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      1 //===-- RegAllocBasic.cpp - Basic Register Allocator ----------------------===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 // This file defines the RABasic function pass, which provides a minimal
     11 // implementation of the basic register allocator.
     12 //
     13 //===----------------------------------------------------------------------===//
     14 
     15 #include "llvm/CodeGen/Passes.h"
     16 #include "AllocationOrder.h"
     17 #include "LiveDebugVariables.h"
     18 #include "RegAllocBase.h"
     19 #include "Spiller.h"
     20 #include "llvm/Analysis/AliasAnalysis.h"
     21 #include "llvm/CodeGen/CalcSpillWeights.h"
     22 #include "llvm/CodeGen/LiveIntervalAnalysis.h"
     23 #include "llvm/CodeGen/LiveRangeEdit.h"
     24 #include "llvm/CodeGen/LiveRegMatrix.h"
     25 #include "llvm/CodeGen/LiveStackAnalysis.h"
     26 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
     27 #include "llvm/CodeGen/MachineFunctionPass.h"
     28 #include "llvm/CodeGen/MachineInstr.h"
     29 #include "llvm/CodeGen/MachineLoopInfo.h"
     30 #include "llvm/CodeGen/MachineRegisterInfo.h"
     31 #include "llvm/CodeGen/RegAllocRegistry.h"
     32 #include "llvm/CodeGen/VirtRegMap.h"
     33 #include "llvm/PassAnalysisSupport.h"
     34 #include "llvm/Support/Debug.h"
     35 #include "llvm/Support/raw_ostream.h"
     36 #include "llvm/Target/TargetRegisterInfo.h"
     37 #include <cstdlib>
     38 #include <queue>
     39 
     40 using namespace llvm;
     41 
     42 #define DEBUG_TYPE "regalloc"
     43 
     44 static RegisterRegAlloc basicRegAlloc("basic", "basic register allocator",
     45                                       createBasicRegisterAllocator);
     46 
     47 namespace {
     48   struct CompSpillWeight {
     49     bool operator()(LiveInterval *A, LiveInterval *B) const {
     50       return A->weight < B->weight;
     51     }
     52   };
     53 }
     54 
     55 namespace {
     56 /// RABasic provides a minimal implementation of the basic register allocation
     57 /// algorithm. It prioritizes live virtual registers by spill weight and spills
     58 /// whenever a register is unavailable. This is not practical in production but
     59 /// provides a useful baseline both for measuring other allocators and comparing
     60 /// the speed of the basic algorithm against other styles of allocators.
     61 class RABasic : public MachineFunctionPass, public RegAllocBase
     62 {
     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 public:
     76   RABasic();
     77 
     78   /// Return the pass name.
     79   const char* getPassName() const override {
     80     return "Basic Register Allocator";
     81   }
     82 
     83   /// RABasic analysis usage.
     84   void getAnalysisUsage(AnalysisUsage &AU) const override;
     85 
     86   void releaseMemory() override;
     87 
     88   Spiller &spiller() override { return *SpillerInstance; }
     89 
     90   void enqueue(LiveInterval *LI) override {
     91     Queue.push(LI);
     92   }
     93 
     94   LiveInterval *dequeue() override {
     95     if (Queue.empty())
     96       return nullptr;
     97     LiveInterval *LI = Queue.top();
     98     Queue.pop();
     99     return LI;
    100   }
    101 
    102   unsigned selectOrSplit(LiveInterval &VirtReg,
    103                          SmallVectorImpl<unsigned> &SplitVRegs) override;
    104 
    105   /// Perform register allocation.
    106   bool runOnMachineFunction(MachineFunction &mf) override;
    107 
    108   // Helper for spilling all live virtual registers currently unified under preg
    109   // that interfere with the most recently queried lvr.  Return true if spilling
    110   // was successful, and append any new spilled/split intervals to splitLVRs.
    111   bool spillInterferences(LiveInterval &VirtReg, unsigned PhysReg,
    112                           SmallVectorImpl<unsigned> &SplitVRegs);
    113 
    114   static char ID;
    115 };
    116 
    117 char RABasic::ID = 0;
    118 
    119 } // end anonymous namespace
    120 
    121 RABasic::RABasic(): MachineFunctionPass(ID) {
    122   initializeLiveDebugVariablesPass(*PassRegistry::getPassRegistry());
    123   initializeLiveIntervalsPass(*PassRegistry::getPassRegistry());
    124   initializeSlotIndexesPass(*PassRegistry::getPassRegistry());
    125   initializeRegisterCoalescerPass(*PassRegistry::getPassRegistry());
    126   initializeMachineSchedulerPass(*PassRegistry::getPassRegistry());
    127   initializeLiveStacksPass(*PassRegistry::getPassRegistry());
    128   initializeMachineDominatorTreePass(*PassRegistry::getPassRegistry());
    129   initializeMachineLoopInfoPass(*PassRegistry::getPassRegistry());
    130   initializeVirtRegMapPass(*PassRegistry::getPassRegistry());
    131   initializeLiveRegMatrixPass(*PassRegistry::getPassRegistry());
    132 }
    133 
    134 void RABasic::getAnalysisUsage(AnalysisUsage &AU) const {
    135   AU.setPreservesCFG();
    136   AU.addRequired<AAResultsWrapperPass>();
    137   AU.addPreserved<AAResultsWrapperPass>();
    138   AU.addRequired<LiveIntervals>();
    139   AU.addPreserved<LiveIntervals>();
    140   AU.addPreserved<SlotIndexes>();
    141   AU.addRequired<LiveDebugVariables>();
    142   AU.addPreserved<LiveDebugVariables>();
    143   AU.addRequired<LiveStacks>();
    144   AU.addPreserved<LiveStacks>();
    145   AU.addRequired<MachineBlockFrequencyInfo>();
    146   AU.addPreserved<MachineBlockFrequencyInfo>();
    147   AU.addRequiredID(MachineDominatorsID);
    148   AU.addPreservedID(MachineDominatorsID);
    149   AU.addRequired<MachineLoopInfo>();
    150   AU.addPreserved<MachineLoopInfo>();
    151   AU.addRequired<VirtRegMap>();
    152   AU.addPreserved<VirtRegMap>();
    153   AU.addRequired<LiveRegMatrix>();
    154   AU.addPreserved<LiveRegMatrix>();
    155   MachineFunctionPass::getAnalysisUsage(AU);
    156 }
    157 
    158 void RABasic::releaseMemory() {
    159   SpillerInstance.reset();
    160 }
    161 
    162 
    163 // Spill or split all live virtual registers currently unified under PhysReg
    164 // that interfere with VirtReg. The newly spilled or split live intervals are
    165 // returned by appending them to SplitVRegs.
    166 bool RABasic::spillInterferences(LiveInterval &VirtReg, unsigned PhysReg,
    167                                  SmallVectorImpl<unsigned> &SplitVRegs) {
    168   // Record each interference and determine if all are spillable before mutating
    169   // either the union or live intervals.
    170   SmallVector<LiveInterval*, 8> Intfs;
    171 
    172   // Collect interferences assigned to any alias of the physical register.
    173   for (MCRegUnitIterator Units(PhysReg, TRI); Units.isValid(); ++Units) {
    174     LiveIntervalUnion::Query &Q = Matrix->query(VirtReg, *Units);
    175     Q.collectInterferingVRegs();
    176     if (Q.seenUnspillableVReg())
    177       return false;
    178     for (unsigned i = Q.interferingVRegs().size(); i; --i) {
    179       LiveInterval *Intf = Q.interferingVRegs()[i - 1];
    180       if (!Intf->isSpillable() || Intf->weight > VirtReg.weight)
    181         return false;
    182       Intfs.push_back(Intf);
    183     }
    184   }
    185   DEBUG(dbgs() << "spilling " << TRI->getName(PhysReg) <<
    186         " interferences with " << VirtReg << "\n");
    187   assert(!Intfs.empty() && "expected interference");
    188 
    189   // Spill each interfering vreg allocated to PhysReg or an alias.
    190   for (unsigned i = 0, e = Intfs.size(); i != e; ++i) {
    191     LiveInterval &Spill = *Intfs[i];
    192 
    193     // Skip duplicates.
    194     if (!VRM->hasPhys(Spill.reg))
    195       continue;
    196 
    197     // Deallocate the interfering vreg by removing it from the union.
    198     // A LiveInterval instance may not be in a union during modification!
    199     Matrix->unassign(Spill);
    200 
    201     // Spill the extracted interval.
    202     LiveRangeEdit LRE(&Spill, SplitVRegs, *MF, *LIS, VRM);
    203     spiller().spill(LRE);
    204   }
    205   return true;
    206 }
    207 
    208 // Driver for the register assignment and splitting heuristics.
    209 // Manages iteration over the LiveIntervalUnions.
    210 //
    211 // This is a minimal implementation of register assignment and splitting that
    212 // spills whenever we run out of registers.
    213 //
    214 // selectOrSplit can only be called once per live virtual register. We then do a
    215 // single interference test for each register the correct class until we find an
    216 // available register. So, the number of interference tests in the worst case is
    217 // |vregs| * |machineregs|. And since the number of interference tests is
    218 // minimal, there is no value in caching them outside the scope of
    219 // selectOrSplit().
    220 unsigned RABasic::selectOrSplit(LiveInterval &VirtReg,
    221                                 SmallVectorImpl<unsigned> &SplitVRegs) {
    222   // Populate a list of physical register spill candidates.
    223   SmallVector<unsigned, 8> PhysRegSpillCands;
    224 
    225   // Check for an available register in this class.
    226   AllocationOrder Order(VirtReg.reg, *VRM, RegClassInfo, Matrix);
    227   while (unsigned PhysReg = Order.next()) {
    228     // Check for interference in PhysReg
    229     switch (Matrix->checkInterference(VirtReg, PhysReg)) {
    230     case LiveRegMatrix::IK_Free:
    231       // PhysReg is available, allocate it.
    232       return PhysReg;
    233 
    234     case LiveRegMatrix::IK_VirtReg:
    235       // Only virtual registers in the way, we may be able to spill them.
    236       PhysRegSpillCands.push_back(PhysReg);
    237       continue;
    238 
    239     default:
    240       // RegMask or RegUnit interference.
    241       continue;
    242     }
    243   }
    244 
    245   // Try to spill another interfering reg with less spill weight.
    246   for (SmallVectorImpl<unsigned>::iterator PhysRegI = PhysRegSpillCands.begin(),
    247        PhysRegE = PhysRegSpillCands.end(); PhysRegI != PhysRegE; ++PhysRegI) {
    248     if (!spillInterferences(VirtReg, *PhysRegI, SplitVRegs))
    249       continue;
    250 
    251     assert(!Matrix->checkInterference(VirtReg, *PhysRegI) &&
    252            "Interference after spill.");
    253     // Tell the caller to allocate to this newly freed physical register.
    254     return *PhysRegI;
    255   }
    256 
    257   // No other spill candidates were found, so spill the current VirtReg.
    258   DEBUG(dbgs() << "spilling: " << VirtReg << '\n');
    259   if (!VirtReg.isSpillable())
    260     return ~0u;
    261   LiveRangeEdit LRE(&VirtReg, SplitVRegs, *MF, *LIS, VRM);
    262   spiller().spill(LRE);
    263 
    264   // The live virtual register requesting allocation was spilled, so tell
    265   // the caller not to allocate anything during this round.
    266   return 0;
    267 }
    268 
    269 bool RABasic::runOnMachineFunction(MachineFunction &mf) {
    270   DEBUG(dbgs() << "********** BASIC REGISTER ALLOCATION **********\n"
    271                << "********** Function: "
    272                << mf.getName() << '\n');
    273 
    274   MF = &mf;
    275   RegAllocBase::init(getAnalysis<VirtRegMap>(),
    276                      getAnalysis<LiveIntervals>(),
    277                      getAnalysis<LiveRegMatrix>());
    278 
    279   calculateSpillWeightsAndHints(*LIS, *MF, VRM,
    280                                 getAnalysis<MachineLoopInfo>(),
    281                                 getAnalysis<MachineBlockFrequencyInfo>());
    282 
    283   SpillerInstance.reset(createInlineSpiller(*this, *MF, *VRM));
    284 
    285   allocatePhysRegs();
    286 
    287   // Diagnostic output before rewriting
    288   DEBUG(dbgs() << "Post alloc VirtRegMap:\n" << *VRM << "\n");
    289 
    290   releaseMemory();
    291   return true;
    292 }
    293 
    294 FunctionPass* llvm::createBasicRegisterAllocator()
    295 {
    296   return new RABasic();
    297 }
    298