1 //===- StackSlotColoring.cpp - Stack slot coloring pass. ------------------===//
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
7 //===----------------------------------------------------------------------===//
9 // This file implements the stack slot coloring pass.
11 //===----------------------------------------------------------------------===//
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"
47 #define DEBUG_TYPE "stack-slot-coloring"
50 DisableSharing("no-stack-slot-sharing",
51 cl::init(false), cl::Hidden,
52 cl::desc("Suppress slot sharing during stack coloring"));
54 static cl::opt<int> DCELimit("ssc-dce-limit", cl::init(-1), cl::Hidden);
56 STATISTIC(NumEliminated, "Number of stack slots eliminated due to coloring");
57 STATISTIC(NumDead, "Number of trivially dead stack accesses eliminated");
61 class StackSlotColoring : public MachineFunctionPass {
63 MachineFrameInfo *MFI;
64 const TargetInstrInfo *TII;
65 const MachineBlockFrequencyInfo *MBFI;
67 // SSIntervals - Spill slot intervals.
68 std::vector<LiveInterval*> SSIntervals;
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;
76 // OrigAlignments - Alignments of stack objects before coloring.
77 SmallVector<unsigned, 16> OrigAlignments;
79 // OrigSizes - Sizess of stack objects before coloring.
80 SmallVector<unsigned, 16> OrigSizes;
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;
88 // NextColor - Next "color" that's not yet used. This is per stack ID.
89 SmallVector<int, 2> NextColors = { -1 };
91 // UsedColors - "Colors" that have been assigned. This is per stack ID
92 SmallVector<BitVector, 2> UsedColors;
94 // Assignments - Color to intervals mapping.
95 SmallVector<SmallVector<LiveInterval*,4>, 16> Assignments;
98 static char ID; // Pass identification
100 StackSlotColoring() : MachineFunctionPass(ID) {
101 initializeStackSlotColoringPass(*PassRegistry::getPassRegistry());
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);
115 bool runOnMachineFunction(MachineFunction &MF) override;
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);
128 } // end anonymous namespace
130 char StackSlotColoring::ID = 0;
132 char &llvm::StackSlotColoringID = StackSlotColoring::ID;
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)
144 // IntervalSorter - Comparison predicate that sort live intervals by
146 struct IntervalSorter {
147 bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
148 return LHS->weight > RHS->weight;
152 } // end anonymous namespace
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());
159 // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
160 for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
162 MachineBasicBlock *MBB = &*MBBI;
163 for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
165 MachineInstr &MI = *MII;
166 for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
167 MachineOperand &MO = MI.getOperand(i);
170 int FI = MO.getIndex();
173 if (!LS->hasInterval(FI))
175 LiveInterval &li = LS->getInterval(FI);
176 if (!MI.isDebugValue())
177 li.weight += LiveIntervals::getSpillWeight(false, true, MBFI, MI);
179 for (MachineInstr::mmo_iterator MMOI = MI.memoperands_begin(),
180 EE = MI.memoperands_end();
181 MMOI != EE; ++MMOI) {
182 MachineMemOperand *MMO = *MMOI;
183 if (const FixedStackPseudoSourceValue *FSV =
184 dyn_cast_or_null<FixedStackPseudoSourceValue>(
185 MMO->getPseudoValue())) {
186 int FI = FSV->getFrameIndex();
188 SSRefs[FI].push_back(MMO);
195 /// InitializeSlots - Process all spill stack slot liveintervals and add them
196 /// to a sorted (by weight) list.
197 void StackSlotColoring::InitializeSlots() {
198 int LastFI = MFI->getObjectIndexEnd();
200 // There is always at least one stack ID.
202 UsedColors.resize(1);
204 OrigAlignments.resize(LastFI);
205 OrigSizes.resize(LastFI);
206 AllColors[0].resize(LastFI);
207 UsedColors[0].resize(LastFI);
208 Assignments.resize(LastFI);
210 using Pair = std::iterator_traits<LiveStacks::iterator>::value_type;
212 SmallVector<Pair *, 16> Intervals;
214 Intervals.reserve(LS->getNumIntervals());
216 Intervals.push_back(&I);
217 llvm::sort(Intervals,
218 [](Pair *LHS, Pair *RHS) { return LHS->first < RHS->first; });
220 // Gather all spill slots into a list.
221 LLVM_DEBUG(dbgs() << "Spill slot intervals:\n");
222 for (auto *I : Intervals) {
223 LiveInterval &li = I->second;
224 LLVM_DEBUG(li.dump());
225 int FI = Register::stackSlot2Index(li.reg);
226 if (MFI->isDeadObjectIndex(FI))
229 SSIntervals.push_back(&li);
230 OrigAlignments[FI] = MFI->getObjectAlignment(FI);
231 OrigSizes[FI] = MFI->getObjectSize(FI);
233 auto StackID = MFI->getStackID(FI);
235 AllColors.resize(StackID + 1);
236 UsedColors.resize(StackID + 1);
237 AllColors[StackID].resize(LastFI);
238 UsedColors[StackID].resize(LastFI);
241 AllColors[StackID].set(FI);
243 LLVM_DEBUG(dbgs() << '\n');
245 // Sort them by weight.
246 llvm::stable_sort(SSIntervals, IntervalSorter());
248 NextColors.resize(AllColors.size());
250 // Get first "color".
251 for (unsigned I = 0, E = AllColors.size(); I != E; ++I)
252 NextColors[I] = AllColors[I].find_first();
255 /// OverlapWithAssignments - Return true if LiveInterval overlaps with any
256 /// LiveIntervals that have already been assigned to the specified color.
258 StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
259 const SmallVectorImpl<LiveInterval *> &OtherLIs = Assignments[Color];
260 for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
261 LiveInterval *OtherLI = OtherLIs[i];
262 if (OtherLI->overlaps(*li))
268 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
269 int StackSlotColoring::ColorSlot(LiveInterval *li) {
272 int FI = Register::stackSlot2Index(li->reg);
273 uint8_t StackID = MFI->getStackID(FI);
275 if (!DisableSharing) {
277 // Check if it's possible to reuse any of the used colors.
278 Color = UsedColors[StackID].find_first();
279 while (Color != -1) {
280 if (!OverlapWithAssignments(li, Color)) {
285 Color = UsedColors[StackID].find_next(Color);
289 if (Color != -1 && MFI->getStackID(Color) != MFI->getStackID(FI)) {
290 LLVM_DEBUG(dbgs() << "cannot share FIs with different stack IDs\n");
294 // Assign it to the first available color (assumed to be the best) if it's
295 // not possible to share a used color with other objects.
297 assert(NextColors[StackID] != -1 && "No more spill slots?");
298 Color = NextColors[StackID];
299 UsedColors[StackID].set(Color);
300 NextColors[StackID] = AllColors[StackID].find_next(NextColors[StackID]);
303 assert(MFI->getStackID(Color) == MFI->getStackID(FI));
305 // Record the assignment.
306 Assignments[Color].push_back(li);
307 LLVM_DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n");
309 // Change size and alignment of the allocated slot. If there are multiple
310 // objects sharing the same slot, then make sure the size and alignment
311 // are large enough for all.
312 unsigned Align = OrigAlignments[FI];
313 if (!Share || Align > MFI->getObjectAlignment(Color))
314 MFI->setObjectAlignment(Color, Align);
315 int64_t Size = OrigSizes[FI];
316 if (!Share || Size > MFI->getObjectSize(Color))
317 MFI->setObjectSize(Color, Size);
321 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine
322 /// operands in the function.
323 bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
324 unsigned NumObjs = MFI->getObjectIndexEnd();
325 SmallVector<int, 16> SlotMapping(NumObjs, -1);
326 SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
327 SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
328 BitVector UsedColors(NumObjs);
330 LLVM_DEBUG(dbgs() << "Color spill slot intervals:\n");
331 bool Changed = false;
332 for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
333 LiveInterval *li = SSIntervals[i];
334 int SS = Register::stackSlot2Index(li->reg);
335 int NewSS = ColorSlot(li);
336 assert(NewSS >= 0 && "Stack coloring failed?");
337 SlotMapping[SS] = NewSS;
338 RevMap[NewSS].push_back(SS);
339 SlotWeights[NewSS] += li->weight;
340 UsedColors.set(NewSS);
341 Changed |= (SS != NewSS);
344 LLVM_DEBUG(dbgs() << "\nSpill slots after coloring:\n");
345 for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
346 LiveInterval *li = SSIntervals[i];
347 int SS = Register::stackSlot2Index(li->reg);
348 li->weight = SlotWeights[SS];
350 // Sort them by new weight.
351 llvm::stable_sort(SSIntervals, IntervalSorter());
354 for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i)
355 LLVM_DEBUG(SSIntervals[i]->dump());
356 LLVM_DEBUG(dbgs() << '\n');
362 // Rewrite all MachineMemOperands.
363 for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
364 int NewFI = SlotMapping[SS];
365 if (NewFI == -1 || (NewFI == (int)SS))
368 const PseudoSourceValue *NewSV = MF.getPSVManager().getFixedStack(NewFI);
369 SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS];
370 for (unsigned i = 0, e = RefMMOs.size(); i != e; ++i)
371 RefMMOs[i]->setValue(NewSV);
374 // Rewrite all MO_FrameIndex operands. Look for dead stores.
375 for (MachineBasicBlock &MBB : MF) {
376 for (MachineInstr &MI : MBB)
377 RewriteInstruction(MI, SlotMapping, MF);
378 RemoveDeadStores(&MBB);
381 // Delete unused stack slots.
382 for (int StackID = 0, E = AllColors.size(); StackID != E; ++StackID) {
383 int NextColor = NextColors[StackID];
384 while (NextColor != -1) {
385 LLVM_DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n");
386 MFI->RemoveStackObject(NextColor);
387 NextColor = AllColors[StackID].find_next(NextColor);
394 /// RewriteInstruction - Rewrite specified instruction by replacing references
395 /// to old frame index with new one.
396 void StackSlotColoring::RewriteInstruction(MachineInstr &MI,
397 SmallVectorImpl<int> &SlotMapping,
398 MachineFunction &MF) {
399 // Update the operands.
400 for (unsigned i = 0, ee = MI.getNumOperands(); i != ee; ++i) {
401 MachineOperand &MO = MI.getOperand(i);
404 int OldFI = MO.getIndex();
407 int NewFI = SlotMapping[OldFI];
408 if (NewFI == -1 || NewFI == OldFI)
411 assert(MFI->getStackID(OldFI) == MFI->getStackID(NewFI));
415 // The MachineMemOperands have already been updated.
418 /// RemoveDeadStores - Scan through a basic block and look for loads followed
419 /// by stores. If they're both using the same stack slot, then the store is
420 /// definitely dead. This could obviously be much more aggressive (consider
421 /// pairs with instructions between them), but such extensions might have a
422 /// considerable compile time impact.
423 bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
424 // FIXME: This could be much more aggressive, but we need to investigate
425 // the compile time impact of doing so.
426 bool changed = false;
428 SmallVector<MachineInstr*, 4> toErase;
430 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
432 if (DCELimit != -1 && (int)NumDead >= DCELimit)
434 int FirstSS, SecondSS;
435 if (TII->isStackSlotCopy(*I, FirstSS, SecondSS) && FirstSS == SecondSS &&
439 toErase.push_back(&*I);
443 MachineBasicBlock::iterator NextMI = std::next(I);
444 MachineBasicBlock::iterator ProbableLoadMI = I;
446 unsigned LoadReg = 0;
447 unsigned StoreReg = 0;
448 unsigned LoadSize = 0;
449 unsigned StoreSize = 0;
450 if (!(LoadReg = TII->isLoadFromStackSlot(*I, FirstSS, LoadSize)))
452 // Skip the ...pseudo debugging... instructions between a load and store.
453 while ((NextMI != E) && NextMI->isDebugInstr()) {
457 if (NextMI == E) continue;
458 if (!(StoreReg = TII->isStoreToStackSlot(*NextMI, SecondSS, StoreSize)))
460 if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1 ||
461 LoadSize != StoreSize)
467 if (NextMI->findRegisterUseOperandIdx(LoadReg, true, nullptr) != -1) {
469 toErase.push_back(&*ProbableLoadMI);
472 toErase.push_back(&*NextMI);
476 for (SmallVectorImpl<MachineInstr *>::iterator I = toErase.begin(),
477 E = toErase.end(); I != E; ++I)
478 (*I)->eraseFromParent();
483 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
485 dbgs() << "********** Stack Slot Coloring **********\n"
486 << "********** Function: " << MF.getName() << '\n';
489 if (skipFunction(MF.getFunction()))
492 MFI = &MF.getFrameInfo();
493 TII = MF.getSubtarget().getInstrInfo();
494 LS = &getAnalysis<LiveStacks>();
495 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
497 bool Changed = false;
499 unsigned NumSlots = LS->getNumIntervals();
504 // If there are calls to setjmp or sigsetjmp, don't perform stack slot
505 // coloring. The stack could be modified before the longjmp is executed,
506 // resulting in the wrong value being used afterwards. (See
507 // <rdar://problem/8007500>.)
508 if (MF.exposesReturnsTwice())
511 // Gather spill slot references
512 ScanForSpillSlotRefs(MF);
514 Changed = ColorSlots(MF);
516 for (int &Next : NextColors)
520 for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
523 OrigAlignments.clear();
527 for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
528 Assignments[i].clear();