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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/SmallSet.h"
     18 #include "llvm/ADT/SmallVector.h"
     19 #include "llvm/ADT/Statistic.h"
     20 #include "llvm/CodeGen/LiveIntervalAnalysis.h"
     21 #include "llvm/CodeGen/LiveStackAnalysis.h"
     22 #include "llvm/CodeGen/MachineFrameInfo.h"
     23 #include "llvm/CodeGen/MachineInstrBuilder.h"
     24 #include "llvm/CodeGen/MachineLoopInfo.h"
     25 #include "llvm/CodeGen/MachineMemOperand.h"
     26 #include "llvm/CodeGen/MachineRegisterInfo.h"
     27 #include "llvm/CodeGen/PseudoSourceValue.h"
     28 #include "llvm/IR/Module.h"
     29 #include "llvm/Support/CommandLine.h"
     30 #include "llvm/Support/Debug.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 MachineLoopInfo *loopInfo;
     52 
     53     // SSIntervals - Spill slot intervals.
     54     std::vector<LiveInterval*> SSIntervals;
     55 
     56     // SSRefs - Keep a list of frame index references for each spill slot.
     57     SmallVector<SmallVector<MachineInstr*, 8>, 16> SSRefs;
     58 
     59     // OrigAlignments - Alignments of stack objects before coloring.
     60     SmallVector<unsigned, 16> OrigAlignments;
     61 
     62     // OrigSizes - Sizess of stack objects before coloring.
     63     SmallVector<unsigned, 16> OrigSizes;
     64 
     65     // AllColors - If index is set, it's a spill slot, i.e. color.
     66     // FIXME: This assumes PEI locate spill slot with smaller indices
     67     // closest to stack pointer / frame pointer. Therefore, smaller
     68     // index == better color.
     69     BitVector AllColors;
     70 
     71     // NextColor - Next "color" that's not yet used.
     72     int NextColor;
     73 
     74     // UsedColors - "Colors" that have been assigned.
     75     BitVector UsedColors;
     76 
     77     // Assignments - Color to intervals mapping.
     78     SmallVector<SmallVector<LiveInterval*,4>, 16> Assignments;
     79 
     80   public:
     81     static char ID; // Pass identification
     82     StackSlotColoring() :
     83       MachineFunctionPass(ID), NextColor(-1) {
     84         initializeStackSlotColoringPass(*PassRegistry::getPassRegistry());
     85       }
     86 
     87     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
     88       AU.setPreservesCFG();
     89       AU.addRequired<SlotIndexes>();
     90       AU.addPreserved<SlotIndexes>();
     91       AU.addRequired<LiveStacks>();
     92       AU.addRequired<MachineLoopInfo>();
     93       AU.addPreserved<MachineLoopInfo>();
     94       AU.addPreservedID(MachineDominatorsID);
     95       MachineFunctionPass::getAnalysisUsage(AU);
     96     }
     97 
     98     virtual bool runOnMachineFunction(MachineFunction &MF);
     99 
    100   private:
    101     void InitializeSlots();
    102     void ScanForSpillSlotRefs(MachineFunction &MF);
    103     bool OverlapWithAssignments(LiveInterval *li, int Color) const;
    104     int ColorSlot(LiveInterval *li);
    105     bool ColorSlots(MachineFunction &MF);
    106     void RewriteInstruction(MachineInstr *MI, int OldFI, int NewFI,
    107                             MachineFunction &MF);
    108     bool RemoveDeadStores(MachineBasicBlock* MBB);
    109   };
    110 } // end anonymous namespace
    111 
    112 char StackSlotColoring::ID = 0;
    113 char &llvm::StackSlotColoringID = StackSlotColoring::ID;
    114 
    115 INITIALIZE_PASS_BEGIN(StackSlotColoring, "stack-slot-coloring",
    116                 "Stack Slot Coloring", false, false)
    117 INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
    118 INITIALIZE_PASS_DEPENDENCY(LiveStacks)
    119 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
    120 INITIALIZE_PASS_END(StackSlotColoring, "stack-slot-coloring",
    121                 "Stack Slot Coloring", false, false)
    122 
    123 namespace {
    124   // IntervalSorter - Comparison predicate that sort live intervals by
    125   // their weight.
    126   struct IntervalSorter {
    127     bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
    128       return LHS->weight > RHS->weight;
    129     }
    130   };
    131 }
    132 
    133 /// ScanForSpillSlotRefs - Scan all the machine instructions for spill slot
    134 /// references and update spill slot weights.
    135 void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) {
    136   SSRefs.resize(MFI->getObjectIndexEnd());
    137 
    138   // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
    139   for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
    140        MBBI != E; ++MBBI) {
    141     MachineBasicBlock *MBB = &*MBBI;
    142     unsigned loopDepth = loopInfo->getLoopDepth(MBB);
    143     for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
    144          MII != EE; ++MII) {
    145       MachineInstr *MI = &*MII;
    146       for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    147         MachineOperand &MO = MI->getOperand(i);
    148         if (!MO.isFI())
    149           continue;
    150         int FI = MO.getIndex();
    151         if (FI < 0)
    152           continue;
    153         if (!LS->hasInterval(FI))
    154           continue;
    155         LiveInterval &li = LS->getInterval(FI);
    156         if (!MI->isDebugValue())
    157           li.weight += LiveIntervals::getSpillWeight(false, true, loopDepth);
    158         SSRefs[FI].push_back(MI);
    159       }
    160     }
    161   }
    162 }
    163 
    164 /// InitializeSlots - Process all spill stack slot liveintervals and add them
    165 /// to a sorted (by weight) list.
    166 void StackSlotColoring::InitializeSlots() {
    167   int LastFI = MFI->getObjectIndexEnd();
    168   OrigAlignments.resize(LastFI);
    169   OrigSizes.resize(LastFI);
    170   AllColors.resize(LastFI);
    171   UsedColors.resize(LastFI);
    172   Assignments.resize(LastFI);
    173 
    174   // Gather all spill slots into a list.
    175   DEBUG(dbgs() << "Spill slot intervals:\n");
    176   for (LiveStacks::iterator i = LS->begin(), e = LS->end(); i != e; ++i) {
    177     LiveInterval &li = i->second;
    178     DEBUG(li.dump());
    179     int FI = TargetRegisterInfo::stackSlot2Index(li.reg);
    180     if (MFI->isDeadObjectIndex(FI))
    181       continue;
    182     SSIntervals.push_back(&li);
    183     OrigAlignments[FI] = MFI->getObjectAlignment(FI);
    184     OrigSizes[FI]      = MFI->getObjectSize(FI);
    185     AllColors.set(FI);
    186   }
    187   DEBUG(dbgs() << '\n');
    188 
    189   // Sort them by weight.
    190   std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
    191 
    192   // Get first "color".
    193   NextColor = AllColors.find_first();
    194 }
    195 
    196 /// OverlapWithAssignments - Return true if LiveInterval overlaps with any
    197 /// LiveIntervals that have already been assigned to the specified color.
    198 bool
    199 StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
    200   const SmallVector<LiveInterval*,4> &OtherLIs = Assignments[Color];
    201   for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
    202     LiveInterval *OtherLI = OtherLIs[i];
    203     if (OtherLI->overlaps(*li))
    204       return true;
    205   }
    206   return false;
    207 }
    208 
    209 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
    210 ///
    211 int StackSlotColoring::ColorSlot(LiveInterval *li) {
    212   int Color = -1;
    213   bool Share = false;
    214   if (!DisableSharing) {
    215     // Check if it's possible to reuse any of the used colors.
    216     Color = UsedColors.find_first();
    217     while (Color != -1) {
    218       if (!OverlapWithAssignments(li, Color)) {
    219         Share = true;
    220         ++NumEliminated;
    221         break;
    222       }
    223       Color = UsedColors.find_next(Color);
    224     }
    225   }
    226 
    227   // Assign it to the first available color (assumed to be the best) if it's
    228   // not possible to share a used color with other objects.
    229   if (!Share) {
    230     assert(NextColor != -1 && "No more spill slots?");
    231     Color = NextColor;
    232     UsedColors.set(Color);
    233     NextColor = AllColors.find_next(NextColor);
    234   }
    235 
    236   // Record the assignment.
    237   Assignments[Color].push_back(li);
    238   int FI = TargetRegisterInfo::stackSlot2Index(li->reg);
    239   DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n");
    240 
    241   // Change size and alignment of the allocated slot. If there are multiple
    242   // objects sharing the same slot, then make sure the size and alignment
    243   // are large enough for all.
    244   unsigned Align = OrigAlignments[FI];
    245   if (!Share || Align > MFI->getObjectAlignment(Color))
    246     MFI->setObjectAlignment(Color, Align);
    247   int64_t Size = OrigSizes[FI];
    248   if (!Share || Size > MFI->getObjectSize(Color))
    249     MFI->setObjectSize(Color, Size);
    250   return Color;
    251 }
    252 
    253 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine
    254 /// operands in the function.
    255 bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
    256   unsigned NumObjs = MFI->getObjectIndexEnd();
    257   SmallVector<int, 16> SlotMapping(NumObjs, -1);
    258   SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
    259   SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
    260   BitVector UsedColors(NumObjs);
    261 
    262   DEBUG(dbgs() << "Color spill slot intervals:\n");
    263   bool Changed = false;
    264   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
    265     LiveInterval *li = SSIntervals[i];
    266     int SS = TargetRegisterInfo::stackSlot2Index(li->reg);
    267     int NewSS = ColorSlot(li);
    268     assert(NewSS >= 0 && "Stack coloring failed?");
    269     SlotMapping[SS] = NewSS;
    270     RevMap[NewSS].push_back(SS);
    271     SlotWeights[NewSS] += li->weight;
    272     UsedColors.set(NewSS);
    273     Changed |= (SS != NewSS);
    274   }
    275 
    276   DEBUG(dbgs() << "\nSpill slots after coloring:\n");
    277   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
    278     LiveInterval *li = SSIntervals[i];
    279     int SS = TargetRegisterInfo::stackSlot2Index(li->reg);
    280     li->weight = SlotWeights[SS];
    281   }
    282   // Sort them by new weight.
    283   std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
    284 
    285 #ifndef NDEBUG
    286   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i)
    287     DEBUG(SSIntervals[i]->dump());
    288   DEBUG(dbgs() << '\n');
    289 #endif
    290 
    291   if (!Changed)
    292     return false;
    293 
    294   // Rewrite all MO_FrameIndex operands.
    295   SmallVector<SmallSet<unsigned, 4>, 4> NewDefs(MF.getNumBlockIDs());
    296   for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
    297     int NewFI = SlotMapping[SS];
    298     if (NewFI == -1 || (NewFI == (int)SS))
    299       continue;
    300 
    301     SmallVector<MachineInstr*, 8> &RefMIs = SSRefs[SS];
    302     for (unsigned i = 0, e = RefMIs.size(); i != e; ++i)
    303       RewriteInstruction(RefMIs[i], SS, NewFI, MF);
    304   }
    305 
    306   // Delete unused stack slots.
    307   while (NextColor != -1) {
    308     DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n");
    309     MFI->RemoveStackObject(NextColor);
    310     NextColor = AllColors.find_next(NextColor);
    311   }
    312 
    313   return true;
    314 }
    315 
    316 /// RewriteInstruction - Rewrite specified instruction by replacing references
    317 /// to old frame index with new one.
    318 void StackSlotColoring::RewriteInstruction(MachineInstr *MI, int OldFI,
    319                                            int NewFI, MachineFunction &MF) {
    320   // Update the operands.
    321   for (unsigned i = 0, ee = MI->getNumOperands(); i != ee; ++i) {
    322     MachineOperand &MO = MI->getOperand(i);
    323     if (!MO.isFI())
    324       continue;
    325     int FI = MO.getIndex();
    326     if (FI != OldFI)
    327       continue;
    328     MO.setIndex(NewFI);
    329   }
    330 
    331   // Update the memory references. This changes the MachineMemOperands
    332   // directly. They may be in use by multiple instructions, however all
    333   // instructions using OldFI are being rewritten to use NewFI.
    334   const Value *OldSV = PseudoSourceValue::getFixedStack(OldFI);
    335   const Value *NewSV = PseudoSourceValue::getFixedStack(NewFI);
    336   for (MachineInstr::mmo_iterator I = MI->memoperands_begin(),
    337        E = MI->memoperands_end(); I != E; ++I)
    338     if ((*I)->getValue() == OldSV)
    339       (*I)->setValue(NewSV);
    340 }
    341 
    342 
    343 /// RemoveDeadStores - Scan through a basic block and look for loads followed
    344 /// by stores.  If they're both using the same stack slot, then the store is
    345 /// definitely dead.  This could obviously be much more aggressive (consider
    346 /// pairs with instructions between them), but such extensions might have a
    347 /// considerable compile time impact.
    348 bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
    349   // FIXME: This could be much more aggressive, but we need to investigate
    350   // the compile time impact of doing so.
    351   bool changed = false;
    352 
    353   SmallVector<MachineInstr*, 4> toErase;
    354 
    355   for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
    356        I != E; ++I) {
    357     if (DCELimit != -1 && (int)NumDead >= DCELimit)
    358       break;
    359 
    360     MachineBasicBlock::iterator NextMI = llvm::next(I);
    361     if (NextMI == MBB->end()) continue;
    362 
    363     int FirstSS, SecondSS;
    364     unsigned LoadReg = 0;
    365     unsigned StoreReg = 0;
    366     if (!(LoadReg = TII->isLoadFromStackSlot(I, FirstSS))) continue;
    367     if (!(StoreReg = TII->isStoreToStackSlot(NextMI, SecondSS))) continue;
    368     if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1) continue;
    369 
    370     ++NumDead;
    371     changed = true;
    372 
    373     if (NextMI->findRegisterUseOperandIdx(LoadReg, true, 0) != -1) {
    374       ++NumDead;
    375       toErase.push_back(I);
    376     }
    377 
    378     toErase.push_back(NextMI);
    379     ++I;
    380   }
    381 
    382   for (SmallVector<MachineInstr*, 4>::iterator I = toErase.begin(),
    383        E = toErase.end(); I != E; ++I)
    384     (*I)->eraseFromParent();
    385 
    386   return changed;
    387 }
    388 
    389 
    390 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
    391   DEBUG({
    392       dbgs() << "********** Stack Slot Coloring **********\n"
    393              << "********** Function: " << MF.getName() << '\n';
    394     });
    395 
    396   MFI = MF.getFrameInfo();
    397   TII = MF.getTarget().getInstrInfo();
    398   LS = &getAnalysis<LiveStacks>();
    399   loopInfo = &getAnalysis<MachineLoopInfo>();
    400 
    401   bool Changed = false;
    402 
    403   unsigned NumSlots = LS->getNumIntervals();
    404   if (NumSlots == 0)
    405     // Nothing to do!
    406     return false;
    407 
    408   // If there are calls to setjmp or sigsetjmp, don't perform stack slot
    409   // coloring. The stack could be modified before the longjmp is executed,
    410   // resulting in the wrong value being used afterwards. (See
    411   // <rdar://problem/8007500>.)
    412   if (MF.exposesReturnsTwice())
    413     return false;
    414 
    415   // Gather spill slot references
    416   ScanForSpillSlotRefs(MF);
    417   InitializeSlots();
    418   Changed = ColorSlots(MF);
    419 
    420   NextColor = -1;
    421   SSIntervals.clear();
    422   for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
    423     SSRefs[i].clear();
    424   SSRefs.clear();
    425   OrigAlignments.clear();
    426   OrigSizes.clear();
    427   AllColors.clear();
    428   UsedColors.clear();
    429   for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
    430     Assignments[i].clear();
    431   Assignments.clear();
    432 
    433   if (Changed) {
    434     for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
    435       Changed |= RemoveDeadStores(I);
    436   }
    437 
    438   return Changed;
    439 }
    440