llvm/lib/CodeGen/RegAllocBasic.cpp

   1 //===-- RegAllocBasic.cpp - Basic Register Allocator ----------------------===//
   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 // This file defines the RABasic function pass, which provides a minimal
  10 // implementation of the basic register allocator.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "AllocationOrder.h"
  15 #include "LiveDebugVariables.h"
  16 #include "RegAllocBase.h"
  17 #include "llvm/Analysis/AliasAnalysis.h"
  18 #include "llvm/CodeGen/CalcSpillWeights.h"
  19 #include "llvm/CodeGen/LiveIntervals.h"
  20 #include "llvm/CodeGen/LiveRangeEdit.h"
  21 #include "llvm/CodeGen/LiveRegMatrix.h"
  22 #include "llvm/CodeGen/LiveStacks.h"
  23 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
  24 #include "llvm/CodeGen/MachineFunctionPass.h"
  25 #include "llvm/CodeGen/MachineLoopInfo.h"
  26 #include "llvm/CodeGen/Passes.h"
  27 #include "llvm/CodeGen/RegAllocRegistry.h"
  28 #include "llvm/CodeGen/Spiller.h"
  29 #include "llvm/CodeGen/TargetRegisterInfo.h"
  30 #include "llvm/CodeGen/VirtRegMap.h"
  31 #include "llvm/Pass.h"
  32 #include "llvm/Support/Debug.h"
  33 #include "llvm/Support/raw_ostream.h"
  34 #include <queue>
  35
  36 using namespace llvm;
  37
  38 #define DEBUG_TYPE "regalloc"
  39
  40 static RegisterRegAlloc basicRegAlloc("basic", "basic register allocator",
  41                                       createBasicRegisterAllocator);
  42
  43 namespace {
  44   struct CompSpillWeight {
  45     bool operator()(const LiveInterval *A, const LiveInterval *B) const {
  46       return A->weight() < B->weight();
  47     }
  48   };
  49 }
  50
  51 namespace {
  52 /// RABasic provides a minimal implementation of the basic register allocation
  53 /// algorithm. It prioritizes live virtual registers by spill weight and spills
  54 /// whenever a register is unavailable. This is not practical in production but
  55 /// provides a useful baseline both for measuring other allocators and comparing
  56 /// the speed of the basic algorithm against other styles of allocators.
  57 class RABasic : public MachineFunctionPass,
  58                 public RegAllocBase,
  59                 private LiveRangeEdit::Delegate {
  60   // context
  61   MachineFunction *MF = nullptr;
  62
  63   // state
  64   std::unique_ptr<Spiller> SpillerInstance;
  65   std::priority_queue<const LiveInterval *, std::vector<const LiveInterval *>,
  66                       CompSpillWeight>
  67       Queue;
  68
  69   // Scratch space.  Allocated here to avoid repeated malloc calls in
  70   // selectOrSplit().
  71   BitVector UsableRegs;
  72
  73   bool LRE_CanEraseVirtReg(Register) override;
  74   void LRE_WillShrinkVirtReg(Register) override;
  75
  76 public:
  77   RABasic(const RegClassFilterFunc F = allocateAllRegClasses);
  78
  79   /// Return the pass name.
  80   StringRef getPassName() const override { return "Basic Register Allocator"; }
  81
  82   /// RABasic analysis usage.
  83   void getAnalysisUsage(AnalysisUsage &AU) const override;
  84
  85   void releaseMemory() override;
  86
  87   Spiller &spiller() override { return *SpillerInstance; }
  88
  89   void enqueueImpl(const LiveInterval *LI) override { Queue.push(LI); }
  90
  91   const LiveInterval *dequeue() override {
  92     if (Queue.empty())
  93       return nullptr;
  94     const LiveInterval *LI = Queue.top();
  95     Queue.pop();
  96     return LI;
  97   }
  98
  99   MCRegister selectOrSplit(const LiveInterval &VirtReg,
 100                            SmallVectorImpl<Register> &SplitVRegs) override;
 101
 102   /// Perform register allocation.
 103   bool runOnMachineFunction(MachineFunction &mf) override;
 104
 105   MachineFunctionProperties getRequiredProperties() const override {
 106     return MachineFunctionProperties().set(
 107         MachineFunctionProperties::Property::NoPHIs);
 108   }
 109
 110   MachineFunctionProperties getClearedProperties() const override {
 111     return MachineFunctionProperties().set(
 112       MachineFunctionProperties::Property::IsSSA);
 113   }
 114
 115   // Helper for spilling all live virtual registers currently unified under preg
 116   // that interfere with the most recently queried lvr.  Return true if spilling
 117   // was successful, and append any new spilled/split intervals to splitLVRs.
 118   bool spillInterferences(const LiveInterval &VirtReg, MCRegister PhysReg,
 119                           SmallVectorImpl<Register> &SplitVRegs);
 120
 121   static char ID;
 122 };
 123
 124 char RABasic::ID = 0;
 125
 126 } // end anonymous namespace
 127
 128 char &llvm::RABasicID = RABasic::ID;
 129
 130 INITIALIZE_PASS_BEGIN(RABasic, "regallocbasic", "Basic Register Allocator",
 131                       false, false)
 132 INITIALIZE_PASS_DEPENDENCY(LiveDebugVariables)
 133 INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
 134 INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
 135 INITIALIZE_PASS_DEPENDENCY(RegisterCoalescer)
 136 INITIALIZE_PASS_DEPENDENCY(MachineScheduler)
 137 INITIALIZE_PASS_DEPENDENCY(LiveStacks)
 138 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
 139 INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
 140 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
 141 INITIALIZE_PASS_DEPENDENCY(VirtRegMap)
 142 INITIALIZE_PASS_DEPENDENCY(LiveRegMatrix)
 143 INITIALIZE_PASS_END(RABasic, "regallocbasic", "Basic Register Allocator", false,
 144                     false)
 145
 146 bool RABasic::LRE_CanEraseVirtReg(Register VirtReg) {
 147   LiveInterval &LI = LIS->getInterval(VirtReg);
 148   if (VRM->hasPhys(VirtReg)) {
 149     Matrix->unassign(LI);
 150     aboutToRemoveInterval(LI);
 151     return true;
 152   }
 153   // Unassigned virtreg is probably in the priority queue.
 154   // RegAllocBase will erase it after dequeueing.
 155   // Nonetheless, clear the live-range so that the debug
 156   // dump will show the right state for that VirtReg.
 157   LI.clear();
 158   return false;
 159 }
 160
 161 void RABasic::LRE_WillShrinkVirtReg(Register VirtReg) {
 162   if (!VRM->hasPhys(VirtReg))
 163     return;
 164
 165   // Register is assigned, put it back on the queue for reassignment.
 166   LiveInterval &LI = LIS->getInterval(VirtReg);
 167   Matrix->unassign(LI);
 168   enqueue(&LI);
 169 }
 170
 171 RABasic::RABasic(RegClassFilterFunc F):
 172   MachineFunctionPass(ID),
 173   RegAllocBase(F) {
 174 }
 175
 176 void RABasic::getAnalysisUsage(AnalysisUsage &AU) const {
 177   AU.setPreservesCFG();
 178   AU.addRequired<AAResultsWrapperPass>();
 179   AU.addPreserved<AAResultsWrapperPass>();
 180   AU.addRequired<LiveIntervals>();
 181   AU.addPreserved<LiveIntervals>();
 182   AU.addPreserved<SlotIndexes>();
 183   AU.addRequired<LiveDebugVariables>();
 184   AU.addPreserved<LiveDebugVariables>();
 185   AU.addRequired<LiveStacks>();
 186   AU.addPreserved<LiveStacks>();
 187   AU.addRequired<MachineBlockFrequencyInfo>();
 188   AU.addPreserved<MachineBlockFrequencyInfo>();
 189   AU.addRequiredID(MachineDominatorsID);
 190   AU.addPreservedID(MachineDominatorsID);
 191   AU.addRequired<MachineLoopInfo>();
 192   AU.addPreserved<MachineLoopInfo>();
 193   AU.addRequired<VirtRegMap>();
 194   AU.addPreserved<VirtRegMap>();
 195   AU.addRequired<LiveRegMatrix>();
 196   AU.addPreserved<LiveRegMatrix>();
 197   MachineFunctionPass::getAnalysisUsage(AU);
 198 }
 199
 200 void RABasic::releaseMemory() {
 201   SpillerInstance.reset();
 202 }
 203
 204
 205 // Spill or split all live virtual registers currently unified under PhysReg
 206 // that interfere with VirtReg. The newly spilled or split live intervals are
 207 // returned by appending them to SplitVRegs.
 208 bool RABasic::spillInterferences(const LiveInterval &VirtReg,
 209                                  MCRegister PhysReg,
 210                                  SmallVectorImpl<Register> &SplitVRegs) {
 211   // Record each interference and determine if all are spillable before mutating
 212   // either the union or live intervals.
 213   SmallVector<const LiveInterval *, 8> Intfs;
 214
 215   // Collect interferences assigned to any alias of the physical register.
 216   for (MCRegUnit Unit : TRI->regunits(PhysReg)) {
 217     LiveIntervalUnion::Query &Q = Matrix->query(VirtReg, Unit);
 218     for (const auto *Intf : reverse(Q.interferingVRegs())) {
 219       if (!Intf->isSpillable() || Intf->weight() > VirtReg.weight())
 220         return false;
 221       Intfs.push_back(Intf);
 222     }
 223   }
 224   LLVM_DEBUG(dbgs() << "spilling " << printReg(PhysReg, TRI)
 225                     << " interferences with " << VirtReg << "\n");
 226   assert(!Intfs.empty() && "expected interference");
 227
 228   // Spill each interfering vreg allocated to PhysReg or an alias.
 229   for (unsigned i = 0, e = Intfs.size(); i != e; ++i) {
 230     const LiveInterval &Spill = *Intfs[i];
 231
 232     // Skip duplicates.
 233     if (!VRM->hasPhys(Spill.reg()))
 234       continue;
 235
 236     // Deallocate the interfering vreg by removing it from the union.
 237     // A LiveInterval instance may not be in a union during modification!
 238     Matrix->unassign(Spill);
 239
 240     // Spill the extracted interval.
 241     LiveRangeEdit LRE(&Spill, SplitVRegs, *MF, *LIS, VRM, this, &DeadRemats);
 242     spiller().spill(LRE);
 243   }
 244   return true;
 245 }
 246
 247 // Driver for the register assignment and splitting heuristics.
 248 // Manages iteration over the LiveIntervalUnions.
 249 //
 250 // This is a minimal implementation of register assignment and splitting that
 251 // spills whenever we run out of registers.
 252 //
 253 // selectOrSplit can only be called once per live virtual register. We then do a
 254 // single interference test for each register the correct class until we find an
 255 // available register. So, the number of interference tests in the worst case is
 256 // |vregs| * |machineregs|. And since the number of interference tests is
 257 // minimal, there is no value in caching them outside the scope of
 258 // selectOrSplit().
 259 MCRegister RABasic::selectOrSplit(const LiveInterval &VirtReg,
 260                                   SmallVectorImpl<Register> &SplitVRegs) {
 261   // Populate a list of physical register spill candidates.
 262   SmallVector<MCRegister, 8> PhysRegSpillCands;
 263
 264   // Check for an available register in this class.
 265   auto Order =
 266       AllocationOrder::create(VirtReg.reg(), *VRM, RegClassInfo, Matrix);
 267   for (MCRegister PhysReg : Order) {
 268     assert(PhysReg.isValid());
 269     // Check for interference in PhysReg
 270     switch (Matrix->checkInterference(VirtReg, PhysReg)) {
 271     case LiveRegMatrix::IK_Free:
 272       // PhysReg is available, allocate it.
 273       return PhysReg;
 274
 275     case LiveRegMatrix::IK_VirtReg:
 276       // Only virtual registers in the way, we may be able to spill them.
 277       PhysRegSpillCands.push_back(PhysReg);
 278       continue;
 279
 280     default:
 281       // RegMask or RegUnit interference.
 282       continue;
 283     }
 284   }
 285
 286   // Try to spill another interfering reg with less spill weight.
 287   for (MCRegister &PhysReg : PhysRegSpillCands) {
 288     if (!spillInterferences(VirtReg, PhysReg, SplitVRegs))
 289       continue;
 290
 291     assert(!Matrix->checkInterference(VirtReg, PhysReg) &&
 292            "Interference after spill.");
 293     // Tell the caller to allocate to this newly freed physical register.
 294     return PhysReg;
 295   }
 296
 297   // No other spill candidates were found, so spill the current VirtReg.
 298   LLVM_DEBUG(dbgs() << "spilling: " << VirtReg << '\n');
 299   if (!VirtReg.isSpillable())
 300     return ~0u;
 301   LiveRangeEdit LRE(&VirtReg, SplitVRegs, *MF, *LIS, VRM, this, &DeadRemats);
 302   spiller().spill(LRE);
 303
 304   // The live virtual register requesting allocation was spilled, so tell
 305   // the caller not to allocate anything during this round.
 306   return 0;
 307 }
 308
 309 bool RABasic::runOnMachineFunction(MachineFunction &mf) {
 310   LLVM_DEBUG(dbgs() << "********** BASIC REGISTER ALLOCATION **********\n"
 311                     << "********** Function: " << mf.getName() << '\n');
 312
 313   MF = &mf;
 314   RegAllocBase::init(getAnalysis<VirtRegMap>(),
 315                      getAnalysis<LiveIntervals>(),
 316                      getAnalysis<LiveRegMatrix>());
 317   VirtRegAuxInfo VRAI(*MF, *LIS, *VRM, getAnalysis<MachineLoopInfo>(),
 318                       getAnalysis<MachineBlockFrequencyInfo>());
 319   VRAI.calculateSpillWeightsAndHints();
 320
 321   SpillerInstance.reset(createInlineSpiller(*this, *MF, *VRM, VRAI));
 322
 323   allocatePhysRegs();
 324   postOptimization();
 325
 326   // Diagnostic output before rewriting
 327   LLVM_DEBUG(dbgs() << "Post alloc VirtRegMap:\n" << *VRM << "\n");
 328
 329   releaseMemory();
 330   return true;
 331 }
 332
 333 FunctionPass* llvm::createBasicRegisterAllocator() {
 334   return new RABasic();
 335 }
 336
 337 FunctionPass* llvm::createBasicRegisterAllocator(RegClassFilterFunc F) {
 338   return new RABasic(F);
 339 }