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