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      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();
    164            MMOI != EE; ++MMOI) {
    165         MachineMemOperand *MMO = *MMOI;
    166         if (const FixedStackPseudoSourceValue *FSV =
    167             dyn_cast_or_null<FixedStackPseudoSourceValue>(
    168                 MMO->getPseudoValue())) {
    169           int FI = FSV->getFrameIndex();
    170           if (FI >= 0)
    171             SSRefs[FI].push_back(MMO);
    172         }
    173       }
    174     }
    175   }
    176 }
    177 
    178 /// InitializeSlots - Process all spill stack slot liveintervals and add them
    179 /// to a sorted (by weight) list.
    180 void StackSlotColoring::InitializeSlots() {
    181   int LastFI = MFI->getObjectIndexEnd();
    182   OrigAlignments.resize(LastFI);
    183   OrigSizes.resize(LastFI);
    184   AllColors.resize(LastFI);
    185   UsedColors.resize(LastFI);
    186   Assignments.resize(LastFI);
    187 
    188   typedef std::iterator_traits<LiveStacks::iterator>::value_type Pair;
    189   SmallVector<Pair *, 16> Intervals;
    190   Intervals.reserve(LS->getNumIntervals());
    191   for (auto &I : *LS)
    192     Intervals.push_back(&I);
    193   std::sort(Intervals.begin(), Intervals.end(),
    194             [](Pair *LHS, Pair *RHS) { return LHS->first < RHS->first; });
    195 
    196   // Gather all spill slots into a list.
    197   DEBUG(dbgs() << "Spill slot intervals:\n");
    198   for (auto *I : Intervals) {
    199     LiveInterval &li = I->second;
    200     DEBUG(li.dump());
    201     int FI = TargetRegisterInfo::stackSlot2Index(li.reg);
    202     if (MFI->isDeadObjectIndex(FI))
    203       continue;
    204     SSIntervals.push_back(&li);
    205     OrigAlignments[FI] = MFI->getObjectAlignment(FI);
    206     OrigSizes[FI]      = MFI->getObjectSize(FI);
    207     AllColors.set(FI);
    208   }
    209   DEBUG(dbgs() << '\n');
    210 
    211   // Sort them by weight.
    212   std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
    213 
    214   // Get first "color".
    215   NextColor = AllColors.find_first();
    216 }
    217 
    218 /// OverlapWithAssignments - Return true if LiveInterval overlaps with any
    219 /// LiveIntervals that have already been assigned to the specified color.
    220 bool
    221 StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
    222   const SmallVectorImpl<LiveInterval *> &OtherLIs = Assignments[Color];
    223   for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
    224     LiveInterval *OtherLI = OtherLIs[i];
    225     if (OtherLI->overlaps(*li))
    226       return true;
    227   }
    228   return false;
    229 }
    230 
    231 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
    232 ///
    233 int StackSlotColoring::ColorSlot(LiveInterval *li) {
    234   int Color = -1;
    235   bool Share = false;
    236   if (!DisableSharing) {
    237     // Check if it's possible to reuse any of the used colors.
    238     Color = UsedColors.find_first();
    239     while (Color != -1) {
    240       if (!OverlapWithAssignments(li, Color)) {
    241         Share = true;
    242         ++NumEliminated;
    243         break;
    244       }
    245       Color = UsedColors.find_next(Color);
    246     }
    247   }
    248 
    249   // Assign it to the first available color (assumed to be the best) if it's
    250   // not possible to share a used color with other objects.
    251   if (!Share) {
    252     assert(NextColor != -1 && "No more spill slots?");
    253     Color = NextColor;
    254     UsedColors.set(Color);
    255     NextColor = AllColors.find_next(NextColor);
    256   }
    257 
    258   // Record the assignment.
    259   Assignments[Color].push_back(li);
    260   int FI = TargetRegisterInfo::stackSlot2Index(li->reg);
    261   DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n");
    262 
    263   // Change size and alignment of the allocated slot. If there are multiple
    264   // objects sharing the same slot, then make sure the size and alignment
    265   // are large enough for all.
    266   unsigned Align = OrigAlignments[FI];
    267   if (!Share || Align > MFI->getObjectAlignment(Color))
    268     MFI->setObjectAlignment(Color, Align);
    269   int64_t Size = OrigSizes[FI];
    270   if (!Share || Size > MFI->getObjectSize(Color))
    271     MFI->setObjectSize(Color, Size);
    272   return Color;
    273 }
    274 
    275 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine
    276 /// operands in the function.
    277 bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
    278   unsigned NumObjs = MFI->getObjectIndexEnd();
    279   SmallVector<int, 16> SlotMapping(NumObjs, -1);
    280   SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
    281   SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
    282   BitVector UsedColors(NumObjs);
    283 
    284   DEBUG(dbgs() << "Color spill slot intervals:\n");
    285   bool Changed = false;
    286   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
    287     LiveInterval *li = SSIntervals[i];
    288     int SS = TargetRegisterInfo::stackSlot2Index(li->reg);
    289     int NewSS = ColorSlot(li);
    290     assert(NewSS >= 0 && "Stack coloring failed?");
    291     SlotMapping[SS] = NewSS;
    292     RevMap[NewSS].push_back(SS);
    293     SlotWeights[NewSS] += li->weight;
    294     UsedColors.set(NewSS);
    295     Changed |= (SS != NewSS);
    296   }
    297 
    298   DEBUG(dbgs() << "\nSpill slots after coloring:\n");
    299   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
    300     LiveInterval *li = SSIntervals[i];
    301     int SS = TargetRegisterInfo::stackSlot2Index(li->reg);
    302     li->weight = SlotWeights[SS];
    303   }
    304   // Sort them by new weight.
    305   std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
    306 
    307 #ifndef NDEBUG
    308   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i)
    309     DEBUG(SSIntervals[i]->dump());
    310   DEBUG(dbgs() << '\n');
    311 #endif
    312 
    313   if (!Changed)
    314     return false;
    315 
    316   // Rewrite all MachineMemOperands.
    317   for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
    318     int NewFI = SlotMapping[SS];
    319     if (NewFI == -1 || (NewFI == (int)SS))
    320       continue;
    321 
    322     const PseudoSourceValue *NewSV = MF.getPSVManager().getFixedStack(NewFI);
    323     SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS];
    324     for (unsigned i = 0, e = RefMMOs.size(); i != e; ++i)
    325       RefMMOs[i]->setValue(NewSV);
    326   }
    327 
    328   // Rewrite all MO_FrameIndex operands.  Look for dead stores.
    329   for (MachineBasicBlock &MBB : MF) {
    330     for (MachineInstr &MI : MBB)
    331       RewriteInstruction(MI, SlotMapping, MF);
    332     RemoveDeadStores(&MBB);
    333   }
    334 
    335   // Delete unused stack slots.
    336   while (NextColor != -1) {
    337     DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n");
    338     MFI->RemoveStackObject(NextColor);
    339     NextColor = AllColors.find_next(NextColor);
    340   }
    341 
    342   return true;
    343 }
    344 
    345 /// RewriteInstruction - Rewrite specified instruction by replacing references
    346 /// to old frame index with new one.
    347 void StackSlotColoring::RewriteInstruction(MachineInstr &MI,
    348                                            SmallVectorImpl<int> &SlotMapping,
    349                                            MachineFunction &MF) {
    350   // Update the operands.
    351   for (unsigned i = 0, ee = MI.getNumOperands(); i != ee; ++i) {
    352     MachineOperand &MO = MI.getOperand(i);
    353     if (!MO.isFI())
    354       continue;
    355     int OldFI = MO.getIndex();
    356     if (OldFI < 0)
    357       continue;
    358     int NewFI = SlotMapping[OldFI];
    359     if (NewFI == -1 || NewFI == OldFI)
    360       continue;
    361     MO.setIndex(NewFI);
    362   }
    363 
    364   // The MachineMemOperands have already been updated.
    365 }
    366 
    367 
    368 /// RemoveDeadStores - Scan through a basic block and look for loads followed
    369 /// by stores.  If they're both using the same stack slot, then the store is
    370 /// definitely dead.  This could obviously be much more aggressive (consider
    371 /// pairs with instructions between them), but such extensions might have a
    372 /// considerable compile time impact.
    373 bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
    374   // FIXME: This could be much more aggressive, but we need to investigate
    375   // the compile time impact of doing so.
    376   bool changed = false;
    377 
    378   SmallVector<MachineInstr*, 4> toErase;
    379 
    380   for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
    381        I != E; ++I) {
    382     if (DCELimit != -1 && (int)NumDead >= DCELimit)
    383       break;
    384 
    385     int FirstSS, SecondSS;
    386     if (TII->isStackSlotCopy(*I, FirstSS, SecondSS) && FirstSS == SecondSS &&
    387         FirstSS != -1) {
    388       ++NumDead;
    389       changed = true;
    390       toErase.push_back(&*I);
    391       continue;
    392     }
    393 
    394     MachineBasicBlock::iterator NextMI = std::next(I);
    395     if (NextMI == MBB->end()) continue;
    396 
    397     unsigned LoadReg = 0;
    398     unsigned StoreReg = 0;
    399     if (!(LoadReg = TII->isLoadFromStackSlot(*I, FirstSS)))
    400       continue;
    401     if (!(StoreReg = TII->isStoreToStackSlot(*NextMI, SecondSS)))
    402       continue;
    403     if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1) continue;
    404 
    405     ++NumDead;
    406     changed = true;
    407 
    408     if (NextMI->findRegisterUseOperandIdx(LoadReg, true, nullptr) != -1) {
    409       ++NumDead;
    410       toErase.push_back(&*I);
    411     }
    412 
    413     toErase.push_back(&*NextMI);
    414     ++I;
    415   }
    416 
    417   for (SmallVectorImpl<MachineInstr *>::iterator I = toErase.begin(),
    418        E = toErase.end(); I != E; ++I)
    419     (*I)->eraseFromParent();
    420 
    421   return changed;
    422 }
    423 
    424 
    425 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
    426   DEBUG({
    427       dbgs() << "********** Stack Slot Coloring **********\n"
    428              << "********** Function: " << MF.getName() << '\n';
    429     });
    430 
    431   MFI = MF.getFrameInfo();
    432   TII = MF.getSubtarget().getInstrInfo();
    433   LS = &getAnalysis<LiveStacks>();
    434   MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
    435 
    436   bool Changed = false;
    437 
    438   unsigned NumSlots = LS->getNumIntervals();
    439   if (NumSlots == 0)
    440     // Nothing to do!
    441     return false;
    442 
    443   // If there are calls to setjmp or sigsetjmp, don't perform stack slot
    444   // coloring. The stack could be modified before the longjmp is executed,
    445   // resulting in the wrong value being used afterwards. (See
    446   // <rdar://problem/8007500>.)
    447   if (MF.exposesReturnsTwice())
    448     return false;
    449 
    450   // Gather spill slot references
    451   ScanForSpillSlotRefs(MF);
    452   InitializeSlots();
    453   Changed = ColorSlots(MF);
    454 
    455   NextColor = -1;
    456   SSIntervals.clear();
    457   for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
    458     SSRefs[i].clear();
    459   SSRefs.clear();
    460   OrigAlignments.clear();
    461   OrigSizes.clear();
    462   AllColors.clear();
    463   UsedColors.clear();
    464   for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
    465     Assignments[i].clear();
    466   Assignments.clear();
    467 
    468   return Changed;
    469 }
    470