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/LiveIntervals.h"
  18 #include "llvm/CodeGen/LiveStacks.h"
  19 #include "llvm/CodeGen/MachineBasicBlock.h"
  20 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
  21 #include "llvm/CodeGen/MachineFrameInfo.h"
  22 #include "llvm/CodeGen/MachineFunction.h"
  23 #include "llvm/CodeGen/MachineFunctionPass.h"
  24 #include "llvm/CodeGen/MachineInstr.h"
  25 #include "llvm/CodeGen/MachineMemOperand.h"
  26 #include "llvm/CodeGen/MachineOperand.h"
  27 #include "llvm/CodeGen/Passes.h"
  28 #include "llvm/CodeGen/PseudoSourceValue.h"
  29 #include "llvm/CodeGen/SlotIndexes.h"
  30 #include "llvm/CodeGen/TargetInstrInfo.h"
  31 #include "llvm/CodeGen/TargetRegisterInfo.h"
  32 #include "llvm/CodeGen/TargetSubtargetInfo.h"
  33 #include "llvm/Pass.h"
  34 #include "llvm/Support/Casting.h"
  35 #include "llvm/Support/CommandLine.h"
  36 #include "llvm/Support/Debug.h"
  37 #include "llvm/Support/raw_ostream.h"
  38 #include <algorithm>
  39 #include <cassert>
  40 #include <cstdint>
  41 #include <iterator>
  42 #include <vector>
  43
  44 using namespace llvm;
  45
  46 #define DEBUG_TYPE "stack-slot-coloring"
  47
  48 static cl::opt<bool>
  49 DisableSharing("no-stack-slot-sharing",
  50              cl::init(false), cl::Hidden,
  51              cl::desc("Suppress slot sharing during stack coloring"));
  52
  53 static cl::opt<int> DCELimit("ssc-dce-limit", cl::init(-1), cl::Hidden);
  54
  55 STATISTIC(NumEliminated, "Number of stack slots eliminated due to coloring");
  56 STATISTIC(NumDead,       "Number of trivially dead stack accesses eliminated");
  57
  58 namespace {
  59
  60   class StackSlotColoring : public MachineFunctionPass {
  61     LiveStacks* LS;
  62     MachineFrameInfo *MFI;
  63     const TargetInstrInfo  *TII;
  64     const MachineBlockFrequencyInfo *MBFI;
  65
  66     // SSIntervals - Spill slot intervals.
  67     std::vector<LiveInterval*> SSIntervals;
  68
  69     // SSRefs - Keep a list of MachineMemOperands for each spill slot.
  70     // MachineMemOperands can be shared between instructions, so we need
  71     // to be careful that renames like [FI0, FI1] -> [FI1, FI2] do not
  72     // become FI0 -> FI1 -> FI2.
  73     SmallVector<SmallVector<MachineMemOperand *, 8>, 16> SSRefs;
  74
  75     // OrigAlignments - Alignments of stack objects before coloring.
  76     SmallVector<unsigned, 16> OrigAlignments;
  77
  78     // OrigSizes - Sizess of stack objects before coloring.
  79     SmallVector<unsigned, 16> OrigSizes;
  80
  81     // AllColors - If index is set, it's a spill slot, i.e. color.
  82     // FIXME: This assumes PEI locate spill slot with smaller indices
  83     // closest to stack pointer / frame pointer. Therefore, smaller
  84     // index == better color. This is per stack ID.
  85     SmallVector<BitVector, 2> AllColors;
  86
  87     // NextColor - Next "color" that's not yet used. This is per stack ID.
  88     SmallVector<int, 2> NextColors = { -1 };
  89
  90     // UsedColors - "Colors" that have been assigned. This is per stack ID
  91     SmallVector<BitVector, 2> UsedColors;
  92
  93     // Assignments - Color to intervals mapping.
  94     SmallVector<SmallVector<LiveInterval*,4>, 16> Assignments;
  95
  96   public:
  97     static char ID; // Pass identification
  98
  99     StackSlotColoring() : MachineFunctionPass(ID) {
 100       initializeStackSlotColoringPass(*PassRegistry::getPassRegistry());
 101     }
 102
 103     void getAnalysisUsage(AnalysisUsage &AU) const override {
 104       AU.setPreservesCFG();
 105       AU.addRequired<SlotIndexes>();
 106       AU.addPreserved<SlotIndexes>();
 107       AU.addRequired<LiveStacks>();
 108       AU.addRequired<MachineBlockFrequencyInfo>();
 109       AU.addPreserved<MachineBlockFrequencyInfo>();
 110       AU.addPreservedID(MachineDominatorsID);
 111       MachineFunctionPass::getAnalysisUsage(AU);
 112     }
 113
 114     bool runOnMachineFunction(MachineFunction &MF) override;
 115
 116   private:
 117     void InitializeSlots();
 118     void ScanForSpillSlotRefs(MachineFunction &MF);
 119     bool OverlapWithAssignments(LiveInterval *li, int Color) const;
 120     int ColorSlot(LiveInterval *li);
 121     bool ColorSlots(MachineFunction &MF);
 122     void RewriteInstruction(MachineInstr &MI, SmallVectorImpl<int> &SlotMapping,
 123                             MachineFunction &MF);
 124     bool RemoveDeadStores(MachineBasicBlock* MBB);
 125   };
 126
 127 } // end anonymous namespace
 128
 129 char StackSlotColoring::ID = 0;
 130
 131 char &llvm::StackSlotColoringID = StackSlotColoring::ID;
 132
 133 INITIALIZE_PASS_BEGIN(StackSlotColoring, DEBUG_TYPE,
 134                 "Stack Slot Coloring", false, false)
 135 INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
 136 INITIALIZE_PASS_DEPENDENCY(LiveStacks)
 137 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
 138 INITIALIZE_PASS_END(StackSlotColoring, DEBUG_TYPE,
 139                 "Stack Slot Coloring", false, false)
 140
 141 namespace {
 142
 143 // IntervalSorter - Comparison predicate that sort live intervals by
 144 // their weight.
 145 struct IntervalSorter {
 146   bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
 147     return LHS->weight > RHS->weight;
 148   }
 149 };
 150
 151 } // end anonymous namespace
 152
 153 /// ScanForSpillSlotRefs - Scan all the machine instructions for spill slot
 154 /// references and update spill slot weights.
 155 void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) {
 156   SSRefs.resize(MFI->getObjectIndexEnd());
 157
 158   // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
 159   for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
 160        MBBI != E; ++MBBI) {
 161     MachineBasicBlock *MBB = &*MBBI;
 162     for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
 163          MII != EE; ++MII) {
 164       MachineInstr &MI = *MII;
 165       for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
 166         MachineOperand &MO = MI.getOperand(i);
 167         if (!MO.isFI())
 168           continue;
 169         int FI = MO.getIndex();
 170         if (FI < 0)
 171           continue;
 172         if (!LS->hasInterval(FI))
 173           continue;
 174         LiveInterval &li = LS->getInterval(FI);
 175         if (!MI.isDebugValue())
 176           li.weight += LiveIntervals::getSpillWeight(false, true, MBFI, MI);
 177       }
 178       for (MachineInstr::mmo_iterator MMOI = MI.memoperands_begin(),
 179                                       EE = MI.memoperands_end();
 180            MMOI != EE; ++MMOI) {
 181         MachineMemOperand *MMO = *MMOI;
 182         if (const FixedStackPseudoSourceValue *FSV =
 183             dyn_cast_or_null<FixedStackPseudoSourceValue>(
 184                 MMO->getPseudoValue())) {
 185           int FI = FSV->getFrameIndex();
 186           if (FI >= 0)
 187             SSRefs[FI].push_back(MMO);
 188         }
 189       }
 190     }
 191   }
 192 }
 193
 194 /// InitializeSlots - Process all spill stack slot liveintervals and add them
 195 /// to a sorted (by weight) list.
 196 void StackSlotColoring::InitializeSlots() {
 197   int LastFI = MFI->getObjectIndexEnd();
 198
 199   // There is always at least one stack ID.
 200   AllColors.resize(1);
 201   UsedColors.resize(1);
 202
 203   OrigAlignments.resize(LastFI);
 204   OrigSizes.resize(LastFI);
 205   AllColors[0].resize(LastFI);
 206   UsedColors[0].resize(LastFI);
 207   Assignments.resize(LastFI);
 208
 209   using Pair = std::iterator_traits<LiveStacks::iterator>::value_type;
 210
 211   SmallVector<Pair *, 16> Intervals;
 212
 213   Intervals.reserve(LS->getNumIntervals());
 214   for (auto &I : *LS)
 215     Intervals.push_back(&I);
 216   llvm::sort(Intervals,
 217              [](Pair *LHS, Pair *RHS) { return LHS->first < RHS->first; });
 218
 219   // Gather all spill slots into a list.
 220   LLVM_DEBUG(dbgs() << "Spill slot intervals:\n");
 221   for (auto *I : Intervals) {
 222     LiveInterval &li = I->second;
 223     LLVM_DEBUG(li.dump());
 224     int FI = Register::stackSlot2Index(li.reg);
 225     if (MFI->isDeadObjectIndex(FI))
 226       continue;
 227
 228     SSIntervals.push_back(&li);
 229     OrigAlignments[FI] = MFI->getObjectAlignment(FI);
 230     OrigSizes[FI]      = MFI->getObjectSize(FI);
 231
 232     auto StackID = MFI->getStackID(FI);
 233     if (StackID != 0) {
 234       AllColors.resize(StackID + 1);
 235       UsedColors.resize(StackID + 1);
 236       AllColors[StackID].resize(LastFI);
 237       UsedColors[StackID].resize(LastFI);
 238     }
 239
 240     AllColors[StackID].set(FI);
 241   }
 242   LLVM_DEBUG(dbgs() << '\n');
 243
 244   // Sort them by weight.
 245   llvm::stable_sort(SSIntervals, IntervalSorter());
 246
 247   NextColors.resize(AllColors.size());
 248
 249   // Get first "color".
 250   for (unsigned I = 0, E = AllColors.size(); I != E; ++I)
 251     NextColors[I] = AllColors[I].find_first();
 252 }
 253
 254 /// OverlapWithAssignments - Return true if LiveInterval overlaps with any
 255 /// LiveIntervals that have already been assigned to the specified color.
 256 bool
 257 StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
 258   const SmallVectorImpl<LiveInterval *> &OtherLIs = Assignments[Color];
 259   for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
 260     LiveInterval *OtherLI = OtherLIs[i];
 261     if (OtherLI->overlaps(*li))
 262       return true;
 263   }
 264   return false;
 265 }
 266
 267 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
 268 int StackSlotColoring::ColorSlot(LiveInterval *li) {
 269   int Color = -1;
 270   bool Share = false;
 271   int FI = Register::stackSlot2Index(li->reg);
 272   uint8_t StackID = MFI->getStackID(FI);
 273
 274   if (!DisableSharing) {
 275
 276     // Check if it's possible to reuse any of the used colors.
 277     Color = UsedColors[StackID].find_first();
 278     while (Color != -1) {
 279       if (!OverlapWithAssignments(li, Color)) {
 280         Share = true;
 281         ++NumEliminated;
 282         break;
 283       }
 284       Color = UsedColors[StackID].find_next(Color);
 285     }
 286   }
 287
 288   if (Color != -1 && MFI->getStackID(Color) != MFI->getStackID(FI)) {
 289     LLVM_DEBUG(dbgs() << "cannot share FIs with different stack IDs\n");
 290     Share = false;
 291   }
 292
 293   // Assign it to the first available color (assumed to be the best) if it's
 294   // not possible to share a used color with other objects.
 295   if (!Share) {
 296     assert(NextColors[StackID] != -1 && "No more spill slots?");
 297     Color = NextColors[StackID];
 298     UsedColors[StackID].set(Color);
 299     NextColors[StackID] = AllColors[StackID].find_next(NextColors[StackID]);
 300   }
 301
 302   assert(MFI->getStackID(Color) == MFI->getStackID(FI));
 303
 304   // Record the assignment.
 305   Assignments[Color].push_back(li);
 306   LLVM_DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n");
 307
 308   // Change size and alignment of the allocated slot. If there are multiple
 309   // objects sharing the same slot, then make sure the size and alignment
 310   // are large enough for all.
 311   unsigned Align = OrigAlignments[FI];
 312   if (!Share || Align > MFI->getObjectAlignment(Color))
 313     MFI->setObjectAlignment(Color, Align);
 314   int64_t Size = OrigSizes[FI];
 315   if (!Share || Size > MFI->getObjectSize(Color))
 316     MFI->setObjectSize(Color, Size);
 317   return Color;
 318 }
 319
 320 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine
 321 /// operands in the function.
 322 bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
 323   unsigned NumObjs = MFI->getObjectIndexEnd();
 324   SmallVector<int, 16> SlotMapping(NumObjs, -1);
 325   SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
 326   SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
 327   BitVector UsedColors(NumObjs);
 328
 329   LLVM_DEBUG(dbgs() << "Color spill slot intervals:\n");
 330   bool Changed = false;
 331   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
 332     LiveInterval *li = SSIntervals[i];
 333     int SS = Register::stackSlot2Index(li->reg);
 334     int NewSS = ColorSlot(li);
 335     assert(NewSS >= 0 && "Stack coloring failed?");
 336     SlotMapping[SS] = NewSS;
 337     RevMap[NewSS].push_back(SS);
 338     SlotWeights[NewSS] += li->weight;
 339     UsedColors.set(NewSS);
 340     Changed |= (SS != NewSS);
 341   }
 342
 343   LLVM_DEBUG(dbgs() << "\nSpill slots after coloring:\n");
 344   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
 345     LiveInterval *li = SSIntervals[i];
 346     int SS = Register::stackSlot2Index(li->reg);
 347     li->weight = SlotWeights[SS];
 348   }
 349   // Sort them by new weight.
 350   llvm::stable_sort(SSIntervals, IntervalSorter());
 351
 352 #ifndef NDEBUG
 353   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i)
 354     LLVM_DEBUG(SSIntervals[i]->dump());
 355   LLVM_DEBUG(dbgs() << '\n');
 356 #endif
 357
 358   if (!Changed)
 359     return false;
 360
 361   // Rewrite all MachineMemOperands.
 362   for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
 363     int NewFI = SlotMapping[SS];
 364     if (NewFI == -1 || (NewFI == (int)SS))
 365       continue;
 366
 367     const PseudoSourceValue *NewSV = MF.getPSVManager().getFixedStack(NewFI);
 368     SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS];
 369     for (unsigned i = 0, e = RefMMOs.size(); i != e; ++i)
 370       RefMMOs[i]->setValue(NewSV);
 371   }
 372
 373   // Rewrite all MO_FrameIndex operands.  Look for dead stores.
 374   for (MachineBasicBlock &MBB : MF) {
 375     for (MachineInstr &MI : MBB)
 376       RewriteInstruction(MI, SlotMapping, MF);
 377     RemoveDeadStores(&MBB);
 378   }
 379
 380   // Delete unused stack slots.
 381   for (int StackID = 0, E = AllColors.size(); StackID != E; ++StackID) {
 382     int NextColor = NextColors[StackID];
 383     while (NextColor != -1) {
 384       LLVM_DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n");
 385       MFI->RemoveStackObject(NextColor);
 386       NextColor = AllColors[StackID].find_next(NextColor);
 387     }
 388   }
 389
 390   return true;
 391 }
 392
 393 /// RewriteInstruction - Rewrite specified instruction by replacing references
 394 /// to old frame index with new one.
 395 void StackSlotColoring::RewriteInstruction(MachineInstr &MI,
 396                                            SmallVectorImpl<int> &SlotMapping,
 397                                            MachineFunction &MF) {
 398   // Update the operands.
 399   for (unsigned i = 0, ee = MI.getNumOperands(); i != ee; ++i) {
 400     MachineOperand &MO = MI.getOperand(i);
 401     if (!MO.isFI())
 402       continue;
 403     int OldFI = MO.getIndex();
 404     if (OldFI < 0)
 405       continue;
 406     int NewFI = SlotMapping[OldFI];
 407     if (NewFI == -1 || NewFI == OldFI)
 408       continue;
 409
 410     assert(MFI->getStackID(OldFI) == MFI->getStackID(NewFI));
 411     MO.setIndex(NewFI);
 412   }
 413
 414   // The MachineMemOperands have already been updated.
 415 }
 416
 417 /// RemoveDeadStores - Scan through a basic block and look for loads followed
 418 /// by stores.  If they're both using the same stack slot, then the store is
 419 /// definitely dead.  This could obviously be much more aggressive (consider
 420 /// pairs with instructions between them), but such extensions might have a
 421 /// considerable compile time impact.
 422 bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
 423   // FIXME: This could be much more aggressive, but we need to investigate
 424   // the compile time impact of doing so.
 425   bool changed = false;
 426
 427   SmallVector<MachineInstr*, 4> toErase;
 428
 429   for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
 430        I != E; ++I) {
 431     if (DCELimit != -1 && (int)NumDead >= DCELimit)
 432       break;
 433     int FirstSS, SecondSS;
 434     if (TII->isStackSlotCopy(*I, FirstSS, SecondSS) && FirstSS == SecondSS &&
 435         FirstSS != -1) {
 436       ++NumDead;
 437       changed = true;
 438       toErase.push_back(&*I);
 439       continue;
 440     }
 441
 442     MachineBasicBlock::iterator NextMI = std::next(I);
 443     MachineBasicBlock::iterator ProbableLoadMI = I;
 444
 445     unsigned LoadReg = 0;
 446     unsigned StoreReg = 0;
 447     unsigned LoadSize = 0;
 448     unsigned StoreSize = 0;
 449     if (!(LoadReg = TII->isLoadFromStackSlot(*I, FirstSS, LoadSize)))
 450       continue;
 451     // Skip the ...pseudo debugging... instructions between a load and store.
 452     while ((NextMI != E) && NextMI->isDebugInstr()) {
 453       ++NextMI;
 454       ++I;
 455     }
 456     if (NextMI == E) continue;
 457     if (!(StoreReg = TII->isStoreToStackSlot(*NextMI, SecondSS, StoreSize)))
 458       continue;
 459     if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1 ||
 460         LoadSize != StoreSize)
 461       continue;
 462
 463     ++NumDead;
 464     changed = true;
 465
 466     if (NextMI->findRegisterUseOperandIdx(LoadReg, true, nullptr) != -1) {
 467       ++NumDead;
 468       toErase.push_back(&*ProbableLoadMI);
 469     }
 470
 471     toErase.push_back(&*NextMI);
 472     ++I;
 473   }
 474
 475   for (SmallVectorImpl<MachineInstr *>::iterator I = toErase.begin(),
 476        E = toErase.end(); I != E; ++I)
 477     (*I)->eraseFromParent();
 478
 479   return changed;
 480 }
 481
 482 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
 483   LLVM_DEBUG({
 484     dbgs() << "********** Stack Slot Coloring **********\n"
 485            << "********** Function: " << MF.getName() << '\n';
 486   });
 487
 488   if (skipFunction(MF.getFunction()))
 489     return false;
 490
 491   MFI = &MF.getFrameInfo();
 492   TII = MF.getSubtarget().getInstrInfo();
 493   LS = &getAnalysis<LiveStacks>();
 494   MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
 495
 496   bool Changed = false;
 497
 498   unsigned NumSlots = LS->getNumIntervals();
 499   if (NumSlots == 0)
 500     // Nothing to do!
 501     return false;
 502
 503   // If there are calls to setjmp or sigsetjmp, don't perform stack slot
 504   // coloring. The stack could be modified before the longjmp is executed,
 505   // resulting in the wrong value being used afterwards. (See
 506   // <rdar://problem/8007500>.)
 507   if (MF.exposesReturnsTwice())
 508     return false;
 509
 510   // Gather spill slot references
 511   ScanForSpillSlotRefs(MF);
 512   InitializeSlots();
 513   Changed = ColorSlots(MF);
 514
 515   for (int &Next : NextColors)
 516     Next = -1;
 517
 518   SSIntervals.clear();
 519   for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
 520     SSRefs[i].clear();
 521   SSRefs.clear();
 522   OrigAlignments.clear();
 523   OrigSizes.clear();
 524   AllColors.clear();
 525   UsedColors.clear();
 526   for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
 527     Assignments[i].clear();
 528   Assignments.clear();
 529
 530   return Changed;
 531 }