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