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/LiveIntervals.h"
18 #include "llvm/CodeGen/LiveStacks.h"
19 #include "llvm/CodeGen/MachineBasicBlock.h"
20 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
21 #include "llvm/CodeGen/MachineFrameInfo.h"
22 #include "llvm/CodeGen/MachineFunction.h"
23 #include "llvm/CodeGen/MachineFunctionPass.h"
24 #include "llvm/CodeGen/MachineInstr.h"
25 #include "llvm/CodeGen/MachineMemOperand.h"
26 #include "llvm/CodeGen/MachineOperand.h"
27 #include "llvm/CodeGen/Passes.h"
28 #include "llvm/CodeGen/PseudoSourceValue.h"
29 #include "llvm/CodeGen/SlotIndexes.h"
30 #include "llvm/CodeGen/TargetInstrInfo.h"
31 #include "llvm/CodeGen/TargetRegisterInfo.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;
63     MachineFrameInfo *MFI;
64     const TargetInstrInfo  *TII;
65     const MachineBlockFrequencyInfo *MBFI;
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 - Sizess 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     // Assignments - Color to intervals mapping.
95     SmallVector<SmallVector<LiveInterval*,4>, 16> Assignments;
96 
97   public:
98     static char ID; // Pass identification
99 
100     StackSlotColoring() : MachineFunctionPass(ID) {
101       initializeStackSlotColoringPass(*PassRegistry::getPassRegistry());
102     }
103 
104     void getAnalysisUsage(AnalysisUsage &AU) const override {
105       AU.setPreservesCFG();
106       AU.addRequired<SlotIndexes>();
107       AU.addPreserved<SlotIndexes>();
108       AU.addRequired<LiveStacks>();
109       AU.addRequired<MachineBlockFrequencyInfo>();
110       AU.addPreserved<MachineBlockFrequencyInfo>();
111       AU.addPreservedID(MachineDominatorsID);
112       MachineFunctionPass::getAnalysisUsage(AU);
113     }
114 
115     bool runOnMachineFunction(MachineFunction &MF) override;
116 
117   private:
118     void InitializeSlots();
119     void ScanForSpillSlotRefs(MachineFunction &MF);
120     bool OverlapWithAssignments(LiveInterval *li, int Color) const;
121     int ColorSlot(LiveInterval *li);
122     bool ColorSlots(MachineFunction &MF);
123     void RewriteInstruction(MachineInstr &MI, SmallVectorImpl<int> &SlotMapping,
124                             MachineFunction &MF);
125     bool RemoveDeadStores(MachineBasicBlock* MBB);
126   };
127 
128 } // end anonymous namespace
129 
130 char StackSlotColoring::ID = 0;
131 
132 char &llvm::StackSlotColoringID = StackSlotColoring::ID;
133 
134 INITIALIZE_PASS_BEGIN(StackSlotColoring, DEBUG_TYPE,
135                 "Stack Slot Coloring", false, false)
136 INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
137 INITIALIZE_PASS_DEPENDENCY(LiveStacks)
138 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
139 INITIALIZE_PASS_END(StackSlotColoring, DEBUG_TYPE,
140                 "Stack Slot Coloring", false, false)
141 
142 namespace {
143 
144 // IntervalSorter - Comparison predicate that sort live intervals by
145 // their weight.
146 struct IntervalSorter {
147   bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
148     return LHS->weight() > RHS->weight();
149   }
150 };
151 
152 } // end anonymous namespace
153 
154 /// ScanForSpillSlotRefs - Scan all the machine instructions for spill slot
155 /// references and update spill slot weights.
156 void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) {
157   SSRefs.resize(MFI->getObjectIndexEnd());
158 
159   // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
160   for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
161        MBBI != E; ++MBBI) {
162     MachineBasicBlock *MBB = &*MBBI;
163     for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
164          MII != EE; ++MII) {
165       MachineInstr &MI = *MII;
166       for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
167         MachineOperand &MO = MI.getOperand(i);
168         if (!MO.isFI())
169           continue;
170         int FI = MO.getIndex();
171         if (FI < 0)
172           continue;
173         if (!LS->hasInterval(FI))
174           continue;
175         LiveInterval &li = LS->getInterval(FI);
176         if (!MI.isDebugValue())
177           li.incrementWeight(
178               LiveIntervals::getSpillWeight(false, true, MBFI, MI));
179       }
180       for (MachineInstr::mmo_iterator MMOI = MI.memoperands_begin(),
181                                       EE = MI.memoperands_end();
182            MMOI != EE; ++MMOI) {
183         MachineMemOperand *MMO = *MMOI;
184         if (const FixedStackPseudoSourceValue *FSV =
185             dyn_cast_or_null<FixedStackPseudoSourceValue>(
186                 MMO->getPseudoValue())) {
187           int FI = FSV->getFrameIndex();
188           if (FI >= 0)
189             SSRefs[FI].push_back(MMO);
190         }
191       }
192     }
193   }
194 }
195 
196 /// InitializeSlots - Process all spill stack slot liveintervals and add them
197 /// to a sorted (by weight) list.
198 void StackSlotColoring::InitializeSlots() {
199   int LastFI = MFI->getObjectIndexEnd();
200 
201   // There is always at least one stack ID.
202   AllColors.resize(1);
203   UsedColors.resize(1);
204 
205   OrigAlignments.resize(LastFI);
206   OrigSizes.resize(LastFI);
207   AllColors[0].resize(LastFI);
208   UsedColors[0].resize(LastFI);
209   Assignments.resize(LastFI);
210 
211   using Pair = std::iterator_traits<LiveStacks::iterator>::value_type;
212 
213   SmallVector<Pair *, 16> Intervals;
214 
215   Intervals.reserve(LS->getNumIntervals());
216   for (auto &I : *LS)
217     Intervals.push_back(&I);
218   llvm::sort(Intervals,
219              [](Pair *LHS, Pair *RHS) { return LHS->first < RHS->first; });
220 
221   // Gather all spill slots into a list.
222   LLVM_DEBUG(dbgs() << "Spill slot intervals:\n");
223   for (auto *I : Intervals) {
224     LiveInterval &li = I->second;
225     LLVM_DEBUG(li.dump());
226     int FI = Register::stackSlot2Index(li.reg());
227     if (MFI->isDeadObjectIndex(FI))
228       continue;
229 
230     SSIntervals.push_back(&li);
231     OrigAlignments[FI] = MFI->getObjectAlign(FI);
232     OrigSizes[FI]      = MFI->getObjectSize(FI);
233 
234     auto StackID = MFI->getStackID(FI);
235     if (StackID != 0) {
236       AllColors.resize(StackID + 1);
237       UsedColors.resize(StackID + 1);
238       AllColors[StackID].resize(LastFI);
239       UsedColors[StackID].resize(LastFI);
240     }
241 
242     AllColors[StackID].set(FI);
243   }
244   LLVM_DEBUG(dbgs() << '\n');
245 
246   // Sort them by weight.
247   llvm::stable_sort(SSIntervals, IntervalSorter());
248 
249   NextColors.resize(AllColors.size());
250 
251   // Get first "color".
252   for (unsigned I = 0, E = AllColors.size(); I != E; ++I)
253     NextColors[I] = AllColors[I].find_first();
254 }
255 
256 /// OverlapWithAssignments - Return true if LiveInterval overlaps with any
257 /// LiveIntervals that have already been assigned to the specified color.
258 bool
259 StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
260   const SmallVectorImpl<LiveInterval *> &OtherLIs = Assignments[Color];
261   for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
262     LiveInterval *OtherLI = OtherLIs[i];
263     if (OtherLI->overlaps(*li))
264       return true;
265   }
266   return false;
267 }
268 
269 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
270 int StackSlotColoring::ColorSlot(LiveInterval *li) {
271   int Color = -1;
272   bool Share = false;
273   int FI = Register::stackSlot2Index(li->reg());
274   uint8_t StackID = MFI->getStackID(FI);
275 
276   if (!DisableSharing) {
277 
278     // Check if it's possible to reuse any of the used colors.
279     Color = UsedColors[StackID].find_first();
280     while (Color != -1) {
281       if (!OverlapWithAssignments(li, Color)) {
282         Share = true;
283         ++NumEliminated;
284         break;
285       }
286       Color = UsedColors[StackID].find_next(Color);
287     }
288   }
289 
290   if (Color != -1 && MFI->getStackID(Color) != MFI->getStackID(FI)) {
291     LLVM_DEBUG(dbgs() << "cannot share FIs with different stack IDs\n");
292     Share = false;
293   }
294 
295   // Assign it to the first available color (assumed to be the best) if it's
296   // not possible to share a used color with other objects.
297   if (!Share) {
298     assert(NextColors[StackID] != -1 && "No more spill slots?");
299     Color = NextColors[StackID];
300     UsedColors[StackID].set(Color);
301     NextColors[StackID] = AllColors[StackID].find_next(NextColors[StackID]);
302   }
303 
304   assert(MFI->getStackID(Color) == MFI->getStackID(FI));
305 
306   // Record the assignment.
307   Assignments[Color].push_back(li);
308   LLVM_DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n");
309 
310   // Change size and alignment of the allocated slot. If there are multiple
311   // objects sharing the same slot, then make sure the size and alignment
312   // are large enough for all.
313   Align Alignment = OrigAlignments[FI];
314   if (!Share || Alignment > MFI->getObjectAlign(Color))
315     MFI->setObjectAlignment(Color, Alignment);
316   int64_t Size = OrigSizes[FI];
317   if (!Share || Size > MFI->getObjectSize(Color))
318     MFI->setObjectSize(Color, Size);
319   return Color;
320 }
321 
322 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine
323 /// operands in the function.
324 bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
325   unsigned NumObjs = MFI->getObjectIndexEnd();
326   SmallVector<int, 16> SlotMapping(NumObjs, -1);
327   SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
328   SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
329   BitVector UsedColors(NumObjs);
330 
331   LLVM_DEBUG(dbgs() << "Color spill slot intervals:\n");
332   bool Changed = false;
333   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
334     LiveInterval *li = SSIntervals[i];
335     int SS = Register::stackSlot2Index(li->reg());
336     int NewSS = ColorSlot(li);
337     assert(NewSS >= 0 && "Stack coloring failed?");
338     SlotMapping[SS] = NewSS;
339     RevMap[NewSS].push_back(SS);
340     SlotWeights[NewSS] += li->weight();
341     UsedColors.set(NewSS);
342     Changed |= (SS != NewSS);
343   }
344 
345   LLVM_DEBUG(dbgs() << "\nSpill slots after coloring:\n");
346   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
347     LiveInterval *li = SSIntervals[i];
348     int SS = Register::stackSlot2Index(li->reg());
349     li->setWeight(SlotWeights[SS]);
350   }
351   // Sort them by new weight.
352   llvm::stable_sort(SSIntervals, IntervalSorter());
353 
354 #ifndef NDEBUG
355   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i)
356     LLVM_DEBUG(SSIntervals[i]->dump());
357   LLVM_DEBUG(dbgs() << '\n');
358 #endif
359 
360   if (!Changed)
361     return false;
362 
363   // Rewrite all MachineMemOperands.
364   for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
365     int NewFI = SlotMapping[SS];
366     if (NewFI == -1 || (NewFI == (int)SS))
367       continue;
368 
369     const PseudoSourceValue *NewSV = MF.getPSVManager().getFixedStack(NewFI);
370     SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS];
371     for (unsigned i = 0, e = RefMMOs.size(); i != e; ++i)
372       RefMMOs[i]->setValue(NewSV);
373   }
374 
375   // Rewrite all MO_FrameIndex operands.  Look for dead stores.
376   for (MachineBasicBlock &MBB : MF) {
377     for (MachineInstr &MI : MBB)
378       RewriteInstruction(MI, SlotMapping, MF);
379     RemoveDeadStores(&MBB);
380   }
381 
382   // Delete unused stack slots.
383   for (int StackID = 0, E = AllColors.size(); StackID != E; ++StackID) {
384     int NextColor = NextColors[StackID];
385     while (NextColor != -1) {
386       LLVM_DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n");
387       MFI->RemoveStackObject(NextColor);
388       NextColor = AllColors[StackID].find_next(NextColor);
389     }
390   }
391 
392   return true;
393 }
394 
395 /// RewriteInstruction - Rewrite specified instruction by replacing references
396 /// to old frame index with new one.
397 void StackSlotColoring::RewriteInstruction(MachineInstr &MI,
398                                            SmallVectorImpl<int> &SlotMapping,
399                                            MachineFunction &MF) {
400   // Update the operands.
401   for (unsigned i = 0, ee = MI.getNumOperands(); i != ee; ++i) {
402     MachineOperand &MO = MI.getOperand(i);
403     if (!MO.isFI())
404       continue;
405     int OldFI = MO.getIndex();
406     if (OldFI < 0)
407       continue;
408     int NewFI = SlotMapping[OldFI];
409     if (NewFI == -1 || NewFI == OldFI)
410       continue;
411 
412     assert(MFI->getStackID(OldFI) == MFI->getStackID(NewFI));
413     MO.setIndex(NewFI);
414   }
415 
416   // The MachineMemOperands have already been updated.
417 }
418 
419 /// RemoveDeadStores - Scan through a basic block and look for loads followed
420 /// by stores.  If they're both using the same stack slot, then the store is
421 /// definitely dead.  This could obviously be much more aggressive (consider
422 /// pairs with instructions between them), but such extensions might have a
423 /// considerable compile time impact.
424 bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
425   // FIXME: This could be much more aggressive, but we need to investigate
426   // the compile time impact of doing so.
427   bool changed = false;
428 
429   SmallVector<MachineInstr*, 4> toErase;
430 
431   for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
432        I != E; ++I) {
433     if (DCELimit != -1 && (int)NumDead >= DCELimit)
434       break;
435     int FirstSS, SecondSS;
436     if (TII->isStackSlotCopy(*I, FirstSS, SecondSS) && FirstSS == SecondSS &&
437         FirstSS != -1) {
438       ++NumDead;
439       changed = true;
440       toErase.push_back(&*I);
441       continue;
442     }
443 
444     MachineBasicBlock::iterator NextMI = std::next(I);
445     MachineBasicBlock::iterator ProbableLoadMI = I;
446 
447     unsigned LoadReg = 0;
448     unsigned StoreReg = 0;
449     unsigned LoadSize = 0;
450     unsigned StoreSize = 0;
451     if (!(LoadReg = TII->isLoadFromStackSlot(*I, FirstSS, LoadSize)))
452       continue;
453     // Skip the ...pseudo debugging... instructions between a load and store.
454     while ((NextMI != E) && NextMI->isDebugInstr()) {
455       ++NextMI;
456       ++I;
457     }
458     if (NextMI == E) continue;
459     if (!(StoreReg = TII->isStoreToStackSlot(*NextMI, SecondSS, StoreSize)))
460       continue;
461     if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1 ||
462         LoadSize != StoreSize)
463       continue;
464 
465     ++NumDead;
466     changed = true;
467 
468     if (NextMI->findRegisterUseOperandIdx(LoadReg, true, nullptr) != -1) {
469       ++NumDead;
470       toErase.push_back(&*ProbableLoadMI);
471     }
472 
473     toErase.push_back(&*NextMI);
474     ++I;
475   }
476 
477   for (SmallVectorImpl<MachineInstr *>::iterator I = toErase.begin(),
478        E = toErase.end(); I != E; ++I)
479     (*I)->eraseFromParent();
480 
481   return changed;
482 }
483 
484 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
485   LLVM_DEBUG({
486     dbgs() << "********** Stack Slot Coloring **********\n"
487            << "********** Function: " << MF.getName() << '\n';
488   });
489 
490   if (skipFunction(MF.getFunction()))
491     return false;
492 
493   MFI = &MF.getFrameInfo();
494   TII = MF.getSubtarget().getInstrInfo();
495   LS = &getAnalysis<LiveStacks>();
496   MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
497 
498   bool Changed = false;
499 
500   unsigned NumSlots = LS->getNumIntervals();
501   if (NumSlots == 0)
502     // Nothing to do!
503     return false;
504 
505   // If there are calls to setjmp or sigsetjmp, don't perform stack slot
506   // coloring. The stack could be modified before the longjmp is executed,
507   // resulting in the wrong value being used afterwards. (See
508   // <rdar://problem/8007500>.)
509   if (MF.exposesReturnsTwice())
510     return false;
511 
512   // Gather spill slot references
513   ScanForSpillSlotRefs(MF);
514   InitializeSlots();
515   Changed = ColorSlots(MF);
516 
517   for (int &Next : NextColors)
518     Next = -1;
519 
520   SSIntervals.clear();
521   for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
522     SSRefs[i].clear();
523   SSRefs.clear();
524   OrigAlignments.clear();
525   OrigSizes.clear();
526   AllColors.clear();
527   UsedColors.clear();
528   for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
529     Assignments[i].clear();
530   Assignments.clear();
531 
532   return Changed;
533 }
534