llvm/lib/CodeGen/StackSlotColoring.cpp

   1 //===- StackSlotColoring.cpp - Stack slot coloring pass. ------------------===//
   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 implements the stack slot coloring pass.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "llvm/ADT/BitVector.h"
  14 #include "llvm/ADT/SmallVector.h"
  15 #include "llvm/ADT/Statistic.h"
  16 #include "llvm/CodeGen/LiveInterval.h"
  17 #include "llvm/CodeGen/LiveIntervalUnion.h"
  18 #include "llvm/CodeGen/LiveIntervals.h"
  19 #include "llvm/CodeGen/LiveStacks.h"
  20 #include "llvm/CodeGen/MachineBasicBlock.h"
  21 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
  22 #include "llvm/CodeGen/MachineFrameInfo.h"
  23 #include "llvm/CodeGen/MachineFunction.h"
  24 #include "llvm/CodeGen/MachineFunctionPass.h"
  25 #include "llvm/CodeGen/MachineInstr.h"
  26 #include "llvm/CodeGen/MachineMemOperand.h"
  27 #include "llvm/CodeGen/MachineOperand.h"
  28 #include "llvm/CodeGen/Passes.h"
  29 #include "llvm/CodeGen/PseudoSourceValue.h"
  30 #include "llvm/CodeGen/PseudoSourceValueManager.h"
  31 #include "llvm/CodeGen/SlotIndexes.h"
  32 #include "llvm/CodeGen/TargetInstrInfo.h"
  33 #include "llvm/CodeGen/TargetSubtargetInfo.h"
  34 #include "llvm/InitializePasses.h"
  35 #include "llvm/Pass.h"
  36 #include "llvm/Support/Casting.h"
  37 #include "llvm/Support/CommandLine.h"
  38 #include "llvm/Support/Debug.h"
  39 #include "llvm/Support/raw_ostream.h"
  40 #include <algorithm>
  41 #include <cassert>
  42 #include <cstdint>
  43 #include <iterator>
  44 #include <vector>
  45
  46 using namespace llvm;
  47
  48 #define DEBUG_TYPE "stack-slot-coloring"
  49
  50 static cl::opt<bool>
  51 DisableSharing("no-stack-slot-sharing",
  52              cl::init(false), cl::Hidden,
  53              cl::desc("Suppress slot sharing during stack coloring"));
  54
  55 static cl::opt<int> DCELimit("ssc-dce-limit", cl::init(-1), cl::Hidden);
  56
  57 STATISTIC(NumEliminated, "Number of stack slots eliminated due to coloring");
  58 STATISTIC(NumDead,       "Number of trivially dead stack accesses eliminated");
  59
  60 namespace {
  61
  62   class StackSlotColoring : public MachineFunctionPass {
  63     LiveStacks *LS = nullptr;
  64     MachineFrameInfo *MFI = nullptr;
  65     const TargetInstrInfo *TII = nullptr;
  66     const MachineBlockFrequencyInfo *MBFI = nullptr;
  67
  68     // SSIntervals - Spill slot intervals.
  69     std::vector<LiveInterval*> SSIntervals;
  70
  71     // SSRefs - Keep a list of MachineMemOperands for each spill slot.
  72     // MachineMemOperands can be shared between instructions, so we need
  73     // to be careful that renames like [FI0, FI1] -> [FI1, FI2] do not
  74     // become FI0 -> FI1 -> FI2.
  75     SmallVector<SmallVector<MachineMemOperand *, 8>, 16> SSRefs;
  76
  77     // OrigAlignments - Alignments of stack objects before coloring.
  78     SmallVector<Align, 16> OrigAlignments;
  79
  80     // OrigSizes - Sizes of stack objects before coloring.
  81     SmallVector<unsigned, 16> OrigSizes;
  82
  83     // AllColors - If index is set, it's a spill slot, i.e. color.
  84     // FIXME: This assumes PEI locate spill slot with smaller indices
  85     // closest to stack pointer / frame pointer. Therefore, smaller
  86     // index == better color. This is per stack ID.
  87     SmallVector<BitVector, 2> AllColors;
  88
  89     // NextColor - Next "color" that's not yet used. This is per stack ID.
  90     SmallVector<int, 2> NextColors = { -1 };
  91
  92     // UsedColors - "Colors" that have been assigned. This is per stack ID
  93     SmallVector<BitVector, 2> UsedColors;
  94
  95     // Join all intervals sharing one color into a single LiveIntervalUnion to
  96     // speedup range overlap test.
  97     class ColorAssignmentInfo {
  98       // Single liverange (used to avoid creation of LiveIntervalUnion).
  99       LiveInterval *SingleLI = nullptr;
 100       // LiveIntervalUnion to perform overlap test.
 101       LiveIntervalUnion *LIU = nullptr;
 102       // LiveIntervalUnion has a parameter in its constructor so doing this
 103       // dirty magic.
 104       uint8_t LIUPad[sizeof(LiveIntervalUnion)];
 105
 106     public:
 107       ~ColorAssignmentInfo() {
 108         if (LIU)
 109           LIU->~LiveIntervalUnion(); // Dirty magic again.
 110       }
 111
 112       // Return true if LiveInterval overlaps with any
 113       // intervals that have already been assigned to this color.
 114       bool overlaps(LiveInterval *LI) const {
 115         if (LIU)
 116           return LiveIntervalUnion::Query(*LI, *LIU).checkInterference();
 117         return SingleLI ? SingleLI->overlaps(*LI) : false;
 118       }
 119
 120       // Add new LiveInterval to this color.
 121       void add(LiveInterval *LI, LiveIntervalUnion::Allocator &Alloc) {
 122         assert(!overlaps(LI));
 123         if (LIU) {
 124           LIU->unify(*LI, *LI);
 125         } else if (SingleLI) {
 126           LIU = new (LIUPad) LiveIntervalUnion(Alloc);
 127           LIU->unify(*SingleLI, *SingleLI);
 128           LIU->unify(*LI, *LI);
 129           SingleLI = nullptr;
 130         } else
 131           SingleLI = LI;
 132       }
 133     };
 134
 135     LiveIntervalUnion::Allocator LIUAlloc;
 136
 137     // Assignments - Color to intervals mapping.
 138     SmallVector<ColorAssignmentInfo, 16> Assignments;
 139
 140   public:
 141     static char ID; // Pass identification
 142
 143     StackSlotColoring() : MachineFunctionPass(ID) {
 144       initializeStackSlotColoringPass(*PassRegistry::getPassRegistry());
 145     }
 146
 147     void getAnalysisUsage(AnalysisUsage &AU) const override {
 148       AU.setPreservesCFG();
 149       AU.addRequired<SlotIndexes>();
 150       AU.addPreserved<SlotIndexes>();
 151       AU.addRequired<LiveStacks>();
 152       AU.addRequired<MachineBlockFrequencyInfo>();
 153       AU.addPreserved<MachineBlockFrequencyInfo>();
 154       AU.addPreservedID(MachineDominatorsID);
 155       MachineFunctionPass::getAnalysisUsage(AU);
 156     }
 157
 158     bool runOnMachineFunction(MachineFunction &MF) override;
 159
 160   private:
 161     void InitializeSlots();
 162     void ScanForSpillSlotRefs(MachineFunction &MF);
 163     int ColorSlot(LiveInterval *li);
 164     bool ColorSlots(MachineFunction &MF);
 165     void RewriteInstruction(MachineInstr &MI, SmallVectorImpl<int> &SlotMapping,
 166                             MachineFunction &MF);
 167     bool RemoveDeadStores(MachineBasicBlock* MBB);
 168   };
 169
 170 } // end anonymous namespace
 171
 172 char StackSlotColoring::ID = 0;
 173
 174 char &llvm::StackSlotColoringID = StackSlotColoring::ID;
 175
 176 INITIALIZE_PASS_BEGIN(StackSlotColoring, DEBUG_TYPE,
 177                 "Stack Slot Coloring", false, false)
 178 INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
 179 INITIALIZE_PASS_DEPENDENCY(LiveStacks)
 180 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
 181 INITIALIZE_PASS_END(StackSlotColoring, DEBUG_TYPE,
 182                 "Stack Slot Coloring", false, false)
 183
 184 namespace {
 185
 186 // IntervalSorter - Comparison predicate that sort live intervals by
 187 // their weight.
 188 struct IntervalSorter {
 189   bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
 190     return LHS->weight() > RHS->weight();
 191   }
 192 };
 193
 194 } // end anonymous namespace
 195
 196 /// ScanForSpillSlotRefs - Scan all the machine instructions for spill slot
 197 /// references and update spill slot weights.
 198 void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) {
 199   SSRefs.resize(MFI->getObjectIndexEnd());
 200
 201   // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
 202   for (MachineBasicBlock &MBB : MF) {
 203     for (MachineInstr &MI : MBB) {
 204       for (const MachineOperand &MO : MI.operands()) {
 205         if (!MO.isFI())
 206           continue;
 207         int FI = MO.getIndex();
 208         if (FI < 0)
 209           continue;
 210         if (!LS->hasInterval(FI))
 211           continue;
 212         LiveInterval &li = LS->getInterval(FI);
 213         if (!MI.isDebugInstr())
 214           li.incrementWeight(
 215               LiveIntervals::getSpillWeight(false, true, MBFI, MI));
 216       }
 217       for (MachineInstr::mmo_iterator MMOI = MI.memoperands_begin(),
 218                                       EE = MI.memoperands_end();
 219            MMOI != EE; ++MMOI) {
 220         MachineMemOperand *MMO = *MMOI;
 221         if (const FixedStackPseudoSourceValue *FSV =
 222             dyn_cast_or_null<FixedStackPseudoSourceValue>(
 223                 MMO->getPseudoValue())) {
 224           int FI = FSV->getFrameIndex();
 225           if (FI >= 0)
 226             SSRefs[FI].push_back(MMO);
 227         }
 228       }
 229     }
 230   }
 231 }
 232
 233 /// InitializeSlots - Process all spill stack slot liveintervals and add them
 234 /// to a sorted (by weight) list.
 235 void StackSlotColoring::InitializeSlots() {
 236   int LastFI = MFI->getObjectIndexEnd();
 237
 238   // There is always at least one stack ID.
 239   AllColors.resize(1);
 240   UsedColors.resize(1);
 241
 242   OrigAlignments.resize(LastFI);
 243   OrigSizes.resize(LastFI);
 244   AllColors[0].resize(LastFI);
 245   UsedColors[0].resize(LastFI);
 246   Assignments.resize(LastFI);
 247
 248   using Pair = std::iterator_traits<LiveStacks::iterator>::value_type;
 249
 250   SmallVector<Pair *, 16> Intervals;
 251
 252   Intervals.reserve(LS->getNumIntervals());
 253   for (auto &I : *LS)
 254     Intervals.push_back(&I);
 255   llvm::sort(Intervals,
 256              [](Pair *LHS, Pair *RHS) { return LHS->first < RHS->first; });
 257
 258   // Gather all spill slots into a list.
 259   LLVM_DEBUG(dbgs() << "Spill slot intervals:\n");
 260   for (auto *I : Intervals) {
 261     LiveInterval &li = I->second;
 262     LLVM_DEBUG(li.dump());
 263     int FI = Register::stackSlot2Index(li.reg());
 264     if (MFI->isDeadObjectIndex(FI))
 265       continue;
 266
 267     SSIntervals.push_back(&li);
 268     OrigAlignments[FI] = MFI->getObjectAlign(FI);
 269     OrigSizes[FI]      = MFI->getObjectSize(FI);
 270
 271     auto StackID = MFI->getStackID(FI);
 272     if (StackID != 0) {
 273       AllColors.resize(StackID + 1);
 274       UsedColors.resize(StackID + 1);
 275       AllColors[StackID].resize(LastFI);
 276       UsedColors[StackID].resize(LastFI);
 277     }
 278
 279     AllColors[StackID].set(FI);
 280   }
 281   LLVM_DEBUG(dbgs() << '\n');
 282
 283   // Sort them by weight.
 284   llvm::stable_sort(SSIntervals, IntervalSorter());
 285
 286   NextColors.resize(AllColors.size());
 287
 288   // Get first "color".
 289   for (unsigned I = 0, E = AllColors.size(); I != E; ++I)
 290     NextColors[I] = AllColors[I].find_first();
 291 }
 292
 293 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
 294 int StackSlotColoring::ColorSlot(LiveInterval *li) {
 295   int Color = -1;
 296   bool Share = false;
 297   int FI = Register::stackSlot2Index(li->reg());
 298   uint8_t StackID = MFI->getStackID(FI);
 299
 300   if (!DisableSharing) {
 301
 302     // Check if it's possible to reuse any of the used colors.
 303     Color = UsedColors[StackID].find_first();
 304     while (Color != -1) {
 305       if (!Assignments[Color].overlaps(li)) {
 306         Share = true;
 307         ++NumEliminated;
 308         break;
 309       }
 310       Color = UsedColors[StackID].find_next(Color);
 311     }
 312   }
 313
 314   if (Color != -1 && MFI->getStackID(Color) != MFI->getStackID(FI)) {
 315     LLVM_DEBUG(dbgs() << "cannot share FIs with different stack IDs\n");
 316     Share = false;
 317   }
 318
 319   // Assign it to the first available color (assumed to be the best) if it's
 320   // not possible to share a used color with other objects.
 321   if (!Share) {
 322     assert(NextColors[StackID] != -1 && "No more spill slots?");
 323     Color = NextColors[StackID];
 324     UsedColors[StackID].set(Color);
 325     NextColors[StackID] = AllColors[StackID].find_next(NextColors[StackID]);
 326   }
 327
 328   assert(MFI->getStackID(Color) == MFI->getStackID(FI));
 329
 330   // Record the assignment.
 331   Assignments[Color].add(li, LIUAlloc);
 332   LLVM_DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n");
 333
 334   // Change size and alignment of the allocated slot. If there are multiple
 335   // objects sharing the same slot, then make sure the size and alignment
 336   // are large enough for all.
 337   Align Alignment = OrigAlignments[FI];
 338   if (!Share || Alignment > MFI->getObjectAlign(Color))
 339     MFI->setObjectAlignment(Color, Alignment);
 340   int64_t Size = OrigSizes[FI];
 341   if (!Share || Size > MFI->getObjectSize(Color))
 342     MFI->setObjectSize(Color, Size);
 343   return Color;
 344 }
 345
 346 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine
 347 /// operands in the function.
 348 bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
 349   unsigned NumObjs = MFI->getObjectIndexEnd();
 350   SmallVector<int, 16> SlotMapping(NumObjs, -1);
 351   SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
 352   SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
 353   BitVector UsedColors(NumObjs);
 354
 355   LLVM_DEBUG(dbgs() << "Color spill slot intervals:\n");
 356   bool Changed = false;
 357   for (LiveInterval *li : SSIntervals) {
 358     int SS = Register::stackSlot2Index(li->reg());
 359     int NewSS = ColorSlot(li);
 360     assert(NewSS >= 0 && "Stack coloring failed?");
 361     SlotMapping[SS] = NewSS;
 362     RevMap[NewSS].push_back(SS);
 363     SlotWeights[NewSS] += li->weight();
 364     UsedColors.set(NewSS);
 365     Changed |= (SS != NewSS);
 366   }
 367
 368   LLVM_DEBUG(dbgs() << "\nSpill slots after coloring:\n");
 369   for (LiveInterval *li : SSIntervals) {
 370     int SS = Register::stackSlot2Index(li->reg());
 371     li->setWeight(SlotWeights[SS]);
 372   }
 373   // Sort them by new weight.
 374   llvm::stable_sort(SSIntervals, IntervalSorter());
 375
 376 #ifndef NDEBUG
 377   for (LiveInterval *li : SSIntervals)
 378     LLVM_DEBUG(li->dump());
 379   LLVM_DEBUG(dbgs() << '\n');
 380 #endif
 381
 382   if (!Changed)
 383     return false;
 384
 385   // Rewrite all MachineMemOperands.
 386   for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
 387     int NewFI = SlotMapping[SS];
 388     if (NewFI == -1 || (NewFI == (int)SS))
 389       continue;
 390
 391     const PseudoSourceValue *NewSV = MF.getPSVManager().getFixedStack(NewFI);
 392     SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS];
 393     for (unsigned i = 0, e = RefMMOs.size(); i != e; ++i)
 394       RefMMOs[i]->setValue(NewSV);
 395   }
 396
 397   // Rewrite all MO_FrameIndex operands.  Look for dead stores.
 398   for (MachineBasicBlock &MBB : MF) {
 399     for (MachineInstr &MI : MBB)
 400       RewriteInstruction(MI, SlotMapping, MF);
 401     RemoveDeadStores(&MBB);
 402   }
 403
 404   // Delete unused stack slots.
 405   for (int StackID = 0, E = AllColors.size(); StackID != E; ++StackID) {
 406     int NextColor = NextColors[StackID];
 407     while (NextColor != -1) {
 408       LLVM_DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n");
 409       MFI->RemoveStackObject(NextColor);
 410       NextColor = AllColors[StackID].find_next(NextColor);
 411     }
 412   }
 413
 414   return true;
 415 }
 416
 417 /// RewriteInstruction - Rewrite specified instruction by replacing references
 418 /// to old frame index with new one.
 419 void StackSlotColoring::RewriteInstruction(MachineInstr &MI,
 420                                            SmallVectorImpl<int> &SlotMapping,
 421                                            MachineFunction &MF) {
 422   // Update the operands.
 423   for (MachineOperand &MO : MI.operands()) {
 424     if (!MO.isFI())
 425       continue;
 426     int OldFI = MO.getIndex();
 427     if (OldFI < 0)
 428       continue;
 429     int NewFI = SlotMapping[OldFI];
 430     if (NewFI == -1 || NewFI == OldFI)
 431       continue;
 432
 433     assert(MFI->getStackID(OldFI) == MFI->getStackID(NewFI));
 434     MO.setIndex(NewFI);
 435   }
 436
 437   // The MachineMemOperands have already been updated.
 438 }
 439
 440 /// RemoveDeadStores - Scan through a basic block and look for loads followed
 441 /// by stores.  If they're both using the same stack slot, then the store is
 442 /// definitely dead.  This could obviously be much more aggressive (consider
 443 /// pairs with instructions between them), but such extensions might have a
 444 /// considerable compile time impact.
 445 bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
 446   // FIXME: This could be much more aggressive, but we need to investigate
 447   // the compile time impact of doing so.
 448   bool changed = false;
 449
 450   SmallVector<MachineInstr*, 4> toErase;
 451
 452   for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
 453        I != E; ++I) {
 454     if (DCELimit != -1 && (int)NumDead >= DCELimit)
 455       break;
 456     int FirstSS, SecondSS;
 457     if (TII->isStackSlotCopy(*I, FirstSS, SecondSS) && FirstSS == SecondSS &&
 458         FirstSS != -1) {
 459       ++NumDead;
 460       changed = true;
 461       toErase.push_back(&*I);
 462       continue;
 463     }
 464
 465     MachineBasicBlock::iterator NextMI = std::next(I);
 466     MachineBasicBlock::iterator ProbableLoadMI = I;
 467
 468     unsigned LoadReg = 0;
 469     unsigned StoreReg = 0;
 470     unsigned LoadSize = 0;
 471     unsigned StoreSize = 0;
 472     if (!(LoadReg = TII->isLoadFromStackSlot(*I, FirstSS, LoadSize)))
 473       continue;
 474     // Skip the ...pseudo debugging... instructions between a load and store.
 475     while ((NextMI != E) && NextMI->isDebugInstr()) {
 476       ++NextMI;
 477       ++I;
 478     }
 479     if (NextMI == E) continue;
 480     if (!(StoreReg = TII->isStoreToStackSlot(*NextMI, SecondSS, StoreSize)))
 481       continue;
 482     if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1 ||
 483         LoadSize != StoreSize)
 484       continue;
 485
 486     ++NumDead;
 487     changed = true;
 488
 489     if (NextMI->findRegisterUseOperandIdx(LoadReg, true, nullptr) != -1) {
 490       ++NumDead;
 491       toErase.push_back(&*ProbableLoadMI);
 492     }
 493
 494     toErase.push_back(&*NextMI);
 495     ++I;
 496   }
 497
 498   for (MachineInstr *MI : toErase)
 499     MI->eraseFromParent();
 500
 501   return changed;
 502 }
 503
 504 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
 505   LLVM_DEBUG({
 506     dbgs() << "********** Stack Slot Coloring **********\n"
 507            << "********** Function: " << MF.getName() << '\n';
 508   });
 509
 510   if (skipFunction(MF.getFunction()))
 511     return false;
 512
 513   MFI = &MF.getFrameInfo();
 514   TII = MF.getSubtarget().getInstrInfo();
 515   LS = &getAnalysis<LiveStacks>();
 516   MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
 517
 518   bool Changed = false;
 519
 520   unsigned NumSlots = LS->getNumIntervals();
 521   if (NumSlots == 0)
 522     // Nothing to do!
 523     return false;
 524
 525   // If there are calls to setjmp or sigsetjmp, don't perform stack slot
 526   // coloring. The stack could be modified before the longjmp is executed,
 527   // resulting in the wrong value being used afterwards.
 528   if (MF.exposesReturnsTwice())
 529     return false;
 530
 531   // Gather spill slot references
 532   ScanForSpillSlotRefs(MF);
 533   InitializeSlots();
 534   Changed = ColorSlots(MF);
 535
 536   for (int &Next : NextColors)
 537     Next = -1;
 538
 539   SSIntervals.clear();
 540   for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
 541     SSRefs[i].clear();
 542   SSRefs.clear();
 543   OrigAlignments.clear();
 544   OrigSizes.clear();
 545   AllColors.clear();
 546   UsedColors.clear();
 547   Assignments.clear();
 548
 549   return Changed;
 550 }