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/Pass.h"
34 #include "llvm/Support/Casting.h"
35 #include "llvm/Support/CommandLine.h"
36 #include "llvm/Support/Debug.h"
37 #include "llvm/Support/raw_ostream.h"
38 #include <algorithm>
39 #include <cassert>
40 #include <cstdint>
41 #include <iterator>
42 #include <vector>
43 
44 using namespace llvm;
45 
46 #define DEBUG_TYPE "stack-slot-coloring"
47 
48 static cl::opt<bool>
49 DisableSharing("no-stack-slot-sharing",
50              cl::init(false), cl::Hidden,
51              cl::desc("Suppress slot sharing during stack coloring"));
52 
53 static cl::opt<int> DCELimit("ssc-dce-limit", cl::init(-1), cl::Hidden);
54 
55 STATISTIC(NumEliminated, "Number of stack slots eliminated due to coloring");
56 STATISTIC(NumDead,       "Number of trivially dead stack accesses eliminated");
57 
58 namespace {
59 
60   class StackSlotColoring : public MachineFunctionPass {
61     LiveStacks* LS;
62     MachineFrameInfo *MFI;
63     const TargetInstrInfo  *TII;
64     const MachineBlockFrequencyInfo *MBFI;
65 
66     // SSIntervals - Spill slot intervals.
67     std::vector<LiveInterval*> SSIntervals;
68 
69     // SSRefs - Keep a list of MachineMemOperands for each spill slot.
70     // MachineMemOperands can be shared between instructions, so we need
71     // to be careful that renames like [FI0, FI1] -> [FI1, FI2] do not
72     // become FI0 -> FI1 -> FI2.
73     SmallVector<SmallVector<MachineMemOperand *, 8>, 16> SSRefs;
74 
75     // OrigAlignments - Alignments of stack objects before coloring.
76     SmallVector<unsigned, 16> OrigAlignments;
77 
78     // OrigSizes - Sizess of stack objects before coloring.
79     SmallVector<unsigned, 16> OrigSizes;
80 
81     // AllColors - If index is set, it's a spill slot, i.e. color.
82     // FIXME: This assumes PEI locate spill slot with smaller indices
83     // closest to stack pointer / frame pointer. Therefore, smaller
84     // index == better color. This is per stack ID.
85     SmallVector<BitVector, 2> AllColors;
86 
87     // NextColor - Next "color" that's not yet used. This is per stack ID.
88     SmallVector<int, 2> NextColors = { -1 };
89 
90     // UsedColors - "Colors" that have been assigned. This is per stack ID
91     SmallVector<BitVector, 2> UsedColors;
92 
93     // Assignments - Color to intervals mapping.
94     SmallVector<SmallVector<LiveInterval*,4>, 16> Assignments;
95 
96   public:
97     static char ID; // Pass identification
98 
99     StackSlotColoring() : MachineFunctionPass(ID) {
100       initializeStackSlotColoringPass(*PassRegistry::getPassRegistry());
101     }
102 
103     void getAnalysisUsage(AnalysisUsage &AU) const override {
104       AU.setPreservesCFG();
105       AU.addRequired<SlotIndexes>();
106       AU.addPreserved<SlotIndexes>();
107       AU.addRequired<LiveStacks>();
108       AU.addRequired<MachineBlockFrequencyInfo>();
109       AU.addPreserved<MachineBlockFrequencyInfo>();
110       AU.addPreservedID(MachineDominatorsID);
111       MachineFunctionPass::getAnalysisUsage(AU);
112     }
113 
114     bool runOnMachineFunction(MachineFunction &MF) override;
115 
116   private:
117     void InitializeSlots();
118     void ScanForSpillSlotRefs(MachineFunction &MF);
119     bool OverlapWithAssignments(LiveInterval *li, int Color) const;
120     int ColorSlot(LiveInterval *li);
121     bool ColorSlots(MachineFunction &MF);
122     void RewriteInstruction(MachineInstr &MI, SmallVectorImpl<int> &SlotMapping,
123                             MachineFunction &MF);
124     bool RemoveDeadStores(MachineBasicBlock* MBB);
125   };
126 
127 } // end anonymous namespace
128 
129 char StackSlotColoring::ID = 0;
130 
131 char &llvm::StackSlotColoringID = StackSlotColoring::ID;
132 
133 INITIALIZE_PASS_BEGIN(StackSlotColoring, DEBUG_TYPE,
134                 "Stack Slot Coloring", false, false)
135 INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
136 INITIALIZE_PASS_DEPENDENCY(LiveStacks)
137 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
138 INITIALIZE_PASS_END(StackSlotColoring, DEBUG_TYPE,
139                 "Stack Slot Coloring", false, false)
140 
141 namespace {
142 
143 // IntervalSorter - Comparison predicate that sort live intervals by
144 // their weight.
145 struct IntervalSorter {
146   bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
147     return LHS->weight > RHS->weight;
148   }
149 };
150 
151 } // end anonymous namespace
152 
153 /// ScanForSpillSlotRefs - Scan all the machine instructions for spill slot
154 /// references and update spill slot weights.
155 void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) {
156   SSRefs.resize(MFI->getObjectIndexEnd());
157 
158   // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
159   for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
160        MBBI != E; ++MBBI) {
161     MachineBasicBlock *MBB = &*MBBI;
162     for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
163          MII != EE; ++MII) {
164       MachineInstr &MI = *MII;
165       for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
166         MachineOperand &MO = MI.getOperand(i);
167         if (!MO.isFI())
168           continue;
169         int FI = MO.getIndex();
170         if (FI < 0)
171           continue;
172         if (!LS->hasInterval(FI))
173           continue;
174         LiveInterval &li = LS->getInterval(FI);
175         if (!MI.isDebugValue())
176           li.weight += LiveIntervals::getSpillWeight(false, true, MBFI, MI);
177       }
178       for (MachineInstr::mmo_iterator MMOI = MI.memoperands_begin(),
179                                       EE = MI.memoperands_end();
180            MMOI != EE; ++MMOI) {
181         MachineMemOperand *MMO = *MMOI;
182         if (const FixedStackPseudoSourceValue *FSV =
183             dyn_cast_or_null<FixedStackPseudoSourceValue>(
184                 MMO->getPseudoValue())) {
185           int FI = FSV->getFrameIndex();
186           if (FI >= 0)
187             SSRefs[FI].push_back(MMO);
188         }
189       }
190     }
191   }
192 }
193 
194 /// InitializeSlots - Process all spill stack slot liveintervals and add them
195 /// to a sorted (by weight) list.
196 void StackSlotColoring::InitializeSlots() {
197   int LastFI = MFI->getObjectIndexEnd();
198 
199   // There is always at least one stack ID.
200   AllColors.resize(1);
201   UsedColors.resize(1);
202 
203   OrigAlignments.resize(LastFI);
204   OrigSizes.resize(LastFI);
205   AllColors[0].resize(LastFI);
206   UsedColors[0].resize(LastFI);
207   Assignments.resize(LastFI);
208 
209   using Pair = std::iterator_traits<LiveStacks::iterator>::value_type;
210 
211   SmallVector<Pair *, 16> Intervals;
212 
213   Intervals.reserve(LS->getNumIntervals());
214   for (auto &I : *LS)
215     Intervals.push_back(&I);
216   llvm::sort(Intervals,
217              [](Pair *LHS, Pair *RHS) { return LHS->first < RHS->first; });
218 
219   // Gather all spill slots into a list.
220   LLVM_DEBUG(dbgs() << "Spill slot intervals:\n");
221   for (auto *I : Intervals) {
222     LiveInterval &li = I->second;
223     LLVM_DEBUG(li.dump());
224     int FI = Register::stackSlot2Index(li.reg);
225     if (MFI->isDeadObjectIndex(FI))
226       continue;
227 
228     SSIntervals.push_back(&li);
229     OrigAlignments[FI] = MFI->getObjectAlignment(FI);
230     OrigSizes[FI]      = MFI->getObjectSize(FI);
231 
232     auto StackID = MFI->getStackID(FI);
233     if (StackID != 0) {
234       AllColors.resize(StackID + 1);
235       UsedColors.resize(StackID + 1);
236       AllColors[StackID].resize(LastFI);
237       UsedColors[StackID].resize(LastFI);
238     }
239 
240     AllColors[StackID].set(FI);
241   }
242   LLVM_DEBUG(dbgs() << '\n');
243 
244   // Sort them by weight.
245   llvm::stable_sort(SSIntervals, IntervalSorter());
246 
247   NextColors.resize(AllColors.size());
248 
249   // Get first "color".
250   for (unsigned I = 0, E = AllColors.size(); I != E; ++I)
251     NextColors[I] = AllColors[I].find_first();
252 }
253 
254 /// OverlapWithAssignments - Return true if LiveInterval overlaps with any
255 /// LiveIntervals that have already been assigned to the specified color.
256 bool
257 StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
258   const SmallVectorImpl<LiveInterval *> &OtherLIs = Assignments[Color];
259   for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
260     LiveInterval *OtherLI = OtherLIs[i];
261     if (OtherLI->overlaps(*li))
262       return true;
263   }
264   return false;
265 }
266 
267 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
268 int StackSlotColoring::ColorSlot(LiveInterval *li) {
269   int Color = -1;
270   bool Share = false;
271   int FI = Register::stackSlot2Index(li->reg);
272   uint8_t StackID = MFI->getStackID(FI);
273 
274   if (!DisableSharing) {
275 
276     // Check if it's possible to reuse any of the used colors.
277     Color = UsedColors[StackID].find_first();
278     while (Color != -1) {
279       if (!OverlapWithAssignments(li, Color)) {
280         Share = true;
281         ++NumEliminated;
282         break;
283       }
284       Color = UsedColors[StackID].find_next(Color);
285     }
286   }
287 
288   if (Color != -1 && MFI->getStackID(Color) != MFI->getStackID(FI)) {
289     LLVM_DEBUG(dbgs() << "cannot share FIs with different stack IDs\n");
290     Share = false;
291   }
292 
293   // Assign it to the first available color (assumed to be the best) if it's
294   // not possible to share a used color with other objects.
295   if (!Share) {
296     assert(NextColors[StackID] != -1 && "No more spill slots?");
297     Color = NextColors[StackID];
298     UsedColors[StackID].set(Color);
299     NextColors[StackID] = AllColors[StackID].find_next(NextColors[StackID]);
300   }
301 
302   assert(MFI->getStackID(Color) == MFI->getStackID(FI));
303 
304   // Record the assignment.
305   Assignments[Color].push_back(li);
306   LLVM_DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n");
307 
308   // Change size and alignment of the allocated slot. If there are multiple
309   // objects sharing the same slot, then make sure the size and alignment
310   // are large enough for all.
311   unsigned Align = OrigAlignments[FI];
312   if (!Share || Align > MFI->getObjectAlignment(Color))
313     MFI->setObjectAlignment(Color, Align);
314   int64_t Size = OrigSizes[FI];
315   if (!Share || Size > MFI->getObjectSize(Color))
316     MFI->setObjectSize(Color, Size);
317   return Color;
318 }
319 
320 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine
321 /// operands in the function.
322 bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
323   unsigned NumObjs = MFI->getObjectIndexEnd();
324   SmallVector<int, 16> SlotMapping(NumObjs, -1);
325   SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
326   SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
327   BitVector UsedColors(NumObjs);
328 
329   LLVM_DEBUG(dbgs() << "Color spill slot intervals:\n");
330   bool Changed = false;
331   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
332     LiveInterval *li = SSIntervals[i];
333     int SS = Register::stackSlot2Index(li->reg);
334     int NewSS = ColorSlot(li);
335     assert(NewSS >= 0 && "Stack coloring failed?");
336     SlotMapping[SS] = NewSS;
337     RevMap[NewSS].push_back(SS);
338     SlotWeights[NewSS] += li->weight;
339     UsedColors.set(NewSS);
340     Changed |= (SS != NewSS);
341   }
342 
343   LLVM_DEBUG(dbgs() << "\nSpill slots after coloring:\n");
344   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
345     LiveInterval *li = SSIntervals[i];
346     int SS = Register::stackSlot2Index(li->reg);
347     li->weight = SlotWeights[SS];
348   }
349   // Sort them by new weight.
350   llvm::stable_sort(SSIntervals, IntervalSorter());
351 
352 #ifndef NDEBUG
353   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i)
354     LLVM_DEBUG(SSIntervals[i]->dump());
355   LLVM_DEBUG(dbgs() << '\n');
356 #endif
357 
358   if (!Changed)
359     return false;
360 
361   // Rewrite all MachineMemOperands.
362   for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
363     int NewFI = SlotMapping[SS];
364     if (NewFI == -1 || (NewFI == (int)SS))
365       continue;
366 
367     const PseudoSourceValue *NewSV = MF.getPSVManager().getFixedStack(NewFI);
368     SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS];
369     for (unsigned i = 0, e = RefMMOs.size(); i != e; ++i)
370       RefMMOs[i]->setValue(NewSV);
371   }
372 
373   // Rewrite all MO_FrameIndex operands.  Look for dead stores.
374   for (MachineBasicBlock &MBB : MF) {
375     for (MachineInstr &MI : MBB)
376       RewriteInstruction(MI, SlotMapping, MF);
377     RemoveDeadStores(&MBB);
378   }
379 
380   // Delete unused stack slots.
381   for (int StackID = 0, E = AllColors.size(); StackID != E; ++StackID) {
382     int NextColor = NextColors[StackID];
383     while (NextColor != -1) {
384       LLVM_DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n");
385       MFI->RemoveStackObject(NextColor);
386       NextColor = AllColors[StackID].find_next(NextColor);
387     }
388   }
389 
390   return true;
391 }
392 
393 /// RewriteInstruction - Rewrite specified instruction by replacing references
394 /// to old frame index with new one.
395 void StackSlotColoring::RewriteInstruction(MachineInstr &MI,
396                                            SmallVectorImpl<int> &SlotMapping,
397                                            MachineFunction &MF) {
398   // Update the operands.
399   for (unsigned i = 0, ee = MI.getNumOperands(); i != ee; ++i) {
400     MachineOperand &MO = MI.getOperand(i);
401     if (!MO.isFI())
402       continue;
403     int OldFI = MO.getIndex();
404     if (OldFI < 0)
405       continue;
406     int NewFI = SlotMapping[OldFI];
407     if (NewFI == -1 || NewFI == OldFI)
408       continue;
409 
410     assert(MFI->getStackID(OldFI) == MFI->getStackID(NewFI));
411     MO.setIndex(NewFI);
412   }
413 
414   // The MachineMemOperands have already been updated.
415 }
416 
417 /// RemoveDeadStores - Scan through a basic block and look for loads followed
418 /// by stores.  If they're both using the same stack slot, then the store is
419 /// definitely dead.  This could obviously be much more aggressive (consider
420 /// pairs with instructions between them), but such extensions might have a
421 /// considerable compile time impact.
422 bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
423   // FIXME: This could be much more aggressive, but we need to investigate
424   // the compile time impact of doing so.
425   bool changed = false;
426 
427   SmallVector<MachineInstr*, 4> toErase;
428 
429   for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
430        I != E; ++I) {
431     if (DCELimit != -1 && (int)NumDead >= DCELimit)
432       break;
433     int FirstSS, SecondSS;
434     if (TII->isStackSlotCopy(*I, FirstSS, SecondSS) && FirstSS == SecondSS &&
435         FirstSS != -1) {
436       ++NumDead;
437       changed = true;
438       toErase.push_back(&*I);
439       continue;
440     }
441 
442     MachineBasicBlock::iterator NextMI = std::next(I);
443     MachineBasicBlock::iterator ProbableLoadMI = I;
444 
445     unsigned LoadReg = 0;
446     unsigned StoreReg = 0;
447     unsigned LoadSize = 0;
448     unsigned StoreSize = 0;
449     if (!(LoadReg = TII->isLoadFromStackSlot(*I, FirstSS, LoadSize)))
450       continue;
451     // Skip the ...pseudo debugging... instructions between a load and store.
452     while ((NextMI != E) && NextMI->isDebugInstr()) {
453       ++NextMI;
454       ++I;
455     }
456     if (NextMI == E) continue;
457     if (!(StoreReg = TII->isStoreToStackSlot(*NextMI, SecondSS, StoreSize)))
458       continue;
459     if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1 ||
460         LoadSize != StoreSize)
461       continue;
462 
463     ++NumDead;
464     changed = true;
465 
466     if (NextMI->findRegisterUseOperandIdx(LoadReg, true, nullptr) != -1) {
467       ++NumDead;
468       toErase.push_back(&*ProbableLoadMI);
469     }
470 
471     toErase.push_back(&*NextMI);
472     ++I;
473   }
474 
475   for (SmallVectorImpl<MachineInstr *>::iterator I = toErase.begin(),
476        E = toErase.end(); I != E; ++I)
477     (*I)->eraseFromParent();
478 
479   return changed;
480 }
481 
482 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
483   LLVM_DEBUG({
484     dbgs() << "********** Stack Slot Coloring **********\n"
485            << "********** Function: " << MF.getName() << '\n';
486   });
487 
488   if (skipFunction(MF.getFunction()))
489     return false;
490 
491   MFI = &MF.getFrameInfo();
492   TII = MF.getSubtarget().getInstrInfo();
493   LS = &getAnalysis<LiveStacks>();
494   MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
495 
496   bool Changed = false;
497 
498   unsigned NumSlots = LS->getNumIntervals();
499   if (NumSlots == 0)
500     // Nothing to do!
501     return false;
502 
503   // If there are calls to setjmp or sigsetjmp, don't perform stack slot
504   // coloring. The stack could be modified before the longjmp is executed,
505   // resulting in the wrong value being used afterwards. (See
506   // <rdar://problem/8007500>.)
507   if (MF.exposesReturnsTwice())
508     return false;
509 
510   // Gather spill slot references
511   ScanForSpillSlotRefs(MF);
512   InitializeSlots();
513   Changed = ColorSlots(MF);
514 
515   for (int &Next : NextColors)
516     Next = -1;
517 
518   SSIntervals.clear();
519   for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
520     SSRefs[i].clear();
521   SSRefs.clear();
522   OrigAlignments.clear();
523   OrigSizes.clear();
524   AllColors.clear();
525   UsedColors.clear();
526   for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
527     Assignments[i].clear();
528   Assignments.clear();
529 
530   return Changed;
531 }
532