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