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/MachineInstr.h"
  26 #include "llvm/CodeGen/MachineLoopInfo.h"
  27 #include "llvm/CodeGen/MachineRegisterInfo.h"
  28 #include "llvm/CodeGen/Passes.h"
  29 #include "llvm/CodeGen/RegAllocRegistry.h"
  30 #include "llvm/CodeGen/Spiller.h"
  31 #include "llvm/CodeGen/TargetRegisterInfo.h"
  32 #include "llvm/CodeGen/VirtRegMap.h"
  33 #include "llvm/Pass.h"
  34 #include "llvm/Support/Debug.h"
  35 #include "llvm/Support/raw_ostream.h"
  36 #include <cstdlib>
  37 #include <queue>
  38
  39 using namespace llvm;
  40
  41 #define DEBUG_TYPE "regalloc"
  42
  43 static RegisterRegAlloc basicRegAlloc("basic", "basic register allocator",
  44                                       createBasicRegisterAllocator);
  45
  46 namespace {
  47   struct CompSpillWeight {
  48     bool operator()(LiveInterval *A, LiveInterval *B) const {
  49       return A->weight() < B->weight();
  50     }
  51   };
  52 }
  53
  54 namespace {
  55 /// RABasic provides a minimal implementation of the basic register allocation
  56 /// algorithm. It prioritizes live virtual registers by spill weight and spills
  57 /// whenever a register is unavailable. This is not practical in production but
  58 /// provides a useful baseline both for measuring other allocators and comparing
  59 /// the speed of the basic algorithm against other styles of allocators.
  60 class RABasic : public MachineFunctionPass,
  61                 public RegAllocBase,
  62                 private LiveRangeEdit::Delegate {
  63   // context
  64   MachineFunction *MF;
  65
  66   // state
  67   std::unique_ptr<Spiller> SpillerInstance;
  68   std::priority_queue<LiveInterval*, std::vector<LiveInterval*>,
  69                       CompSpillWeight> Queue;
  70
  71   // Scratch space.  Allocated here to avoid repeated malloc calls in
  72   // selectOrSplit().
  73   BitVector UsableRegs;
  74
  75   bool LRE_CanEraseVirtReg(Register) override;
  76   void LRE_WillShrinkVirtReg(Register) override;
  77
  78 public:
  79   RABasic(const RegClassFilterFunc F = allocateAllRegClasses);
  80
  81   /// Return the pass name.
  82   StringRef getPassName() const override { return "Basic Register Allocator"; }
  83
  84   /// RABasic analysis usage.
  85   void getAnalysisUsage(AnalysisUsage &AU) const override;
  86
  87   void releaseMemory() override;
  88
  89   Spiller &spiller() override { return *SpillerInstance; }
  90
  91   void enqueueImpl(LiveInterval *LI) override {
  92     Queue.push(LI);
  93   }
  94
  95   LiveInterval *dequeue() override {
  96     if (Queue.empty())
  97       return nullptr;
  98     LiveInterval *LI = Queue.top();
  99     Queue.pop();
 100     return LI;
 101   }
 102
 103   MCRegister selectOrSplit(LiveInterval &VirtReg,
 104                            SmallVectorImpl<Register> &SplitVRegs) override;
 105
 106   /// Perform register allocation.
 107   bool runOnMachineFunction(MachineFunction &mf) override;
 108
 109   MachineFunctionProperties getRequiredProperties() const override {
 110     return MachineFunctionProperties().set(
 111         MachineFunctionProperties::Property::NoPHIs);
 112   }
 113
 114   MachineFunctionProperties getClearedProperties() const override {
 115     return MachineFunctionProperties().set(
 116       MachineFunctionProperties::Property::IsSSA);
 117   }
 118
 119   // Helper for spilling all live virtual registers currently unified under preg
 120   // that interfere with the most recently queried lvr.  Return true if spilling
 121   // was successful, and append any new spilled/split intervals to splitLVRs.
 122   bool spillInterferences(LiveInterval &VirtReg, MCRegister PhysReg,
 123                           SmallVectorImpl<Register> &SplitVRegs);
 124
 125   static char ID;
 126 };
 127
 128 char RABasic::ID = 0;
 129
 130 } // end anonymous namespace
 131
 132 char &llvm::RABasicID = RABasic::ID;
 133
 134 INITIALIZE_PASS_BEGIN(RABasic, "regallocbasic", "Basic Register Allocator",
 135                       false, false)
 136 INITIALIZE_PASS_DEPENDENCY(LiveDebugVariables)
 137 INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
 138 INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
 139 INITIALIZE_PASS_DEPENDENCY(RegisterCoalescer)
 140 INITIALIZE_PASS_DEPENDENCY(MachineScheduler)
 141 INITIALIZE_PASS_DEPENDENCY(LiveStacks)
 142 INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
 143 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
 144 INITIALIZE_PASS_DEPENDENCY(VirtRegMap)
 145 INITIALIZE_PASS_DEPENDENCY(LiveRegMatrix)
 146 INITIALIZE_PASS_END(RABasic, "regallocbasic", "Basic Register Allocator", false,
 147                     false)
 148
 149 bool RABasic::LRE_CanEraseVirtReg(Register VirtReg) {
 150   LiveInterval &LI = LIS->getInterval(VirtReg);
 151   if (VRM->hasPhys(VirtReg)) {
 152     Matrix->unassign(LI);
 153     aboutToRemoveInterval(LI);
 154     return true;
 155   }
 156   // Unassigned virtreg is probably in the priority queue.
 157   // RegAllocBase will erase it after dequeueing.
 158   // Nonetheless, clear the live-range so that the debug
 159   // dump will show the right state for that VirtReg.
 160   LI.clear();
 161   return false;
 162 }
 163
 164 void RABasic::LRE_WillShrinkVirtReg(Register VirtReg) {
 165   if (!VRM->hasPhys(VirtReg))
 166     return;
 167
 168   // Register is assigned, put it back on the queue for reassignment.
 169   LiveInterval &LI = LIS->getInterval(VirtReg);
 170   Matrix->unassign(LI);
 171   enqueue(&LI);
 172 }
 173
 174 RABasic::RABasic(RegClassFilterFunc F):
 175   MachineFunctionPass(ID),
 176   RegAllocBase(F) {
 177 }
 178
 179 void RABasic::getAnalysisUsage(AnalysisUsage &AU) const {
 180   AU.setPreservesCFG();
 181   AU.addRequired<AAResultsWrapperPass>();
 182   AU.addPreserved<AAResultsWrapperPass>();
 183   AU.addRequired<LiveIntervals>();
 184   AU.addPreserved<LiveIntervals>();
 185   AU.addPreserved<SlotIndexes>();
 186   AU.addRequired<LiveDebugVariables>();
 187   AU.addPreserved<LiveDebugVariables>();
 188   AU.addRequired<LiveStacks>();
 189   AU.addPreserved<LiveStacks>();
 190   AU.addRequired<MachineBlockFrequencyInfo>();
 191   AU.addPreserved<MachineBlockFrequencyInfo>();
 192   AU.addRequiredID(MachineDominatorsID);
 193   AU.addPreservedID(MachineDominatorsID);
 194   AU.addRequired<MachineLoopInfo>();
 195   AU.addPreserved<MachineLoopInfo>();
 196   AU.addRequired<VirtRegMap>();
 197   AU.addPreserved<VirtRegMap>();
 198   AU.addRequired<LiveRegMatrix>();
 199   AU.addPreserved<LiveRegMatrix>();
 200   MachineFunctionPass::getAnalysisUsage(AU);
 201 }
 202
 203 void RABasic::releaseMemory() {
 204   SpillerInstance.reset();
 205 }
 206
 207
 208 // Spill or split all live virtual registers currently unified under PhysReg
 209 // that interfere with VirtReg. The newly spilled or split live intervals are
 210 // returned by appending them to SplitVRegs.
 211 bool RABasic::spillInterferences(LiveInterval &VirtReg, MCRegister PhysReg,
 212                                  SmallVectorImpl<Register> &SplitVRegs) {
 213   // Record each interference and determine if all are spillable before mutating
 214   // either the union or live intervals.
 215   SmallVector<LiveInterval*, 8> Intfs;
 216
 217   // Collect interferences assigned to any alias of the physical register.
 218   for (MCRegUnitIterator Units(PhysReg, TRI); Units.isValid(); ++Units) {
 219     LiveIntervalUnion::Query &Q = Matrix->query(VirtReg, *Units);
 220     Q.collectInterferingVRegs();
 221     for (unsigned i = Q.interferingVRegs().size(); i; --i) {
 222       LiveInterval *Intf = Q.interferingVRegs()[i - 1];
 223       if (!Intf->isSpillable() || Intf->weight() > VirtReg.weight())
 224         return false;
 225       Intfs.push_back(Intf);
 226     }
 227   }
 228   LLVM_DEBUG(dbgs() << "spilling " << printReg(PhysReg, TRI)
 229                     << " interferences with " << VirtReg << "\n");
 230   assert(!Intfs.empty() && "expected interference");
 231
 232   // Spill each interfering vreg allocated to PhysReg or an alias.
 233   for (unsigned i = 0, e = Intfs.size(); i != e; ++i) {
 234     LiveInterval &Spill = *Intfs[i];
 235
 236     // Skip duplicates.
 237     if (!VRM->hasPhys(Spill.reg()))
 238       continue;
 239
 240     // Deallocate the interfering vreg by removing it from the union.
 241     // A LiveInterval instance may not be in a union during modification!
 242     Matrix->unassign(Spill);
 243
 244     // Spill the extracted interval.
 245     LiveRangeEdit LRE(&Spill, SplitVRegs, *MF, *LIS, VRM, this, &DeadRemats);
 246     spiller().spill(LRE);
 247   }
 248   return true;
 249 }
 250
 251 // Driver for the register assignment and splitting heuristics.
 252 // Manages iteration over the LiveIntervalUnions.
 253 //
 254 // This is a minimal implementation of register assignment and splitting that
 255 // spills whenever we run out of registers.
 256 //
 257 // selectOrSplit can only be called once per live virtual register. We then do a
 258 // single interference test for each register the correct class until we find an
 259 // available register. So, the number of interference tests in the worst case is
 260 // |vregs| * |machineregs|. And since the number of interference tests is
 261 // minimal, there is no value in caching them outside the scope of
 262 // selectOrSplit().
 263 MCRegister RABasic::selectOrSplit(LiveInterval &VirtReg,
 264                                   SmallVectorImpl<Register> &SplitVRegs) {
 265   // Populate a list of physical register spill candidates.
 266   SmallVector<MCRegister, 8> PhysRegSpillCands;
 267
 268   // Check for an available register in this class.
 269   auto Order =
 270       AllocationOrder::create(VirtReg.reg(), *VRM, RegClassInfo, Matrix);
 271   for (MCRegister PhysReg : Order) {
 272     assert(PhysReg.isValid());
 273     // Check for interference in PhysReg
 274     switch (Matrix->checkInterference(VirtReg, PhysReg)) {
 275     case LiveRegMatrix::IK_Free:
 276       // PhysReg is available, allocate it.
 277       return PhysReg;
 278
 279     case LiveRegMatrix::IK_VirtReg:
 280       // Only virtual registers in the way, we may be able to spill them.
 281       PhysRegSpillCands.push_back(PhysReg);
 282       continue;
 283
 284     default:
 285       // RegMask or RegUnit interference.
 286       continue;
 287     }
 288   }
 289
 290   // Try to spill another interfering reg with less spill weight.
 291   for (MCRegister &PhysReg : PhysRegSpillCands) {
 292     if (!spillInterferences(VirtReg, PhysReg, SplitVRegs))
 293       continue;
 294
 295     assert(!Matrix->checkInterference(VirtReg, PhysReg) &&
 296            "Interference after spill.");
 297     // Tell the caller to allocate to this newly freed physical register.
 298     return PhysReg;
 299   }
 300
 301   // No other spill candidates were found, so spill the current VirtReg.
 302   LLVM_DEBUG(dbgs() << "spilling: " << VirtReg << '\n');
 303   if (!VirtReg.isSpillable())
 304     return ~0u;
 305   LiveRangeEdit LRE(&VirtReg, SplitVRegs, *MF, *LIS, VRM, this, &DeadRemats);
 306   spiller().spill(LRE);
 307
 308   // The live virtual register requesting allocation was spilled, so tell
 309   // the caller not to allocate anything during this round.
 310   return 0;
 311 }
 312
 313 bool RABasic::runOnMachineFunction(MachineFunction &mf) {
 314   LLVM_DEBUG(dbgs() << "********** BASIC REGISTER ALLOCATION **********\n"
 315                     << "********** Function: " << mf.getName() << '\n');
 316
 317   MF = &mf;
 318   RegAllocBase::init(getAnalysis<VirtRegMap>(),
 319                      getAnalysis<LiveIntervals>(),
 320                      getAnalysis<LiveRegMatrix>());
 321   VirtRegAuxInfo VRAI(*MF, *LIS, *VRM, getAnalysis<MachineLoopInfo>(),
 322                       getAnalysis<MachineBlockFrequencyInfo>());
 323   VRAI.calculateSpillWeightsAndHints();
 324
 325   SpillerInstance.reset(createInlineSpiller(*this, *MF, *VRM, VRAI));
 326
 327   allocatePhysRegs();
 328   postOptimization();
 329
 330   // Diagnostic output before rewriting
 331   LLVM_DEBUG(dbgs() << "Post alloc VirtRegMap:\n" << *VRM << "\n");
 332
 333   releaseMemory();
 334   return true;
 335 }
 336
 337 FunctionPass* llvm::createBasicRegisterAllocator() {
 338   return new RABasic();
 339 }
 340
 341 FunctionPass* llvm::createBasicRegisterAllocator(RegClassFilterFunc F) {
 342   return new RABasic(F);
 343 }