments for every definition of Reg.
  // Visit all def operands. If the same instruction has multiple defs of Reg,
  // createDeadDef() will deduplicate.
  const TargetRegisterInfo &TRI = *MRI->getTargetRegisterInfo();
  unsigned Reg = LI.reg;
  for (const MachineOperand &MO : MRI->reg_nodbg_operands(Reg)) {
    if (!MO.isDef() && !MO.readsReg())
      continue;

    unsigned SubReg = MO.getSubReg();
    if (LI.hasSubRanges() || (SubReg != 0 && TrackSubRegs)) {
      LaneBitmask SubMask = SubReg != 0 ? TRI.getSubRegIndexLaneMask(SubReg)
                                        : MRI->getMaxLaneMaskForVReg(Reg);
      // If this is the first time we see a subregister def, initialize
      // subranges by creating a copy of the main range.
      if (!LI.hasSubRanges() && !LI.empty()) {
        LaneBitmask ClassMask = MRI->getMaxLaneMaskForVReg(Reg);
        LI.createSubRangeFrom(*Alloc, ClassMask, LI);
      }

      LI.refineSubRanges(*Alloc, SubMask,
                         [&MO, this](LiveInterval::SubRange &SR) {
                           if (MO.isDef())
                             createDeadDef(*Indexes, *Alloc, SR, MO);
                         },
                         *Indexes, TRI);
    }

    // Create the def in the main liverange. We do not have to do this if
    // subranges are tracked as we recreate the main range later in this case.
    if (MO.isDef() && !LI.hasSubRanges())
      createDeadDef(*Indexes, *Alloc, LI, MO);
  }

  // We may have created empty live ranges for partially undefined uses, we
  // can't keep them because we won't find defs in them later.
  LI.removeEmptySubRanges();

  // Step 2: Extend live segments to all uses, constructing SSA form as
  // necessary.
  if (LI.hasSubRanges()) {
    for (LiveInterval::SubRange &S : LI.subranges()) {
      LiveRangeCalc SubLRC;
      SubLRC.reset(MF, Indexes, DomTree, Alloc);
      SubLRC.extendToUses(S, Reg, S.LaneMask, &LI);
    }
    LI.clear();
    constructMainRangeFromSubranges(LI);
  } else {
    resetLiveOutMap();
    extendToUses(LI, Reg, LaneBitmask::getAll());
  }
}

void LiveRangeCalc::constructMainRangeFromSubranges(LiveInterval &LI) {
  // First create dead defs at all defs found in subranges.
  LiveRange &MainRange = LI;
  assert(MainRange.segments.empty() && MainRange.valnos.empty() &&
         "Expect empty main liverange");

  for (const LiveInterval::SubRange &SR : LI.subranges()) {
    for (const VNInfo *VNI : SR.valnos) {
      if (!VNI->isUnused() && !VNI->isPHIDef())
        MainRange.createDeadDef(VNI->def, *Alloc);
    }
  }
  resetLiveOutMap();
  extendToUses(MainRange, LI.reg, LaneBitmask::getAll(), &LI);
}

void LiveRangeCalc::createDeadDefs(LiveRange &LR, unsigned Reg) {
  assert(MRI && Indexes && "call reset() first");

  // Visit all def operands. If the same instruction has multiple defs of Reg,
  // LR.createDeadDef() will deduplicate.
  for (MachineOperand &MO : MRI->def_operands(Reg))
    createDeadDef(*Indexes, *Alloc, LR, MO);
}

void LiveRangeCalc::extendToUses(LiveRange &LR, unsigned Reg, LaneBitmask Mask,
                                 LiveInterval *LI) {
  SmallVector<SlotIndex, 4> Undefs;
  if (LI != nullptr)
    LI->computeSubRangeUndefs(Undefs, Mask, *MRI, *Indexes);

  // Visit all operands that read Reg. This may include partial defs.
  bool IsSubRange = !Mask.all();
  const TargetRegisterInfo &TRI = *MRI->getTargetRegisterInfo();
  for (MachineOperand &MO : MRI->reg_nodbg_operands(Reg)) {
    // Clear all kill flags. They will be reinserted after register allocation
    // by LiveIntervals::addKillFlags().
    if (MO.isUse())
      MO.setIsKill(false);
    // MO::readsReg returns "true" for subregister defs. This is for keeping
    // liveness of the entire register (i.e. for the main range of the live
    // interval). For subranges, definitions of non-overlapping subregisters
    // do not count as uses.
    if (!MO.readsReg() || (IsSubRange && MO.isDef()))
      continue;

    unsigned SubReg = MO.getSubReg();
    if (SubReg != 0) {
      LaneBitmask SLM = TRI.getSubRegIndexLaneMask(SubReg);
      if (MO.isDef())
        SLM = ~SLM;
      // Ignore uses not reading the current (sub)range.
      if ((SLM & Mask).none())
        continue;
    }

    // Determine the actual place of the use.
    const MachineInstr *MI = MO.getParent();
    unsigned OpNo = (&MO - &MI->getOperand(0));
    SlotIndex UseIdx;
    if (MI->isPHI()) {
      assert(!MO.isDef() && "Cannot handle PHI def of partial register.");
      // The actual place where a phi operand is used is the end of the pred
      // MBB. PHI operands are paired: (Reg, PredMBB).
      UseIdx = Indexes->getMBBEndIdx(MI->getOperand(OpNo+1).getMBB());
    } else {
      // Check for early-clobber redefs.
      bool isEarlyClobber = false;
      unsigned DefIdx;
      if (MO.isDef())
        isEarlyClobber = MO.isEarlyClobber();
      else if (MI->isRegTiedToDefOperand(OpNo, &DefIdx)) {
        // FIXME: This would be a lot easier if tied early-clobber uses also
        // had an early-clobber flag.
        isEarlyClobber = MI->getOperand(DefIdx).isEarlyClobber();
      }
      UseIdx = Indexes->getInstructionIndex(*MI).getRegSlot(isEarlyClobber);
    }

    // MI is reading Reg. We may have visited MI before if it happens to be
    // reading Reg multiple times. That is OK, extend() is idempotent.
    extend(LR, UseIdx, Reg, Undefs);
  }
}

void LiveRangeCalc::updateFromLiveIns() {
  LiveRangeUpdater Updater;
  for (const LiveInBlock &I : LiveIn) {
    if (!I.DomNode)
      continue;
    MachineBasicBlock *MBB = I.DomNode->getBlock();
    assert(I.Value && "No live-in value found");
    SlotIndex Start, End;
    std::tie(Start, End) = Indexes->getMBBRange(MBB);

    if (I.Kill.isValid())
      // Value is killed inside this block.
      End = I.Kill;
    else {
      // The value is live-through, update LiveOut as well.
      // Defer the Domtree lookup until it is needed.
      assert(Seen.test(MBB->getNumber()));
      Map[MBB] = LiveOutPair(I.Value, nullptr);
    }
    Updater.setDest(&I.LR);
    Updater.add(Start, End, I.Value);
  }
  LiveIn.clear();
}

void LiveRangeCalc::extend(LiveRange &LR, SlotIndex Use, unsigned PhysReg,
                           ArrayRef<SlotIndex> Undefs) {
  assert(Use.isValid() && "Invalid SlotIndex");
  assert(Indexes && "Missing SlotIndexes");
  assert(DomTree && "Missing dominator tree");

  MachineBasicBlock *UseMBB = Indexes->getMBBFromIndex(Use.getPrevSlot());
  assert(UseMBB && "No MBB at Use");

  // Is there a def in the same MBB we can extend?
  auto EP = LR.extendInBlock(Undefs, Indexes->getMBBStartIdx(UseMBB), Use);
  if (EP.first != nullptr || EP.second)
    return;

  // Find the single reaching def, or determine if Use is jointly dominated by
  // multiple values, and we may need to create even more phi-defs to preserve
  // VNInfo SSA form.  Perform a search for all predecessor blocks where we
  // know the dominating VNInfo.
  if (findReachingDefs(LR, *UseMBB, Use, PhysReg, Undefs))
    return;

  // When there were multiple different values, we may need new PHIs.
  calculateValues();
}

// This function is called by a client after using the low-level API to add
// live-out and live-in blocks.  The unique value optimization is not
// available, SplitEditor::transferValues handles that case directly anyway.
void LiveRangeCalc::calculateValues() {
  assert(Indexes && "Missing SlotIndexes");
  assert(DomTree && "Missing dominator tree");
  updateSSA();
  updateFromLiveIns();
}

bool LiveRangeCalc::isDefOnEntry(LiveRange &LR, ArrayRef<SlotIndex> Undefs,
                                 MachineBasicBlock &MBB, BitVector &DefOnEntry,
                                 BitVector &UndefOnEntry) {
  unsigned BN = MBB.getNumber();
  if (DefOnEntry[BN])
    return true;
  if (UndefOnEntry[BN])
    return false;

  auto MarkDefined = [BN, &DefOnEntry](MachineBasicBlock &B) -> bool {
    for (MachineBasicBlock *S : B.successors())
      DefOnEntry[S->getNumber()] = true;
    DefOnEntry[BN] = true;
    return true;
  };

  SetVector<unsigned> WorkList;
  // Checking if the entry of MBB is reached by some def: add all predecessors
  // that are potentially defined-on-exit to the work list.
  for (MachineBasicBlock *P : MBB.predecessors())
    WorkList.insert(P->getNumber());

  for (unsigned i = 0; i != WorkList.size(); ++i) {
    // Determine if the exit from the block is reached by some def.
    unsigned N = WorkList[i];
    MachineBasicBlock &B = *MF->getBlockNumbered(N);
    if (Seen[N]) {
      const LiveOutPair &LOB = Map[&B];
      if (LOB.first != nullptr && LOB.first != &UndefVNI)
        return MarkDefined(B);
    }
    SlotIndex Begin, End;
    std::tie(Begin, End) = Indexes->getMBBRange(&B);
    // Treat End as not belonging to B.
    // If LR has a segment S that starts at the next block, i.e. [End, ...),
    // std::upper_bound will return the segment following S. Instead,
    // S should be treated as the first segment that does not overlap B.
    LiveRange::iterator UB = std::upper_bound(LR.begin(), LR.end(),
                                              End.getPrevSlot());
    if (UB != LR.begin()) {
      LiveRange::Segment &Seg = *std::prev(UB);
      if (Seg.end > Begin) {
        // There is a segment that overlaps B. If the range is not explicitly
        // undefined between the end of the segment and the end of the block,
        // treat the block as defined on exit. If it is, go to the next block
        // on the work list.
        if (LR.isUndefIn(Undefs, Seg.end, End))
          continue;
        return MarkDefined(B);
      }
    }

    // No segment overlaps with this block. If this block is not defined on
    // entry, or it undefines the range, do not process its predecessors.
    if (UndefOnEntry[N] || LR.isUndefIn(Undefs, Begin, End)) {
      UndefOnEntry[N] = true;
      continue;
    }
    if (DefOnEntry[N])
      return MarkDefined(B);

    // Still don't know: add all predecessors to the work list.
    for (MachineBasicBlock *P : B.predecessors())
      WorkList.insert(P->getNumber());
  }

  UndefOnEntry[BN] = true;
  return false;
}

bool LiveRangeCalc::findReachingDefs(LiveRange &LR, MachineBasicBlock &UseMBB,
                                     SlotIndex Use, unsigned PhysReg,
                                     ArrayRef<SlotIndex> Undefs) {
  unsigned UseMBBNum = UseMBB.getNumber();

  // Block numbers where LR should be live-in.
  SmallVector<unsigned, 16> WorkList(1, UseMBBNum);

  // Remember if we have seen more than one value.
  bool UniqueVNI = true;
  VNInfo *TheVNI = nullptr;

  bool FoundUndef = false;

  // Using Seen as a visited set, perform a BFS for all reaching defs.
  for (unsigned i = 0; i != WorkList.size(); ++i) {
    MachineBasicBlock *MBB = MF->getBlockNumbered(WorkList[i]);

#ifndef NDEBUG
    if (MBB->pred_empty()) {
      MBB->getParent()->verify();
      errs() << "Use of " << printReg(PhysReg, MRI->getTargetRegisterInfo())
             << " does not have a corresponding definition on every path:\n";
      const MachineInstr *MI = Indexes->getInstructionFromIndex(Use);
      if (MI != nullptr)
        errs() << Use << " " << *MI;
      report_fatal_error("Use not jointly dominated by defs.");
    }

    if (TargetRegisterInfo::isPhysicalRegister(PhysReg) &&
        !MBB->isLiveIn(PhysReg)) {
      MBB->getParent()->verify();
      const TargetRegisterInfo *TRI = MRI->getTargetRegisterInfo();
      errs() << "The register " << printReg(PhysReg, TRI)
             << " needs to be live in to " << printMBBReference(*MBB)
             << ", but is missing from the live-in list.\n";
      report_fatal_error("Invalid global physical register");
    }
#endif
    FoundUndef |= MBB->pred_empty();

    for (MachineBasicBlock *Pred : MBB->predecessors()) {
       // Is this a known live-out block?
       if (Seen.test(Pred->getNumber())) {
         if (VNInfo *VNI = Map[Pred].first) {
           if (TheVNI && TheVNI != VNI)
             UniqueVNI = false;
           TheVNI = VNI;
         }
         continue;
       }

       SlotIndex Start, End;
       std::tie(Start, End) = Indexes->getMBBRange(Pred);

       // First time we see Pred.  Try to determine the live-out value, but set
       // it as null if Pred is live-through with an unknown value.
       auto EP = LR.extendInBlock(Undefs, Start, End);
       VNInfo *VNI = EP.first;
       FoundUndef |= EP.second;
       setLiveOutValue(Pred, EP.second ? &UndefVNI : VNI);
       if (VNI) {
         if (TheVNI && TheVNI != VNI)
           UniqueVNI = false;
         TheVNI = VNI;
       }
       if (VNI || EP.second)
         continue;

       // No, we need a live-in value for Pred as well
       if (Pred != &UseMBB)
         WorkList.push_back(Pred->getNumber());
       else
          // Loopback to UseMBB, so value is really live through.
         Use = SlotIndex();
    }
  }

  LiveIn.clear();
  FoundUndef |= (TheVNI == nullptr || TheVNI == &UndefVNI);
  if (!Undefs.empty() && FoundUndef)
    UniqueVNI = false;

  // Both updateSSA() and LiveRangeUpdater benefit from ordered blocks, but
  // neither require it. Skip the sorting overhead for small updates.
  if (WorkList.size() > 4)
    array_pod_sort(WorkList.begin(), WorkList.end());

  // If a unique reaching def was found, blit in the live ranges immediately.
  if (UniqueVNI) {
    assert(TheVNI != nullptr && TheVNI != &UndefVNI);
    LiveRangeUpdater Updater(&LR);
    for (unsigned BN : WorkList) {
      SlotIndex Start, End;
      std::tie(Start, End) = Indexes->getMBBRange(BN);
      // Trim the live range in UseMBB.
      if (BN == UseMBBNum && Use.isValid())
        End = Use;
      else
        Map[MF->getBlockNumbered(BN)] = LiveOutPair(TheVNI, nullptr);
      Updater.add(Start, End, TheVNI);
    }
    return true;
  }

  // Prepare the defined/undefined bit vectors.
  EntryInfoMap::iterator Entry;
  bool DidInsert;
  std::tie(Entry, DidInsert) = EntryInfos.insert(
      std::make_pair(&LR, std::make_pair(BitVector(), BitVector())));
  if (DidInsert) {
    // Initialize newly inserted entries.
    unsigned N = MF->getNumBlockIDs();
    Entry->second.first.resize(N);
    Entry->second.second.resize(N);
  }
  BitVector &DefOnEntry = Entry->second.first;
  BitVector &UndefOnEntry = Entry->second.second;

  // Multiple values were found, so transfer the work list to the LiveIn array
  // where UpdateSSA will use it as a work list.
  LiveIn.reserve(WorkList.size());
  for (unsigned BN : WorkList) {
    MachineBasicBlock *MBB = MF->getBlockNumbered(BN);
    if (!Undefs.empty() &&
        !isDefOnEntry(LR, Undefs, *MBB, DefOnEntry, UndefOnEntry))
      continue;
    addLiveInBlock(LR, DomTree->getNode(MBB));
    if (MBB == &UseMBB)
      LiveIn.back().Kill = Use;
  }

  return false;
}

// This is essentially the same iterative algorithm that SSAUpdater uses,
// except we already have a dominator tree, so we don't have to recompute it.
void LiveRangeCalc::updateSSA() {
  assert(Indexes && "Missing SlotIndexes");
  assert(DomTree && "Missing dominator tree");

  // Interate until convergence.
  bool Changed;
  do {
    Changed = false;
    // Propagate live-out values down the dominator tree, inserting phi-defs
    // when necessary.
    for (LiveInBlock &I : LiveIn) {
      MachineDomTreeNode *Node = I.DomNode;
      // Skip block if the live-in value has already been determined.
      if (!Node)
        continue;
      MachineBasicBlock *MBB = Node->getBlock();
      MachineDomTreeNode *IDom = Node->getIDom();
      LiveOutPair IDomValue;

      // We need a live-in value to a block with no immediate dominator?
      // This is probably an unreachable block that has survived somehow.
      bool needPHI = !IDom || !Seen.test(IDom->getBlock()->getNumber());

      // IDom dominates all of our predecessors, but it may not be their
      // immediate dominator. Check if any of them have live-out values that are
      // properly dominated by IDom. If so, we need a phi-def here.
      if (!needPHI) {
        IDomValue = Map[IDom->getBlock()];

        // Cache the DomTree node that defined the value.
        if (IDomValue.first && IDomValue.first != &UndefVNI &&
            !IDomValue.second) {
          Map[IDom->getBlock()].second = IDomValue.second =
            DomTree->getNode(Indexes->getMBBFromIndex(IDomValue.first->def));
        }

        for (MachineBasicBlock *Pred : MBB->predecessors()) {
          LiveOutPair &Value = Map[Pred];
          if (!Value.first || Value.first == IDomValue.first)
            continue;
          if (Value.first == &UndefVNI) {
            needPHI = true;
            break;
          }

          // Cache the DomTree node that defined the value.
          if (!Value.second)
            Value.second =
              DomTree->getNode(Indexes->getMBBFromIndex(Value.first->def));

          // This predecessor is carrying something other than IDomValue.
          // It could be because IDomValue hasn't propagated yet, or it could be
          // because MBB is in the dominance frontier of that value.
          if (DomTree->dominates(IDom, Value.second)) {
            needPHI = true;
            break;
          }
        }
      }

      // The value may be live-through even if Kill is set, as can happen when
      // we are called from extendRange. In that case LiveOutSeen is true, and
      // LiveOut indicates a foreign or missing value.
      LiveOutPair &LOP = Map[MBB];

      // Create a phi-def if required.
      if (needPHI) {
        Changed = true;
        assert(Alloc && "Need VNInfo allocator to create PHI-defs");
        SlotIndex Start, End;
        std::tie(Start, End) = Indexes->getMBBRange(MBB);
        LiveRange &LR = I.LR;
        VNInfo *VNI = LR.getNextValue(Start, *Alloc);
        I.Value = VNI;
        // This block is done, we know the final value.
        I.DomNode = nullptr;

        // Add liveness since updateFromLiveIns now skips this node.
        if (I.Kill.isValid()) {
          if (VNI)
            LR.addSegment(LiveInterval::Segment(Start, I.Kill, VNI));
        } else {
          if (VNI)
            LR.addSegment(LiveInterval::Segment(Start, End, VNI));
          LOP = LiveOutPair(VNI, Node);
        }
      } else if (IDomValue.first && IDomValue.first != &UndefVNI) {
        // No phi-def here. Remember incoming value.
        I.Value = IDomValue.first;

        // If the IDomValue is killed in the block, don't propagate through.
        if (I.Kill.isValid())
          continue;

        // Propagate IDomValue if it isn't killed:
        // MBB is live-out and doesn't define its own value.
        if (LOP.first == IDomValue.first)
          continue;
        Changed = true;
        LOP = IDomValue;
      }
    }
  } while (Changed);
}

bool LiveRangeCalc::isJointlyDominated(const MachineBasicBlock *MBB,
                                       ArrayRef<SlotIndex> Defs,
                                       const SlotIndexes &Indexes) {
  const MachineFunction &MF = *MBB->getParent();
  BitVector DefBlocks(MF.getNumBlockIDs());
  for (SlotIndex I : Defs)
    DefBlocks.set(Indexes.getMBBFromIndex(I)->getNumber());

  SetVector<unsigned> PredQueue;
  PredQueue.insert(MBB->getNumber());
  for (unsigned i = 0; i != PredQueue.size(); ++i) {
    unsigned BN = PredQueue[i];
    if (DefBlocks[BN])
      return true;
    const MachineBasicBlock *B = MF.getBlockNumbered(BN);
    for (const MachineBasicBlock *P : B->predecessors())
      PredQueue.insert(P->getNumber());
  }
  return false;
}