llvm/lib/CodeGen/LiveRangeCalc.cpp

   1 //===- LiveRangeCalc.cpp - Calculate live ranges -------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // Implementation of the LiveRangeCalc class.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "llvm/CodeGen/LiveRangeCalc.h"
  14 #include "llvm/ADT/BitVector.h"
  15 #include "llvm/ADT/STLExtras.h"
  16 #include "llvm/ADT/SetVector.h"
  17 #include "llvm/ADT/SmallVector.h"
  18 #include "llvm/CodeGen/LiveInterval.h"
  19 #include "llvm/CodeGen/MachineBasicBlock.h"
  20 #include "llvm/CodeGen/MachineDominators.h"
  21 #include "llvm/CodeGen/MachineFunction.h"
  22 #include "llvm/CodeGen/MachineInstr.h"
  23 #include "llvm/CodeGen/MachineRegisterInfo.h"
  24 #include "llvm/CodeGen/SlotIndexes.h"
  25 #include "llvm/CodeGen/TargetRegisterInfo.h"
  26 #include "llvm/Support/ErrorHandling.h"
  27 #include "llvm/Support/raw_ostream.h"
  28 #include <algorithm>
  29 #include <cassert>
  30 #include <iterator>
  31 #include <tuple>
  32 #include <utility>
  33
  34 using namespace llvm;
  35
  36 #define DEBUG_TYPE "regalloc"
  37
  38 // Reserve an address that indicates a value that is known to be "undef".
  39 static VNInfo UndefVNI(0xbad, SlotIndex());
  40
  41 void LiveRangeCalc::resetLiveOutMap() {
  42   unsigned NumBlocks = MF->getNumBlockIDs();
  43   Seen.clear();
  44   Seen.resize(NumBlocks);
  45   EntryInfos.clear();
  46   Map.resize(NumBlocks);
  47 }
  48
  49 void LiveRangeCalc::reset(const MachineFunction *mf,
  50                           SlotIndexes *SI,
  51                           MachineDominatorTree *MDT,
  52                           VNInfo::Allocator *VNIA) {
  53   MF = mf;
  54   MRI = &MF->getRegInfo();
  55   Indexes = SI;
  56   DomTree = MDT;
  57   Alloc = VNIA;
  58   resetLiveOutMap();
  59   LiveIn.clear();
  60 }
  61
  62 void LiveRangeCalc::updateFromLiveIns() {
  63   LiveRangeUpdater Updater;
  64   for (const LiveInBlock &I : LiveIn) {
  65     if (!I.DomNode)
  66       continue;
  67     MachineBasicBlock *MBB = I.DomNode->getBlock();
  68     assert(I.Value && "No live-in value found");
  69     SlotIndex Start, End;
  70     std::tie(Start, End) = Indexes->getMBBRange(MBB);
  71
  72     if (I.Kill.isValid())
  73       // Value is killed inside this block.
  74       End = I.Kill;
  75     else {
  76       // The value is live-through, update LiveOut as well.
  77       // Defer the Domtree lookup until it is needed.
  78       assert(Seen.test(MBB->getNumber()));
  79       Map[MBB] = LiveOutPair(I.Value, nullptr);
  80     }
  81     Updater.setDest(&I.LR);
  82     Updater.add(Start, End, I.Value);
  83   }
  84   LiveIn.clear();
  85 }
  86
  87 void LiveRangeCalc::extend(LiveRange &LR, SlotIndex Use, unsigned PhysReg,
  88                            ArrayRef<SlotIndex> Undefs) {
  89   assert(Use.isValid() && "Invalid SlotIndex");
  90   assert(Indexes && "Missing SlotIndexes");
  91   assert(DomTree && "Missing dominator tree");
  92
  93   MachineBasicBlock *UseMBB = Indexes->getMBBFromIndex(Use.getPrevSlot());
  94   assert(UseMBB && "No MBB at Use");
  95
  96   // Is there a def in the same MBB we can extend?
  97   auto EP = LR.extendInBlock(Undefs, Indexes->getMBBStartIdx(UseMBB), Use);
  98   if (EP.first != nullptr || EP.second)
  99     return;
 100
 101   // Find the single reaching def, or determine if Use is jointly dominated by
 102   // multiple values, and we may need to create even more phi-defs to preserve
 103   // VNInfo SSA form.  Perform a search for all predecessor blocks where we
 104   // know the dominating VNInfo.
 105   if (findReachingDefs(LR, *UseMBB, Use, PhysReg, Undefs))
 106     return;
 107
 108   // When there were multiple different values, we may need new PHIs.
 109   calculateValues();
 110 }
 111
 112 // This function is called by a client after using the low-level API to add
 113 // live-out and live-in blocks.  The unique value optimization is not
 114 // available, SplitEditor::transferValues handles that case directly anyway.
 115 void LiveRangeCalc::calculateValues() {
 116   assert(Indexes && "Missing SlotIndexes");
 117   assert(DomTree && "Missing dominator tree");
 118   updateSSA();
 119   updateFromLiveIns();
 120 }
 121
 122 bool LiveRangeCalc::isDefOnEntry(LiveRange &LR, ArrayRef<SlotIndex> Undefs,
 123                                  MachineBasicBlock &MBB, BitVector &DefOnEntry,
 124                                  BitVector &UndefOnEntry) {
 125   unsigned BN = MBB.getNumber();
 126   if (DefOnEntry[BN])
 127     return true;
 128   if (UndefOnEntry[BN])
 129     return false;
 130
 131   auto MarkDefined = [BN, &DefOnEntry](MachineBasicBlock &B) -> bool {
 132     for (MachineBasicBlock *S : B.successors())
 133       DefOnEntry[S->getNumber()] = true;
 134     DefOnEntry[BN] = true;
 135     return true;
 136   };
 137
 138   SetVector<unsigned> WorkList;
 139   // Checking if the entry of MBB is reached by some def: add all predecessors
 140   // that are potentially defined-on-exit to the work list.
 141   for (MachineBasicBlock *P : MBB.predecessors())
 142     WorkList.insert(P->getNumber());
 143
 144   for (unsigned i = 0; i != WorkList.size(); ++i) {
 145     // Determine if the exit from the block is reached by some def.
 146     unsigned N = WorkList[i];
 147     MachineBasicBlock &B = *MF->getBlockNumbered(N);
 148     if (Seen[N]) {
 149       const LiveOutPair &LOB = Map[&B];
 150       if (LOB.first != nullptr && LOB.first != &UndefVNI)
 151         return MarkDefined(B);
 152     }
 153     SlotIndex Begin, End;
 154     std::tie(Begin, End) = Indexes->getMBBRange(&B);
 155     // Treat End as not belonging to B.
 156     // If LR has a segment S that starts at the next block, i.e. [End, ...),
 157     // std::upper_bound will return the segment following S. Instead,
 158     // S should be treated as the first segment that does not overlap B.
 159     LiveRange::iterator UB = upper_bound(LR, End.getPrevSlot());
 160     if (UB != LR.begin()) {
 161       LiveRange::Segment &Seg = *std::prev(UB);
 162       if (Seg.end > Begin) {
 163         // There is a segment that overlaps B. If the range is not explicitly
 164         // undefined between the end of the segment and the end of the block,
 165         // treat the block as defined on exit. If it is, go to the next block
 166         // on the work list.
 167         if (LR.isUndefIn(Undefs, Seg.end, End))
 168           continue;
 169         return MarkDefined(B);
 170       }
 171     }
 172
 173     // No segment overlaps with this block. If this block is not defined on
 174     // entry, or it undefines the range, do not process its predecessors.
 175     if (UndefOnEntry[N] || LR.isUndefIn(Undefs, Begin, End)) {
 176       UndefOnEntry[N] = true;
 177       continue;
 178     }
 179     if (DefOnEntry[N])
 180       return MarkDefined(B);
 181
 182     // Still don't know: add all predecessors to the work list.
 183     for (MachineBasicBlock *P : B.predecessors())
 184       WorkList.insert(P->getNumber());
 185   }
 186
 187   UndefOnEntry[BN] = true;
 188   return false;
 189 }
 190
 191 bool LiveRangeCalc::findReachingDefs(LiveRange &LR, MachineBasicBlock &UseMBB,
 192                                      SlotIndex Use, unsigned PhysReg,
 193                                      ArrayRef<SlotIndex> Undefs) {
 194   unsigned UseMBBNum = UseMBB.getNumber();
 195
 196   // Block numbers where LR should be live-in.
 197   SmallVector<unsigned, 16> WorkList(1, UseMBBNum);
 198
 199   // Remember if we have seen more than one value.
 200   bool UniqueVNI = true;
 201   VNInfo *TheVNI = nullptr;
 202
 203   bool FoundUndef = false;
 204
 205   // Using Seen as a visited set, perform a BFS for all reaching defs.
 206   for (unsigned i = 0; i != WorkList.size(); ++i) {
 207     MachineBasicBlock *MBB = MF->getBlockNumbered(WorkList[i]);
 208
 209 #ifndef NDEBUG
 210     if (MBB->pred_empty()) {
 211       MBB->getParent()->verify();
 212       errs() << "Use of " << printReg(PhysReg, MRI->getTargetRegisterInfo())
 213              << " does not have a corresponding definition on every path:\n";
 214       const MachineInstr *MI = Indexes->getInstructionFromIndex(Use);
 215       if (MI != nullptr)
 216         errs() << Use << " " << *MI;
 217       report_fatal_error("Use not jointly dominated by defs.");
 218     }
 219
 220     if (Register::isPhysicalRegister(PhysReg)) {
 221       const TargetRegisterInfo *TRI = MRI->getTargetRegisterInfo();
 222       bool IsLiveIn = MBB->isLiveIn(PhysReg);
 223       for (MCRegAliasIterator Alias(PhysReg, TRI, false); !IsLiveIn && Alias.isValid(); ++Alias)
 224         IsLiveIn = MBB->isLiveIn(*Alias);
 225       if (!IsLiveIn) {
 226         MBB->getParent()->verify();
 227         errs() << "The register " << printReg(PhysReg, TRI)
 228                << " needs to be live in to " << printMBBReference(*MBB)
 229                << ", but is missing from the live-in list.\n";
 230         report_fatal_error("Invalid global physical register");
 231       }
 232     }
 233 #endif
 234     FoundUndef |= MBB->pred_empty();
 235
 236     for (MachineBasicBlock *Pred : MBB->predecessors()) {
 237        // Is this a known live-out block?
 238        if (Seen.test(Pred->getNumber())) {
 239          if (VNInfo *VNI = Map[Pred].first) {
 240            if (TheVNI && TheVNI != VNI)
 241              UniqueVNI = false;
 242            TheVNI = VNI;
 243          }
 244          continue;
 245        }
 246
 247        SlotIndex Start, End;
 248        std::tie(Start, End) = Indexes->getMBBRange(Pred);
 249
 250        // First time we see Pred.  Try to determine the live-out value, but set
 251        // it as null if Pred is live-through with an unknown value.
 252        auto EP = LR.extendInBlock(Undefs, Start, End);
 253        VNInfo *VNI = EP.first;
 254        FoundUndef |= EP.second;
 255        setLiveOutValue(Pred, EP.second ? &UndefVNI : VNI);
 256        if (VNI) {
 257          if (TheVNI && TheVNI != VNI)
 258            UniqueVNI = false;
 259          TheVNI = VNI;
 260        }
 261        if (VNI || EP.second)
 262          continue;
 263
 264        // No, we need a live-in value for Pred as well
 265        if (Pred != &UseMBB)
 266          WorkList.push_back(Pred->getNumber());
 267        else
 268           // Loopback to UseMBB, so value is really live through.
 269          Use = SlotIndex();
 270     }
 271   }
 272
 273   LiveIn.clear();
 274   FoundUndef |= (TheVNI == nullptr || TheVNI == &UndefVNI);
 275   if (!Undefs.empty() && FoundUndef)
 276     UniqueVNI = false;
 277
 278   // Both updateSSA() and LiveRangeUpdater benefit from ordered blocks, but
 279   // neither require it. Skip the sorting overhead for small updates.
 280   if (WorkList.size() > 4)
 281     array_pod_sort(WorkList.begin(), WorkList.end());
 282
 283   // If a unique reaching def was found, blit in the live ranges immediately.
 284   if (UniqueVNI) {
 285     assert(TheVNI != nullptr && TheVNI != &UndefVNI);
 286     LiveRangeUpdater Updater(&LR);
 287     for (unsigned BN : WorkList) {
 288       SlotIndex Start, End;
 289       std::tie(Start, End) = Indexes->getMBBRange(BN);
 290       // Trim the live range in UseMBB.
 291       if (BN == UseMBBNum && Use.isValid())
 292         End = Use;
 293       else
 294         Map[MF->getBlockNumbered(BN)] = LiveOutPair(TheVNI, nullptr);
 295       Updater.add(Start, End, TheVNI);
 296     }
 297     return true;
 298   }
 299
 300   // Prepare the defined/undefined bit vectors.
 301   EntryInfoMap::iterator Entry;
 302   bool DidInsert;
 303   std::tie(Entry, DidInsert) = EntryInfos.insert(
 304       std::make_pair(&LR, std::make_pair(BitVector(), BitVector())));
 305   if (DidInsert) {
 306     // Initialize newly inserted entries.
 307     unsigned N = MF->getNumBlockIDs();
 308     Entry->second.first.resize(N);
 309     Entry->second.second.resize(N);
 310   }
 311   BitVector &DefOnEntry = Entry->second.first;
 312   BitVector &UndefOnEntry = Entry->second.second;
 313
 314   // Multiple values were found, so transfer the work list to the LiveIn array
 315   // where UpdateSSA will use it as a work list.
 316   LiveIn.reserve(WorkList.size());
 317   for (unsigned BN : WorkList) {
 318     MachineBasicBlock *MBB = MF->getBlockNumbered(BN);
 319     if (!Undefs.empty() &&
 320         !isDefOnEntry(LR, Undefs, *MBB, DefOnEntry, UndefOnEntry))
 321       continue;
 322     addLiveInBlock(LR, DomTree->getNode(MBB));
 323     if (MBB == &UseMBB)
 324       LiveIn.back().Kill = Use;
 325   }
 326
 327   return false;
 328 }
 329
 330 // This is essentially the same iterative algorithm that SSAUpdater uses,
 331 // except we already have a dominator tree, so we don't have to recompute it.
 332 void LiveRangeCalc::updateSSA() {
 333   assert(Indexes && "Missing SlotIndexes");
 334   assert(DomTree && "Missing dominator tree");
 335
 336   // Interate until convergence.
 337   bool Changed;
 338   do {
 339     Changed = false;
 340     // Propagate live-out values down the dominator tree, inserting phi-defs
 341     // when necessary.
 342     for (LiveInBlock &I : LiveIn) {
 343       MachineDomTreeNode *Node = I.DomNode;
 344       // Skip block if the live-in value has already been determined.
 345       if (!Node)
 346         continue;
 347       MachineBasicBlock *MBB = Node->getBlock();
 348       MachineDomTreeNode *IDom = Node->getIDom();
 349       LiveOutPair IDomValue;
 350
 351       // We need a live-in value to a block with no immediate dominator?
 352       // This is probably an unreachable block that has survived somehow.
 353       bool needPHI = !IDom || !Seen.test(IDom->getBlock()->getNumber());
 354
 355       // IDom dominates all of our predecessors, but it may not be their
 356       // immediate dominator. Check if any of them have live-out values that are
 357       // properly dominated by IDom. If so, we need a phi-def here.
 358       if (!needPHI) {
 359         IDomValue = Map[IDom->getBlock()];
 360
 361         // Cache the DomTree node that defined the value.
 362         if (IDomValue.first && IDomValue.first != &UndefVNI &&
 363             !IDomValue.second) {
 364           Map[IDom->getBlock()].second = IDomValue.second =
 365             DomTree->getNode(Indexes->getMBBFromIndex(IDomValue.first->def));
 366         }
 367
 368         for (MachineBasicBlock *Pred : MBB->predecessors()) {
 369           LiveOutPair &Value = Map[Pred];
 370           if (!Value.first || Value.first == IDomValue.first)
 371             continue;
 372           if (Value.first == &UndefVNI) {
 373             needPHI = true;
 374             break;
 375           }
 376
 377           // Cache the DomTree node that defined the value.
 378           if (!Value.second)
 379             Value.second =
 380               DomTree->getNode(Indexes->getMBBFromIndex(Value.first->def));
 381
 382           // This predecessor is carrying something other than IDomValue.
 383           // It could be because IDomValue hasn't propagated yet, or it could be
 384           // because MBB is in the dominance frontier of that value.
 385           if (DomTree->dominates(IDom, Value.second)) {
 386             needPHI = true;
 387             break;
 388           }
 389         }
 390       }
 391
 392       // The value may be live-through even if Kill is set, as can happen when
 393       // we are called from extendRange. In that case LiveOutSeen is true, and
 394       // LiveOut indicates a foreign or missing value.
 395       LiveOutPair &LOP = Map[MBB];
 396
 397       // Create a phi-def if required.
 398       if (needPHI) {
 399         Changed = true;
 400         assert(Alloc && "Need VNInfo allocator to create PHI-defs");
 401         SlotIndex Start, End;
 402         std::tie(Start, End) = Indexes->getMBBRange(MBB);
 403         LiveRange &LR = I.LR;
 404         VNInfo *VNI = LR.getNextValue(Start, *Alloc);
 405         I.Value = VNI;
 406         // This block is done, we know the final value.
 407         I.DomNode = nullptr;
 408
 409         // Add liveness since updateFromLiveIns now skips this node.
 410         if (I.Kill.isValid()) {
 411           if (VNI)
 412             LR.addSegment(LiveInterval::Segment(Start, I.Kill, VNI));
 413         } else {
 414           if (VNI)
 415             LR.addSegment(LiveInterval::Segment(Start, End, VNI));
 416           LOP = LiveOutPair(VNI, Node);
 417         }
 418       } else if (IDomValue.first && IDomValue.first != &UndefVNI) {
 419         // No phi-def here. Remember incoming value.
 420         I.Value = IDomValue.first;
 421
 422         // If the IDomValue is killed in the block, don't propagate through.
 423         if (I.Kill.isValid())
 424           continue;
 425
 426         // Propagate IDomValue if it isn't killed:
 427         // MBB is live-out and doesn't define its own value.
 428         if (LOP.first == IDomValue.first)
 429           continue;
 430         Changed = true;
 431         LOP = IDomValue;
 432       }
 433     }
 434   } while (Changed);
 435 }
 436
 437 bool LiveRangeCalc::isJointlyDominated(const MachineBasicBlock *MBB,
 438                                        ArrayRef<SlotIndex> Defs,
 439                                        const SlotIndexes &Indexes) {
 440   const MachineFunction &MF = *MBB->getParent();
 441   BitVector DefBlocks(MF.getNumBlockIDs());
 442   for (SlotIndex I : Defs)
 443     DefBlocks.set(Indexes.getMBBFromIndex(I)->getNumber());
 444
 445   SetVector<unsigned> PredQueue;
 446   PredQueue.insert(MBB->getNumber());
 447   for (unsigned i = 0; i != PredQueue.size(); ++i) {
 448     unsigned BN = PredQueue[i];
 449     if (DefBlocks[BN])
 450       return true;
 451     const MachineBasicBlock *B = MF.getBlockNumbered(BN);
 452     for (const MachineBasicBlock *P : B->predecessors())
 453       PredQueue.insert(P->getNumber());
 454   }
 455   return false;
 456 }