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