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