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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 "stackcoloring"
     15 #include "llvm/Function.h"
     16 #include "llvm/Module.h"
     17 #include "llvm/CodeGen/Passes.h"
     18 #include "llvm/CodeGen/LiveIntervalAnalysis.h"
     19 #include "llvm/CodeGen/LiveStackAnalysis.h"
     20 #include "llvm/CodeGen/MachineFrameInfo.h"
     21 #include "llvm/CodeGen/MachineInstrBuilder.h"
     22 #include "llvm/CodeGen/MachineLoopInfo.h"
     23 #include "llvm/CodeGen/MachineMemOperand.h"
     24 #include "llvm/CodeGen/MachineRegisterInfo.h"
     25 #include "llvm/CodeGen/PseudoSourceValue.h"
     26 #include "llvm/Support/CommandLine.h"
     27 #include "llvm/Support/Debug.h"
     28 #include "llvm/Target/TargetInstrInfo.h"
     29 #include "llvm/Target/TargetMachine.h"
     30 #include "llvm/ADT/BitVector.h"
     31 #include "llvm/ADT/SmallSet.h"
     32 #include "llvm/ADT/SmallVector.h"
     33 #include "llvm/ADT/Statistic.h"
     34 #include <vector>
     35 using namespace llvm;
     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     bool ColorWithRegs;
     50     LiveStacks* LS;
     51     MachineFrameInfo *MFI;
     52     const TargetInstrInfo  *TII;
     53     const MachineLoopInfo *loopInfo;
     54 
     55     // SSIntervals - Spill slot intervals.
     56     std::vector<LiveInterval*> SSIntervals;
     57 
     58     // SSRefs - Keep a list of frame index references for each spill slot.
     59     SmallVector<SmallVector<MachineInstr*, 8>, 16> SSRefs;
     60 
     61     // OrigAlignments - Alignments of stack objects before coloring.
     62     SmallVector<unsigned, 16> OrigAlignments;
     63 
     64     // OrigSizes - Sizess of stack objects before coloring.
     65     SmallVector<unsigned, 16> OrigSizes;
     66 
     67     // AllColors - If index is set, it's a spill slot, i.e. color.
     68     // FIXME: This assumes PEI locate spill slot with smaller indices
     69     // closest to stack pointer / frame pointer. Therefore, smaller
     70     // index == better color.
     71     BitVector AllColors;
     72 
     73     // NextColor - Next "color" that's not yet used.
     74     int NextColor;
     75 
     76     // UsedColors - "Colors" that have been assigned.
     77     BitVector UsedColors;
     78 
     79     // Assignments - Color to intervals mapping.
     80     SmallVector<SmallVector<LiveInterval*,4>, 16> Assignments;
     81 
     82   public:
     83     static char ID; // Pass identification
     84     StackSlotColoring() :
     85       MachineFunctionPass(ID), ColorWithRegs(false), NextColor(-1) {
     86         initializeStackSlotColoringPass(*PassRegistry::getPassRegistry());
     87       }
     88 
     89     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
     90       AU.setPreservesCFG();
     91       AU.addRequired<SlotIndexes>();
     92       AU.addPreserved<SlotIndexes>();
     93       AU.addRequired<LiveStacks>();
     94       AU.addRequired<MachineLoopInfo>();
     95       AU.addPreserved<MachineLoopInfo>();
     96       AU.addPreservedID(MachineDominatorsID);
     97       MachineFunctionPass::getAnalysisUsage(AU);
     98     }
     99 
    100     virtual bool runOnMachineFunction(MachineFunction &MF);
    101 
    102   private:
    103     void InitializeSlots();
    104     void ScanForSpillSlotRefs(MachineFunction &MF);
    105     bool OverlapWithAssignments(LiveInterval *li, int Color) const;
    106     int ColorSlot(LiveInterval *li);
    107     bool ColorSlots(MachineFunction &MF);
    108     void RewriteInstruction(MachineInstr *MI, int OldFI, int NewFI,
    109                             MachineFunction &MF);
    110     bool RemoveDeadStores(MachineBasicBlock* MBB);
    111   };
    112 } // end anonymous namespace
    113 
    114 char StackSlotColoring::ID = 0;
    115 char &llvm::StackSlotColoringID = StackSlotColoring::ID;
    116 
    117 INITIALIZE_PASS_BEGIN(StackSlotColoring, "stack-slot-coloring",
    118                 "Stack Slot Coloring", false, false)
    119 INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
    120 INITIALIZE_PASS_DEPENDENCY(LiveStacks)
    121 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
    122 INITIALIZE_PASS_END(StackSlotColoring, "stack-slot-coloring",
    123                 "Stack Slot Coloring", false, false)
    124 
    125 namespace {
    126   // IntervalSorter - Comparison predicate that sort live intervals by
    127   // their weight.
    128   struct IntervalSorter {
    129     bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
    130       return LHS->weight > RHS->weight;
    131     }
    132   };
    133 }
    134 
    135 /// ScanForSpillSlotRefs - Scan all the machine instructions for spill slot
    136 /// references and update spill slot weights.
    137 void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) {
    138   SSRefs.resize(MFI->getObjectIndexEnd());
    139 
    140   // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
    141   for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
    142        MBBI != E; ++MBBI) {
    143     MachineBasicBlock *MBB = &*MBBI;
    144     unsigned loopDepth = loopInfo->getLoopDepth(MBB);
    145     for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
    146          MII != EE; ++MII) {
    147       MachineInstr *MI = &*MII;
    148       for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    149         MachineOperand &MO = MI->getOperand(i);
    150         if (!MO.isFI())
    151           continue;
    152         int FI = MO.getIndex();
    153         if (FI < 0)
    154           continue;
    155         if (!LS->hasInterval(FI))
    156           continue;
    157         LiveInterval &li = LS->getInterval(FI);
    158         if (!MI->isDebugValue())
    159           li.weight += LiveIntervals::getSpillWeight(false, true, loopDepth);
    160         SSRefs[FI].push_back(MI);
    161       }
    162     }
    163   }
    164 }
    165 
    166 /// InitializeSlots - Process all spill stack slot liveintervals and add them
    167 /// to a sorted (by weight) list.
    168 void StackSlotColoring::InitializeSlots() {
    169   int LastFI = MFI->getObjectIndexEnd();
    170   OrigAlignments.resize(LastFI);
    171   OrigSizes.resize(LastFI);
    172   AllColors.resize(LastFI);
    173   UsedColors.resize(LastFI);
    174   Assignments.resize(LastFI);
    175 
    176   // Gather all spill slots into a list.
    177   DEBUG(dbgs() << "Spill slot intervals:\n");
    178   for (LiveStacks::iterator i = LS->begin(), e = LS->end(); i != e; ++i) {
    179     LiveInterval &li = i->second;
    180     DEBUG(li.dump());
    181     int FI = TargetRegisterInfo::stackSlot2Index(li.reg);
    182     if (MFI->isDeadObjectIndex(FI))
    183       continue;
    184     SSIntervals.push_back(&li);
    185     OrigAlignments[FI] = MFI->getObjectAlignment(FI);
    186     OrigSizes[FI]      = MFI->getObjectSize(FI);
    187     AllColors.set(FI);
    188   }
    189   DEBUG(dbgs() << '\n');
    190 
    191   // Sort them by weight.
    192   std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
    193 
    194   // Get first "color".
    195   NextColor = AllColors.find_first();
    196 }
    197 
    198 /// OverlapWithAssignments - Return true if LiveInterval overlaps with any
    199 /// LiveIntervals that have already been assigned to the specified color.
    200 bool
    201 StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
    202   const SmallVector<LiveInterval*,4> &OtherLIs = Assignments[Color];
    203   for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
    204     LiveInterval *OtherLI = OtherLIs[i];
    205     if (OtherLI->overlaps(*li))
    206       return true;
    207   }
    208   return false;
    209 }
    210 
    211 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
    212 ///
    213 int StackSlotColoring::ColorSlot(LiveInterval *li) {
    214   int Color = -1;
    215   bool Share = false;
    216   if (!DisableSharing) {
    217     // Check if it's possible to reuse any of the used colors.
    218     Color = UsedColors.find_first();
    219     while (Color != -1) {
    220       if (!OverlapWithAssignments(li, Color)) {
    221         Share = true;
    222         ++NumEliminated;
    223         break;
    224       }
    225       Color = UsedColors.find_next(Color);
    226     }
    227   }
    228 
    229   // Assign it to the first available color (assumed to be the best) if it's
    230   // not possible to share a used color with other objects.
    231   if (!Share) {
    232     assert(NextColor != -1 && "No more spill slots?");
    233     Color = NextColor;
    234     UsedColors.set(Color);
    235     NextColor = AllColors.find_next(NextColor);
    236   }
    237 
    238   // Record the assignment.
    239   Assignments[Color].push_back(li);
    240   int FI = TargetRegisterInfo::stackSlot2Index(li->reg);
    241   DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n");
    242 
    243   // Change size and alignment of the allocated slot. If there are multiple
    244   // objects sharing the same slot, then make sure the size and alignment
    245   // are large enough for all.
    246   unsigned Align = OrigAlignments[FI];
    247   if (!Share || Align > MFI->getObjectAlignment(Color))
    248     MFI->setObjectAlignment(Color, Align);
    249   int64_t Size = OrigSizes[FI];
    250   if (!Share || Size > MFI->getObjectSize(Color))
    251     MFI->setObjectSize(Color, Size);
    252   return Color;
    253 }
    254 
    255 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine
    256 /// operands in the function.
    257 bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
    258   unsigned NumObjs = MFI->getObjectIndexEnd();
    259   SmallVector<int, 16> SlotMapping(NumObjs, -1);
    260   SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
    261   SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
    262   BitVector UsedColors(NumObjs);
    263 
    264   DEBUG(dbgs() << "Color spill slot intervals:\n");
    265   bool Changed = false;
    266   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
    267     LiveInterval *li = SSIntervals[i];
    268     int SS = TargetRegisterInfo::stackSlot2Index(li->reg);
    269     int NewSS = ColorSlot(li);
    270     assert(NewSS >= 0 && "Stack coloring failed?");
    271     SlotMapping[SS] = NewSS;
    272     RevMap[NewSS].push_back(SS);
    273     SlotWeights[NewSS] += li->weight;
    274     UsedColors.set(NewSS);
    275     Changed |= (SS != NewSS);
    276   }
    277 
    278   DEBUG(dbgs() << "\nSpill slots after coloring:\n");
    279   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
    280     LiveInterval *li = SSIntervals[i];
    281     int SS = TargetRegisterInfo::stackSlot2Index(li->reg);
    282     li->weight = SlotWeights[SS];
    283   }
    284   // Sort them by new weight.
    285   std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
    286 
    287 #ifndef NDEBUG
    288   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i)
    289     DEBUG(SSIntervals[i]->dump());
    290   DEBUG(dbgs() << '\n');
    291 #endif
    292 
    293   if (!Changed)
    294     return false;
    295 
    296   // Rewrite all MO_FrameIndex operands.
    297   SmallVector<SmallSet<unsigned, 4>, 4> NewDefs(MF.getNumBlockIDs());
    298   for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
    299     int NewFI = SlotMapping[SS];
    300     if (NewFI == -1 || (NewFI == (int)SS))
    301       continue;
    302 
    303     SmallVector<MachineInstr*, 8> &RefMIs = SSRefs[SS];
    304     for (unsigned i = 0, e = RefMIs.size(); i != e; ++i)
    305       RewriteInstruction(RefMIs[i], SS, NewFI, MF);
    306   }
    307 
    308   // Delete unused stack slots.
    309   while (NextColor != -1) {
    310     DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n");
    311     MFI->RemoveStackObject(NextColor);
    312     NextColor = AllColors.find_next(NextColor);
    313   }
    314 
    315   return true;
    316 }
    317 
    318 /// RewriteInstruction - Rewrite specified instruction by replacing references
    319 /// to old frame index with new one.
    320 void StackSlotColoring::RewriteInstruction(MachineInstr *MI, int OldFI,
    321                                            int NewFI, MachineFunction &MF) {
    322   // Update the operands.
    323   for (unsigned i = 0, ee = MI->getNumOperands(); i != ee; ++i) {
    324     MachineOperand &MO = MI->getOperand(i);
    325     if (!MO.isFI())
    326       continue;
    327     int FI = MO.getIndex();
    328     if (FI != OldFI)
    329       continue;
    330     MO.setIndex(NewFI);
    331   }
    332 
    333   // Update the memory references. This changes the MachineMemOperands
    334   // directly. They may be in use by multiple instructions, however all
    335   // instructions using OldFI are being rewritten to use NewFI.
    336   const Value *OldSV = PseudoSourceValue::getFixedStack(OldFI);
    337   const Value *NewSV = PseudoSourceValue::getFixedStack(NewFI);
    338   for (MachineInstr::mmo_iterator I = MI->memoperands_begin(),
    339        E = MI->memoperands_end(); I != E; ++I)
    340     if ((*I)->getValue() == OldSV)
    341       (*I)->setValue(NewSV);
    342 }
    343 
    344 
    345 /// RemoveDeadStores - Scan through a basic block and look for loads followed
    346 /// by stores.  If they're both using the same stack slot, then the store is
    347 /// definitely dead.  This could obviously be much more aggressive (consider
    348 /// pairs with instructions between them), but such extensions might have a
    349 /// considerable compile time impact.
    350 bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
    351   // FIXME: This could be much more aggressive, but we need to investigate
    352   // the compile time impact of doing so.
    353   bool changed = false;
    354 
    355   SmallVector<MachineInstr*, 4> toErase;
    356 
    357   for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
    358        I != E; ++I) {
    359     if (DCELimit != -1 && (int)NumDead >= DCELimit)
    360       break;
    361 
    362     MachineBasicBlock::iterator NextMI = llvm::next(I);
    363     if (NextMI == MBB->end()) continue;
    364 
    365     int FirstSS, SecondSS;
    366     unsigned LoadReg = 0;
    367     unsigned StoreReg = 0;
    368     if (!(LoadReg = TII->isLoadFromStackSlot(I, FirstSS))) continue;
    369     if (!(StoreReg = TII->isStoreToStackSlot(NextMI, SecondSS))) continue;
    370     if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1) continue;
    371 
    372     ++NumDead;
    373     changed = true;
    374 
    375     if (NextMI->findRegisterUseOperandIdx(LoadReg, true, 0) != -1) {
    376       ++NumDead;
    377       toErase.push_back(I);
    378     }
    379 
    380     toErase.push_back(NextMI);
    381     ++I;
    382   }
    383 
    384   for (SmallVector<MachineInstr*, 4>::iterator I = toErase.begin(),
    385        E = toErase.end(); I != E; ++I)
    386     (*I)->eraseFromParent();
    387 
    388   return changed;
    389 }
    390 
    391 
    392 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
    393   DEBUG({
    394       dbgs() << "********** Stack Slot Coloring **********\n"
    395              << "********** Function: "
    396              << MF.getFunction()->getName() << '\n';
    397     });
    398 
    399   MFI = MF.getFrameInfo();
    400   TII = MF.getTarget().getInstrInfo();
    401   LS = &getAnalysis<LiveStacks>();
    402   loopInfo = &getAnalysis<MachineLoopInfo>();
    403 
    404   bool Changed = false;
    405 
    406   unsigned NumSlots = LS->getNumIntervals();
    407   if (NumSlots == 0)
    408     // Nothing to do!
    409     return false;
    410 
    411   // If there are calls to setjmp or sigsetjmp, don't perform stack slot
    412   // coloring. The stack could be modified before the longjmp is executed,
    413   // resulting in the wrong value being used afterwards. (See
    414   // <rdar://problem/8007500>.)
    415   if (MF.exposesReturnsTwice())
    416     return false;
    417 
    418   // Gather spill slot references
    419   ScanForSpillSlotRefs(MF);
    420   InitializeSlots();
    421   Changed = ColorSlots(MF);
    422 
    423   NextColor = -1;
    424   SSIntervals.clear();
    425   for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
    426     SSRefs[i].clear();
    427   SSRefs.clear();
    428   OrigAlignments.clear();
    429   OrigSizes.clear();
    430   AllColors.clear();
    431   UsedColors.clear();
    432   for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
    433     Assignments[i].clear();
    434   Assignments.clear();
    435 
    436   if (Changed) {
    437     for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
    438       Changed |= RemoveDeadStores(I);
    439   }
    440 
    441   return Changed;
    442 }
    443