lib/CodeGen/StackSlotColoring.cpp

   1 //===-- StackSlotColoring.cpp - Stack slot coloring pass. -----------------===//
   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 implements the stack slot coloring pass.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #define DEBUG_TYPE "stackcoloring"
  15 #include "llvm/CodeGen/Passes.h"
  16 #include "llvm/CodeGen/LiveStackAnalysis.h"
  17 #include "llvm/CodeGen/MachineFrameInfo.h"
  18 #include "llvm/CodeGen/PseudoSourceValue.h"
  19 #include "llvm/Support/CommandLine.h"
  20 #include "llvm/Support/Compiler.h"
  21 #include "llvm/Support/Debug.h"
  22 #include "llvm/Target/TargetInstrInfo.h"
  23 #include "llvm/Target/TargetMachine.h"
  24 #include "llvm/ADT/BitVector.h"
  25 #include "llvm/ADT/SmallVector.h"
  26 #include "llvm/ADT/Statistic.h"
  27 #include <vector>
  28 using namespace llvm;
  29
  30 static cl::opt<bool>
  31 DisableSharing("no-stack-slot-sharing",
  32              cl::init(false), cl::Hidden,
  33              cl::desc("Suppress slot sharing during stack coloring"));
  34
  35 static cl::opt<int> DCELimit("ssc-dce-limit", cl::init(-1), cl::Hidden);
  36
  37 STATISTIC(NumEliminated,   "Number of stack slots eliminated due to coloring");
  38 STATISTIC(NumDeadAccesses,
  39                           "Number of trivially dead stack accesses eliminated");
  40
  41 namespace {
  42   class VISIBILITY_HIDDEN StackSlotColoring : public MachineFunctionPass {
  43     LiveStacks* LS;
  44     MachineFrameInfo *MFI;
  45     const TargetInstrInfo  *TII;
  46
  47     // SSIntervals - Spill slot intervals.
  48     std::vector<LiveInterval*> SSIntervals;
  49
  50     // OrigAlignments - Alignments of stack objects before coloring.
  51     SmallVector<unsigned, 16> OrigAlignments;
  52
  53     // OrigSizes - Sizess of stack objects before coloring.
  54     SmallVector<unsigned, 16> OrigSizes;
  55
  56     // AllColors - If index is set, it's a spill slot, i.e. color.
  57     // FIXME: This assumes PEI locate spill slot with smaller indices
  58     // closest to stack pointer / frame pointer. Therefore, smaller
  59     // index == better color.
  60     BitVector AllColors;
  61
  62     // NextColor - Next "color" that's not yet used.
  63     int NextColor;
  64
  65     // UsedColors - "Colors" that have been assigned.
  66     BitVector UsedColors;
  67
  68     // Assignments - Color to intervals mapping.
  69     SmallVector<SmallVector<LiveInterval*,4>,16> Assignments;
  70
  71   public:
  72     static char ID; // Pass identification
  73     StackSlotColoring() : MachineFunctionPass(&ID), NextColor(-1) {}
  74
  75     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  76       AU.addRequired<LiveStacks>();
  77
  78       AU.addPreservedID(MachineLoopInfoID);
  79       AU.addPreservedID(MachineDominatorsID);
  80       MachineFunctionPass::getAnalysisUsage(AU);
  81     }
  82
  83     virtual bool runOnMachineFunction(MachineFunction &MF);
  84     virtual const char* getPassName() const {
  85       return "Stack Slot Coloring";
  86     }
  87
  88   private:
  89     bool InitializeSlots();
  90     bool OverlapWithAssignments(LiveInterval *li, int Color) const;
  91     int ColorSlot(LiveInterval *li);
  92     bool ColorSlots(MachineFunction &MF);
  93     bool removeDeadStores(MachineBasicBlock* MBB);
  94   };
  95 } // end anonymous namespace
  96
  97 char StackSlotColoring::ID = 0;
  98
  99 static RegisterPass<StackSlotColoring>
 100 X("stack-slot-coloring", "Stack Slot Coloring");
 101
 102 FunctionPass *llvm::createStackSlotColoringPass() {
 103   return new StackSlotColoring();
 104 }
 105
 106 namespace {
 107   // IntervalSorter - Comparison predicate that sort live intervals by
 108   // their weight.
 109   struct IntervalSorter {
 110     bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
 111       return LHS->weight > RHS->weight;
 112     }
 113   };
 114 }
 115
 116 /// InitializeSlots - Process all spill stack slot liveintervals and add them
 117 /// to a sorted (by weight) list.
 118 bool StackSlotColoring::InitializeSlots() {
 119   if (LS->getNumIntervals() < 2)
 120     return false;
 121
 122   int LastFI = MFI->getObjectIndexEnd();
 123   OrigAlignments.resize(LastFI);
 124   OrigSizes.resize(LastFI);
 125   AllColors.resize(LastFI);
 126   UsedColors.resize(LastFI);
 127   Assignments.resize(LastFI);
 128
 129   // Gather all spill slots into a list.
 130   for (LiveStacks::iterator i = LS->begin(), e = LS->end(); i != e; ++i) {
 131     LiveInterval &li = i->second;
 132     int FI = li.getStackSlotIndex();
 133     if (MFI->isDeadObjectIndex(FI))
 134       continue;
 135     SSIntervals.push_back(&li);
 136     OrigAlignments[FI] = MFI->getObjectAlignment(FI);
 137     OrigSizes[FI]      = MFI->getObjectSize(FI);
 138     AllColors.set(FI);
 139   }
 140
 141   // Sort them by weight.
 142   std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
 143
 144   // Get first "color".
 145   NextColor = AllColors.find_first();
 146   return true;
 147 }
 148
 149 /// OverlapWithAssignments - Return true if LiveInterval overlaps with any
 150 /// LiveIntervals that have already been assigned to the specified color.
 151 bool
 152 StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
 153   const SmallVector<LiveInterval*,4> &OtherLIs = Assignments[Color];
 154   for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
 155     LiveInterval *OtherLI = OtherLIs[i];
 156     if (OtherLI->overlaps(*li))
 157       return true;
 158   }
 159   return false;
 160 }
 161
 162 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
 163 ///
 164 int StackSlotColoring::ColorSlot(LiveInterval *li) {
 165   int Color = -1;
 166   bool Share = false;
 167   if (!DisableSharing) {
 168     // Check if it's possible to reuse any of the used colors.
 169     Color = UsedColors.find_first();
 170     while (Color != -1) {
 171       if (!OverlapWithAssignments(li, Color)) {
 172         Share = true;
 173         ++NumEliminated;
 174         break;
 175       }
 176       Color = UsedColors.find_next(Color);
 177     }
 178   }
 179
 180   // Assign it to the first available color (assumed to be the best) if it's
 181   // not possible to share a used color with other objects.
 182   if (!Share) {
 183     assert(NextColor != -1 && "No more spill slots?");
 184     Color = NextColor;
 185     UsedColors.set(Color);
 186     NextColor = AllColors.find_next(NextColor);
 187   }
 188
 189   // Record the assignment.
 190   Assignments[Color].push_back(li);
 191   int FI = li->getStackSlotIndex();
 192   DOUT << "Assigning fi#" << FI << " to fi#" << Color << "\n";
 193
 194   // Change size and alignment of the allocated slot. If there are multiple
 195   // objects sharing the same slot, then make sure the size and alignment
 196   // are large enough for all.
 197   unsigned Align = OrigAlignments[FI];
 198   if (!Share || Align > MFI->getObjectAlignment(Color))
 199     MFI->setObjectAlignment(Color, Align);
 200   int64_t Size = OrigSizes[FI];
 201   if (!Share || Size > MFI->getObjectSize(Color))
 202     MFI->setObjectSize(Color, Size);
 203   return Color;
 204 }
 205
 206 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine
 207 /// operands in the function.
 208 bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
 209   unsigned NumObjs = MFI->getObjectIndexEnd();
 210   std::vector<int> SlotMapping(NumObjs, -1);
 211
 212   bool Changed = false;
 213   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
 214     LiveInterval *li = SSIntervals[i];
 215     int SS = li->getStackSlotIndex();
 216     int NewSS = ColorSlot(li);
 217     SlotMapping[SS] = NewSS;
 218     Changed |= (SS != NewSS);
 219   }
 220
 221   if (!Changed)
 222     return false;
 223
 224   // Rewrite all MO_FrameIndex operands.
 225   // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
 226   for (MachineFunction::iterator MBB = MF.begin(), E = MF.end();
 227        MBB != E; ++MBB) {
 228     for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
 229          MII != EE; ++MII) {
 230       MachineInstr &MI = *MII;
 231       for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
 232         MachineOperand &MO = MI.getOperand(i);
 233         if (!MO.isFI())
 234           continue;
 235         int FI = MO.getIndex();
 236         if (FI < 0)
 237           continue;
 238         int NewFI = SlotMapping[FI];
 239         if (NewFI == -1)
 240           continue;
 241         MO.setIndex(NewFI);
 242
 243         // Update the MachineMemOperand for the new memory location.
 244         // FIXME: We need a better method of managing these too.
 245         SmallVector<MachineMemOperand, 2> MMOs(MI.memoperands_begin(),
 246                                                MI.memoperands_end());
 247         MI.clearMemOperands(MF);
 248         const Value *OldSV = PseudoSourceValue::getFixedStack(FI);
 249         for (unsigned i = 0, e = MMOs.size(); i != e; ++i) {
 250           if (MMOs[i].getValue() == OldSV) {
 251             MachineMemOperand MMO(PseudoSourceValue::getFixedStack(NewFI),
 252                                   MMOs[i].getFlags(), MMOs[i].getOffset(),
 253                                   MMOs[i].getSize(), MMOs[i].getAlignment());
 254             MI.addMemOperand(MF, MMO);
 255           } else
 256             MI.addMemOperand(MF, MMOs[i]);
 257         }
 258       }
 259     }
 260   }
 261
 262   // Delete unused stack slots.
 263   while (NextColor != -1) {
 264     DOUT << "Removing unused stack object fi#" << NextColor << "\n";
 265     MFI->RemoveStackObject(NextColor);
 266     NextColor = AllColors.find_next(NextColor);
 267   }
 268
 269   return true;
 270 }
 271
 272 /// removeDeadStores - Scan through a basic block and look for loads followed
 273 /// by stores.  If they're both using the same stack slot, then the store is
 274 /// definitely dead.  This could obviously be much more aggressive (consider
 275 /// pairs with instructions between them), but such extensions might have a
 276 /// considerable compile time impact.
 277 bool StackSlotColoring::removeDeadStores(MachineBasicBlock* MBB) {
 278   // FIXME: This could be much more aggressive, but we need to investigate
 279   // the compile time impact of doing so.
 280   bool changed = false;
 281
 282   SmallVector<MachineInstr*, 4> toErase;
 283
 284   for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
 285        I != E; ++I) {
 286     if (DCELimit != -1 && (int)NumDeadAccesses >= DCELimit)
 287       break;
 288
 289     MachineBasicBlock::iterator NextMI = next(I);
 290     if (NextMI == MBB->end()) continue;
 291
 292     int FirstSS, SecondSS;
 293     unsigned LoadReg = 0;
 294     unsigned StoreReg = 0;
 295     if (!(LoadReg = TII->isLoadFromStackSlot(I, FirstSS))) continue;
 296     if (!(StoreReg = TII->isStoreToStackSlot(NextMI, SecondSS))) continue;
 297     if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1) continue;
 298
 299     ++NumDeadAccesses;
 300     changed = true;
 301
 302     if (NextMI->findRegisterUseOperandIdx(LoadReg, true, 0) != -1) {
 303       ++NumDeadAccesses;
 304       toErase.push_back(I);
 305     }
 306
 307     toErase.push_back(NextMI);
 308     ++I;
 309   }
 310
 311   for (SmallVector<MachineInstr*, 4>::iterator I = toErase.begin(),
 312        E = toErase.end(); I != E; ++I)
 313     (*I)->eraseFromParent();
 314
 315   return changed;
 316 }
 317
 318
 319 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
 320   DOUT << "********** Stack Slot Coloring **********\n";
 321
 322   MFI = MF.getFrameInfo();
 323   TII = MF.getTarget().getInstrInfo();
 324   LS = &getAnalysis<LiveStacks>();
 325
 326   bool Changed = false;
 327   if (InitializeSlots())
 328     Changed = ColorSlots(MF);
 329
 330   NextColor = -1;
 331   SSIntervals.clear();
 332   OrigAlignments.clear();
 333   OrigSizes.clear();
 334   AllColors.clear();
 335   UsedColors.clear();
 336   for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
 337     Assignments[i].clear();
 338   Assignments.clear();
 339
 340   if (Changed) {
 341     for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
 342       Changed |= removeDeadStores(I);
 343   }
 344
 345   return Changed;
 346 }