src/llvm/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 #include "llvm/CodeGen/Passes.h"
  15 #include "llvm/ADT/BitVector.h"
  16 #include "llvm/ADT/SmallVector.h"
  17 #include "llvm/ADT/Statistic.h"
  18 #include "llvm/CodeGen/LiveIntervalAnalysis.h"
  19 #include "llvm/CodeGen/LiveStackAnalysis.h"
  20 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
  21 #include "llvm/CodeGen/MachineFrameInfo.h"
  22 #include "llvm/CodeGen/MachineInstrBuilder.h"
  23 #include "llvm/CodeGen/MachineMemOperand.h"
  24 #include "llvm/CodeGen/MachineRegisterInfo.h"
  25 #include "llvm/CodeGen/PseudoSourceValue.h"
  26 #include "llvm/IR/Module.h"
  27 #include "llvm/Support/CommandLine.h"
  28 #include "llvm/Support/Debug.h"
  29 #include "llvm/Support/raw_ostream.h"
  30 #include "llvm/Target/TargetInstrInfo.h"
  31 #include "llvm/Target/TargetSubtargetInfo.h"
  32 #include <vector>
  33 using namespace llvm;
  34
  35 #define DEBUG_TYPE "stackslotcoloring"
  36
  37 static cl::opt<bool>
  38 DisableSharing("no-stack-slot-sharing",
  39              cl::init(false), cl::Hidden,
  40              cl::desc("Suppress slot sharing during stack coloring"));
  41
  42 static cl::opt<int> DCELimit("ssc-dce-limit", cl::init(-1), cl::Hidden);
  43
  44 STATISTIC(NumEliminated, "Number of stack slots eliminated due to coloring");
  45 STATISTIC(NumDead,       "Number of trivially dead stack accesses eliminated");
  46
  47 namespace {
  48   class StackSlotColoring : public MachineFunctionPass {
  49     LiveStacks* LS;
  50     MachineFrameInfo *MFI;
  51     const TargetInstrInfo  *TII;
  52     const MachineBlockFrequencyInfo *MBFI;
  53
  54     // SSIntervals - Spill slot intervals.
  55     std::vector<LiveInterval*> SSIntervals;
  56
  57     // SSRefs - Keep a list of MachineMemOperands for each spill slot.
  58     // MachineMemOperands can be shared between instructions, so we need
  59     // to be careful that renames like [FI0, FI1] -> [FI1, FI2] do not
  60     // become FI0 -> FI1 -> FI2.
  61     SmallVector<SmallVector<MachineMemOperand *, 8>, 16> SSRefs;
  62
  63     // OrigAlignments - Alignments of stack objects before coloring.
  64     SmallVector<unsigned, 16> OrigAlignments;
  65
  66     // OrigSizes - Sizess of stack objects before coloring.
  67     SmallVector<unsigned, 16> OrigSizes;
  68
  69     // AllColors - If index is set, it's a spill slot, i.e. color.
  70     // FIXME: This assumes PEI locate spill slot with smaller indices
  71     // closest to stack pointer / frame pointer. Therefore, smaller
  72     // index == better color.
  73     BitVector AllColors;
  74
  75     // NextColor - Next "color" that's not yet used.
  76     int NextColor;
  77
  78     // UsedColors - "Colors" that have been assigned.
  79     BitVector UsedColors;
  80
  81     // Assignments - Color to intervals mapping.
  82     SmallVector<SmallVector<LiveInterval*,4>, 16> Assignments;
  83
  84   public:
  85     static char ID; // Pass identification
  86     StackSlotColoring() :
  87       MachineFunctionPass(ID), NextColor(-1) {
  88         initializeStackSlotColoringPass(*PassRegistry::getPassRegistry());
  89       }
  90
  91     void getAnalysisUsage(AnalysisUsage &AU) const override {
  92       AU.setPreservesCFG();
  93       AU.addRequired<SlotIndexes>();
  94       AU.addPreserved<SlotIndexes>();
  95       AU.addRequired<LiveStacks>();
  96       AU.addRequired<MachineBlockFrequencyInfo>();
  97       AU.addPreserved<MachineBlockFrequencyInfo>();
  98       AU.addPreservedID(MachineDominatorsID);
  99       MachineFunctionPass::getAnalysisUsage(AU);
 100     }
 101
 102     bool runOnMachineFunction(MachineFunction &MF) override;
 103
 104   private:
 105     void InitializeSlots();
 106     void ScanForSpillSlotRefs(MachineFunction &MF);
 107     bool OverlapWithAssignments(LiveInterval *li, int Color) const;
 108     int ColorSlot(LiveInterval *li);
 109     bool ColorSlots(MachineFunction &MF);
 110     void RewriteInstruction(MachineInstr *MI, SmallVectorImpl<int> &SlotMapping,
 111                             MachineFunction &MF);
 112     bool RemoveDeadStores(MachineBasicBlock* MBB);
 113   };
 114 } // end anonymous namespace
 115
 116 char StackSlotColoring::ID = 0;
 117 char &llvm::StackSlotColoringID = StackSlotColoring::ID;
 118
 119 INITIALIZE_PASS_BEGIN(StackSlotColoring, "stack-slot-coloring",
 120                 "Stack Slot Coloring", false, false)
 121 INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
 122 INITIALIZE_PASS_DEPENDENCY(LiveStacks)
 123 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
 124 INITIALIZE_PASS_END(StackSlotColoring, "stack-slot-coloring",
 125                 "Stack Slot Coloring", false, false)
 126
 127 namespace {
 128   // IntervalSorter - Comparison predicate that sort live intervals by
 129   // their weight.
 130   struct IntervalSorter {
 131     bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
 132       return LHS->weight > RHS->weight;
 133     }
 134   };
 135 }
 136
 137 /// ScanForSpillSlotRefs - Scan all the machine instructions for spill slot
 138 /// references and update spill slot weights.
 139 void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) {
 140   SSRefs.resize(MFI->getObjectIndexEnd());
 141
 142   // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
 143   for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
 144        MBBI != E; ++MBBI) {
 145     MachineBasicBlock *MBB = &*MBBI;
 146     for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
 147          MII != EE; ++MII) {
 148       MachineInstr *MI = &*MII;
 149       for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
 150         MachineOperand &MO = MI->getOperand(i);
 151         if (!MO.isFI())
 152           continue;
 153         int FI = MO.getIndex();
 154         if (FI < 0)
 155           continue;
 156         if (!LS->hasInterval(FI))
 157           continue;
 158         LiveInterval &li = LS->getInterval(FI);
 159         if (!MI->isDebugValue())
 160           li.weight += LiveIntervals::getSpillWeight(false, true, MBFI, MI);
 161       }
 162       for (MachineInstr::mmo_iterator MMOI = MI->memoperands_begin(),
 163            EE = MI->memoperands_end(); MMOI != EE; ++MMOI) {
 164         MachineMemOperand *MMO = *MMOI;
 165         if (const FixedStackPseudoSourceValue *FSV =
 166             dyn_cast_or_null<FixedStackPseudoSourceValue>(
 167                 MMO->getPseudoValue())) {
 168           int FI = FSV->getFrameIndex();
 169           if (FI >= 0)
 170             SSRefs[FI].push_back(MMO);
 171         }
 172       }
 173     }
 174   }
 175 }
 176
 177 /// InitializeSlots - Process all spill stack slot liveintervals and add them
 178 /// to a sorted (by weight) list.
 179 void StackSlotColoring::InitializeSlots() {
 180   int LastFI = MFI->getObjectIndexEnd();
 181   OrigAlignments.resize(LastFI);
 182   OrigSizes.resize(LastFI);
 183   AllColors.resize(LastFI);
 184   UsedColors.resize(LastFI);
 185   Assignments.resize(LastFI);
 186
 187   // Gather all spill slots into a list.
 188   DEBUG(dbgs() << "Spill slot intervals:\n");
 189   for (LiveStacks::iterator i = LS->begin(), e = LS->end(); i != e; ++i) {
 190     LiveInterval &li = i->second;
 191     DEBUG(li.dump());
 192     int FI = TargetRegisterInfo::stackSlot2Index(li.reg);
 193     if (MFI->isDeadObjectIndex(FI))
 194       continue;
 195     SSIntervals.push_back(&li);
 196     OrigAlignments[FI] = MFI->getObjectAlignment(FI);
 197     OrigSizes[FI]      = MFI->getObjectSize(FI);
 198     AllColors.set(FI);
 199   }
 200   DEBUG(dbgs() << '\n');
 201
 202   // Sort them by weight.
 203   std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
 204
 205   // Get first "color".
 206   NextColor = AllColors.find_first();
 207 }
 208
 209 /// OverlapWithAssignments - Return true if LiveInterval overlaps with any
 210 /// LiveIntervals that have already been assigned to the specified color.
 211 bool
 212 StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
 213   const SmallVectorImpl<LiveInterval *> &OtherLIs = Assignments[Color];
 214   for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
 215     LiveInterval *OtherLI = OtherLIs[i];
 216     if (OtherLI->overlaps(*li))
 217       return true;
 218   }
 219   return false;
 220 }
 221
 222 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
 223 ///
 224 int StackSlotColoring::ColorSlot(LiveInterval *li) {
 225   int Color = -1;
 226   bool Share = false;
 227   if (!DisableSharing) {
 228     // Check if it's possible to reuse any of the used colors.
 229     Color = UsedColors.find_first();
 230     while (Color != -1) {
 231       if (!OverlapWithAssignments(li, Color)) {
 232         Share = true;
 233         ++NumEliminated;
 234         break;
 235       }
 236       Color = UsedColors.find_next(Color);
 237     }
 238   }
 239
 240   // Assign it to the first available color (assumed to be the best) if it's
 241   // not possible to share a used color with other objects.
 242   if (!Share) {
 243     assert(NextColor != -1 && "No more spill slots?");
 244     Color = NextColor;
 245     UsedColors.set(Color);
 246     NextColor = AllColors.find_next(NextColor);
 247   }
 248
 249   // Record the assignment.
 250   Assignments[Color].push_back(li);
 251   int FI = TargetRegisterInfo::stackSlot2Index(li->reg);
 252   DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n");
 253
 254   // Change size and alignment of the allocated slot. If there are multiple
 255   // objects sharing the same slot, then make sure the size and alignment
 256   // are large enough for all.
 257   unsigned Align = OrigAlignments[FI];
 258   if (!Share || Align > MFI->getObjectAlignment(Color))
 259     MFI->setObjectAlignment(Color, Align);
 260   int64_t Size = OrigSizes[FI];
 261   if (!Share || Size > MFI->getObjectSize(Color))
 262     MFI->setObjectSize(Color, Size);
 263   return Color;
 264 }
 265
 266 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine
 267 /// operands in the function.
 268 bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
 269   unsigned NumObjs = MFI->getObjectIndexEnd();
 270   SmallVector<int, 16> SlotMapping(NumObjs, -1);
 271   SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
 272   SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
 273   BitVector UsedColors(NumObjs);
 274
 275   DEBUG(dbgs() << "Color spill slot intervals:\n");
 276   bool Changed = false;
 277   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
 278     LiveInterval *li = SSIntervals[i];
 279     int SS = TargetRegisterInfo::stackSlot2Index(li->reg);
 280     int NewSS = ColorSlot(li);
 281     assert(NewSS >= 0 && "Stack coloring failed?");
 282     SlotMapping[SS] = NewSS;
 283     RevMap[NewSS].push_back(SS);
 284     SlotWeights[NewSS] += li->weight;
 285     UsedColors.set(NewSS);
 286     Changed |= (SS != NewSS);
 287   }
 288
 289   DEBUG(dbgs() << "\nSpill slots after coloring:\n");
 290   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
 291     LiveInterval *li = SSIntervals[i];
 292     int SS = TargetRegisterInfo::stackSlot2Index(li->reg);
 293     li->weight = SlotWeights[SS];
 294   }
 295   // Sort them by new weight.
 296   std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
 297
 298 #ifndef NDEBUG
 299   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i)
 300     DEBUG(SSIntervals[i]->dump());
 301   DEBUG(dbgs() << '\n');
 302 #endif
 303
 304   if (!Changed)
 305     return false;
 306
 307   // Rewrite all MachineMemOperands.
 308   for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
 309     int NewFI = SlotMapping[SS];
 310     if (NewFI == -1 || (NewFI == (int)SS))
 311       continue;
 312
 313     const PseudoSourceValue *NewSV = PseudoSourceValue::getFixedStack(NewFI);
 314     SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS];
 315     for (unsigned i = 0, e = RefMMOs.size(); i != e; ++i)
 316       RefMMOs[i]->setValue(NewSV);
 317   }
 318
 319   // Rewrite all MO_FrameIndex operands.  Look for dead stores.
 320   for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
 321        MBBI != E; ++MBBI) {
 322     MachineBasicBlock *MBB = &*MBBI;
 323     for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
 324          MII != EE; ++MII)
 325       RewriteInstruction(MII, SlotMapping, MF);
 326     RemoveDeadStores(MBB);
 327   }
 328
 329   // Delete unused stack slots.
 330   while (NextColor != -1) {
 331     DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n");
 332     MFI->RemoveStackObject(NextColor);
 333     NextColor = AllColors.find_next(NextColor);
 334   }
 335
 336   return true;
 337 }
 338
 339 /// RewriteInstruction - Rewrite specified instruction by replacing references
 340 /// to old frame index with new one.
 341 void StackSlotColoring::RewriteInstruction(MachineInstr *MI,
 342                                            SmallVectorImpl<int> &SlotMapping,
 343                                            MachineFunction &MF) {
 344   // Update the operands.
 345   for (unsigned i = 0, ee = MI->getNumOperands(); i != ee; ++i) {
 346     MachineOperand &MO = MI->getOperand(i);
 347     if (!MO.isFI())
 348       continue;
 349     int OldFI = MO.getIndex();
 350     if (OldFI < 0)
 351       continue;
 352     int NewFI = SlotMapping[OldFI];
 353     if (NewFI == -1 || NewFI == OldFI)
 354       continue;
 355     MO.setIndex(NewFI);
 356   }
 357
 358   // The MachineMemOperands have already been updated.
 359 }
 360
 361
 362 /// RemoveDeadStores - Scan through a basic block and look for loads followed
 363 /// by stores.  If they're both using the same stack slot, then the store is
 364 /// definitely dead.  This could obviously be much more aggressive (consider
 365 /// pairs with instructions between them), but such extensions might have a
 366 /// considerable compile time impact.
 367 bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
 368   // FIXME: This could be much more aggressive, but we need to investigate
 369   // the compile time impact of doing so.
 370   bool changed = false;
 371
 372   SmallVector<MachineInstr*, 4> toErase;
 373
 374   for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
 375        I != E; ++I) {
 376     if (DCELimit != -1 && (int)NumDead >= DCELimit)
 377       break;
 378
 379     int FirstSS, SecondSS;
 380     if (TII->isStackSlotCopy(I, FirstSS, SecondSS) &&
 381         FirstSS == SecondSS &&
 382         FirstSS != -1) {
 383       ++NumDead;
 384       changed = true;
 385       toErase.push_back(I);
 386       continue;
 387     }
 388
 389     MachineBasicBlock::iterator NextMI = std::next(I);
 390     if (NextMI == MBB->end()) continue;
 391
 392     unsigned LoadReg = 0;
 393     unsigned StoreReg = 0;
 394     if (!(LoadReg = TII->isLoadFromStackSlot(I, FirstSS))) continue;
 395     if (!(StoreReg = TII->isStoreToStackSlot(NextMI, SecondSS))) continue;
 396     if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1) continue;
 397
 398     ++NumDead;
 399     changed = true;
 400
 401     if (NextMI->findRegisterUseOperandIdx(LoadReg, true, nullptr) != -1) {
 402       ++NumDead;
 403       toErase.push_back(I);
 404     }
 405
 406     toErase.push_back(NextMI);
 407     ++I;
 408   }
 409
 410   for (SmallVectorImpl<MachineInstr *>::iterator I = toErase.begin(),
 411        E = toErase.end(); I != E; ++I)
 412     (*I)->eraseFromParent();
 413
 414   return changed;
 415 }
 416
 417
 418 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
 419   DEBUG({
 420       dbgs() << "********** Stack Slot Coloring **********\n"
 421              << "********** Function: " << MF.getName() << '\n';
 422     });
 423
 424   MFI = MF.getFrameInfo();
 425   TII = MF.getSubtarget().getInstrInfo();
 426   LS = &getAnalysis<LiveStacks>();
 427   MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
 428
 429   bool Changed = false;
 430
 431   unsigned NumSlots = LS->getNumIntervals();
 432   if (NumSlots == 0)
 433     // Nothing to do!
 434     return false;
 435
 436   // If there are calls to setjmp or sigsetjmp, don't perform stack slot
 437   // coloring. The stack could be modified before the longjmp is executed,
 438   // resulting in the wrong value being used afterwards. (See
 439   // <rdar://problem/8007500>.)
 440   if (MF.exposesReturnsTwice())
 441     return false;
 442
 443   // Gather spill slot references
 444   ScanForSpillSlotRefs(MF);
 445   InitializeSlots();
 446   Changed = ColorSlots(MF);
 447
 448   NextColor = -1;
 449   SSIntervals.clear();
 450   for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
 451     SSRefs[i].clear();
 452   SSRefs.clear();
 453   OrigAlignments.clear();
 454   OrigSizes.clear();
 455   AllColors.clear();
 456   UsedColors.clear();
 457   for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
 458     Assignments[i].clear();
 459   Assignments.clear();
 460
 461   return Changed;
 462 }