1//===- ShrinkWrap.cpp - Compute safe point for prolog/epilog insertion ----===//
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 pass looks for safe point where the prologue and epilogue can be
10// inserted.
11// The safe point for the prologue (resp. epilogue) is called Save
12// (resp. Restore).
13// A point is safe for prologue (resp. epilogue) if and only if
14// it 1) dominates (resp. post-dominates) all the frame related operations and
15// between 2) two executions of the Save (resp. Restore) point there is an
16// execution of the Restore (resp. Save) point.
17//
18// For instance, the following points are safe:
19// for (int i = 0; i < 10; ++i) {
20// Save
21// ...
22// Restore
23// }
24// Indeed, the execution looks like Save -> Restore -> Save -> Restore ...
25// And the following points are not:
26// for (int i = 0; i < 10; ++i) {
27// Save
28// ...
29// }
30// for (int i = 0; i < 10; ++i) {
31// ...
32// Restore
33// }
34// Indeed, the execution looks like Save -> Save -> ... -> Restore -> Restore.
35//
36// This pass also ensures that the safe points are 3) cheaper than the regular
37// entry and exits blocks.
38//
39// Property #1 is ensured via the use of MachineDominatorTree and
40// MachinePostDominatorTree.
41// Property #2 is ensured via property #1 and MachineLoopInfo, i.e., both
42// points must be in the same loop.
43// Property #3 is ensured via the MachineBlockFrequencyInfo.
44//
45// If this pass found points matching all these properties, then
46// MachineFrameInfo is updated with this information.
47//
48//===----------------------------------------------------------------------===//
49
50#include "llvm/CodeGen/ShrinkWrap.h"
51#include "llvm/ADT/BitVector.h"
52#include "llvm/ADT/PostOrderIterator.h"
53#include "llvm/ADT/SetVector.h"
54#include "llvm/ADT/SmallVector.h"
55#include "llvm/ADT/Statistic.h"
56#include "llvm/Analysis/CFG.h"
57#include "llvm/Analysis/ValueTracking.h"
58#include "llvm/CodeGen/MachineBasicBlock.h"
59#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
60#include "llvm/CodeGen/MachineDominators.h"
61#include "llvm/CodeGen/MachineFrameInfo.h"
62#include "llvm/CodeGen/MachineFunction.h"
63#include "llvm/CodeGen/MachineFunctionPass.h"
64#include "llvm/CodeGen/MachineInstr.h"
65#include "llvm/CodeGen/MachineLoopInfo.h"
66#include "llvm/CodeGen/MachineOperand.h"
67#include "llvm/CodeGen/MachineOptimizationRemarkEmitter.h"
68#include "llvm/CodeGen/MachinePostDominators.h"
69#include "llvm/CodeGen/RegisterClassInfo.h"
70#include "llvm/CodeGen/RegisterScavenging.h"
71#include "llvm/CodeGen/TargetFrameLowering.h"
72#include "llvm/CodeGen/TargetInstrInfo.h"
73#include "llvm/CodeGen/TargetLowering.h"
74#include "llvm/CodeGen/TargetRegisterInfo.h"
75#include "llvm/CodeGen/TargetSubtargetInfo.h"
76#include "llvm/IR/Attributes.h"
77#include "llvm/IR/Function.h"
78#include "llvm/InitializePasses.h"
79#include "llvm/MC/MCAsmInfo.h"
80#include "llvm/Pass.h"
81#include "llvm/Support/CommandLine.h"
82#include "llvm/Support/Debug.h"
83#include "llvm/Support/ErrorHandling.h"
84#include "llvm/Support/raw_ostream.h"
85#include "llvm/Target/TargetMachine.h"
86#include <cassert>
87#include <memory>
88
89using namespace llvm;
90
91#define DEBUG_TYPE "shrink-wrap"
92
93STATISTIC(NumFunc, "Number of functions");
94STATISTIC(NumCandidates, "Number of shrink-wrapping candidates");
95STATISTIC(NumCandidatesDropped,
96 "Number of shrink-wrapping candidates dropped because of frequency");
97
98static cl::opt<cl::boolOrDefault>
99EnableShrinkWrapOpt("enable-shrink-wrap", cl::Hidden,
100 cl::desc("enable the shrink-wrapping pass"));
101static cl::opt<bool> EnablePostShrinkWrapOpt(
102 "enable-shrink-wrap-region-split", cl::init(Val: true), cl::Hidden,
103 cl::desc("enable splitting of the restore block if possible"));
104
105namespace {
106
107/// Class to determine where the safe point to insert the
108/// prologue and epilogue are.
109/// Unlike the paper from Fred C. Chow, PLDI'88, that introduces the
110/// shrink-wrapping term for prologue/epilogue placement, this pass
111/// does not rely on expensive data-flow analysis. Instead we use the
112/// dominance properties and loop information to decide which point
113/// are safe for such insertion.
114class ShrinkWrapImpl {
115 /// Hold callee-saved information.
116 RegisterClassInfo RCI;
117 MachineDominatorTree *MDT = nullptr;
118 MachinePostDominatorTree *MPDT = nullptr;
119
120 /// Current safe point found for the prologue.
121 /// The prologue will be inserted before the first instruction
122 /// in this basic block.
123 MachineBasicBlock *Save = nullptr;
124
125 /// Current safe point found for the epilogue.
126 /// The epilogue will be inserted before the first terminator instruction
127 /// in this basic block.
128 MachineBasicBlock *Restore = nullptr;
129
130 /// Hold the information of the basic block frequency.
131 /// Use to check the profitability of the new points.
132 MachineBlockFrequencyInfo *MBFI = nullptr;
133
134 /// Hold the loop information. Used to determine if Save and Restore
135 /// are in the same loop.
136 MachineLoopInfo *MLI = nullptr;
137
138 // Emit remarks.
139 MachineOptimizationRemarkEmitter *ORE = nullptr;
140
141 /// Frequency of the Entry block.
142 BlockFrequency EntryFreq;
143
144 /// Current opcode for frame setup.
145 unsigned FrameSetupOpcode = ~0u;
146
147 /// Current opcode for frame destroy.
148 unsigned FrameDestroyOpcode = ~0u;
149
150 /// Stack pointer register, used by llvm.{savestack,restorestack}
151 Register SP;
152
153 /// Entry block.
154 const MachineBasicBlock *Entry = nullptr;
155
156 using SetOfRegs = SmallSetVector<unsigned, 16>;
157
158 /// Registers that need to be saved for the current function.
159 mutable SetOfRegs CurrentCSRs;
160
161 /// Current MachineFunction.
162 MachineFunction *MachineFunc = nullptr;
163
164 /// Is `true` for the block numbers where we assume possible stack accesses
165 /// or computation of stack-relative addresses on any CFG path including the
166 /// block itself. Is `false` for basic blocks where we can guarantee the
167 /// opposite. False positives won't lead to incorrect analysis results,
168 /// therefore this approach is fair.
169 BitVector StackAddressUsedBlockInfo;
170
171 /// Check if \p MI uses or defines a callee-saved register or
172 /// a frame index. If this is the case, this means \p MI must happen
173 /// after Save and before Restore.
174 bool useOrDefCSROrFI(const MachineInstr &MI, RegScavenger *RS,
175 bool StackAddressUsed) const;
176
177 const SetOfRegs &getCurrentCSRs(RegScavenger *RS) const {
178 if (CurrentCSRs.empty()) {
179 BitVector SavedRegs;
180 const TargetFrameLowering *TFI =
181 MachineFunc->getSubtarget().getFrameLowering();
182
183 TFI->determineCalleeSaves(MF&: *MachineFunc, SavedRegs, RS);
184
185 for (int Reg = SavedRegs.find_first(); Reg != -1;
186 Reg = SavedRegs.find_next(Prev: Reg))
187 CurrentCSRs.insert(X: (unsigned)Reg);
188 }
189 return CurrentCSRs;
190 }
191
192 /// Update the Save and Restore points such that \p MBB is in
193 /// the region that is dominated by Save and post-dominated by Restore
194 /// and Save and Restore still match the safe point definition.
195 /// Such point may not exist and Save and/or Restore may be null after
196 /// this call.
197 void updateSaveRestorePoints(MachineBasicBlock &MBB, RegScavenger *RS);
198
199 // Try to find safe point based on dominance and block frequency without
200 // any change in IR.
201 bool performShrinkWrapping(
202 const ReversePostOrderTraversal<MachineBasicBlock *> &RPOT,
203 RegScavenger *RS);
204
205 /// This function tries to split the restore point if doing so can shrink the
206 /// save point further. \return True if restore point is split.
207 bool postShrinkWrapping(bool HasCandidate, MachineFunction &MF,
208 RegScavenger *RS);
209
210 /// This function analyzes if the restore point can split to create a new
211 /// restore point. This function collects
212 /// 1. Any preds of current restore that are reachable by callee save/FI
213 /// blocks
214 /// - indicated by DirtyPreds
215 /// 2. Any preds of current restore that are not DirtyPreds - indicated by
216 /// CleanPreds
217 /// Both sets should be non-empty for considering restore point split.
218 bool checkIfRestoreSplittable(
219 const MachineBasicBlock *CurRestore,
220 const DenseSet<const MachineBasicBlock *> &ReachableByDirty,
221 SmallVectorImpl<MachineBasicBlock *> &DirtyPreds,
222 SmallVectorImpl<MachineBasicBlock *> &CleanPreds,
223 const TargetInstrInfo *TII, RegScavenger *RS);
224
225 /// Initialize the pass for \p MF.
226 void init(MachineFunction &MF) {
227 RCI.runOnMachineFunction(MF);
228 Save = nullptr;
229 Restore = nullptr;
230 EntryFreq = MBFI->getEntryFreq();
231 const TargetSubtargetInfo &Subtarget = MF.getSubtarget();
232 const TargetInstrInfo &TII = *Subtarget.getInstrInfo();
233 FrameSetupOpcode = TII.getCallFrameSetupOpcode();
234 FrameDestroyOpcode = TII.getCallFrameDestroyOpcode();
235 SP = Subtarget.getTargetLowering()->getStackPointerRegisterToSaveRestore();
236 Entry = &MF.front();
237 CurrentCSRs.clear();
238 MachineFunc = &MF;
239
240 ++NumFunc;
241 }
242
243 /// Check whether or not Save and Restore points are still interesting for
244 /// shrink-wrapping.
245 bool ArePointsInteresting() const { return Save != Entry && Save && Restore; }
246
247public:
248 ShrinkWrapImpl(MachineDominatorTree *MDT, MachinePostDominatorTree *MPDT,
249 MachineBlockFrequencyInfo *MBFI, MachineLoopInfo *MLI,
250 MachineOptimizationRemarkEmitter *ORE)
251 : MDT(MDT), MPDT(MPDT), MBFI(MBFI), MLI(MLI), ORE(ORE) {}
252
253 /// Check if shrink wrapping is enabled for this target and function.
254 static bool isShrinkWrapEnabled(const MachineFunction &MF);
255
256 bool run(MachineFunction &MF);
257};
258
259class ShrinkWrapLegacy : public MachineFunctionPass {
260public:
261 static char ID;
262
263 ShrinkWrapLegacy() : MachineFunctionPass(ID) {
264 initializeShrinkWrapLegacyPass(*PassRegistry::getPassRegistry());
265 }
266
267 void getAnalysisUsage(AnalysisUsage &AU) const override {
268 AU.setPreservesAll();
269 AU.addRequired<MachineBlockFrequencyInfoWrapperPass>();
270 AU.addRequired<MachineDominatorTreeWrapperPass>();
271 AU.addRequired<MachinePostDominatorTreeWrapperPass>();
272 AU.addRequired<MachineLoopInfoWrapperPass>();
273 AU.addRequired<MachineOptimizationRemarkEmitterPass>();
274 MachineFunctionPass::getAnalysisUsage(AU);
275 }
276
277 MachineFunctionProperties getRequiredProperties() const override {
278 return MachineFunctionProperties().setNoVRegs();
279 }
280
281 StringRef getPassName() const override { return "Shrink Wrapping analysis"; }
282
283 /// Perform the shrink-wrapping analysis and update
284 /// the MachineFrameInfo attached to \p MF with the results.
285 bool runOnMachineFunction(MachineFunction &MF) override;
286};
287
288} // end anonymous namespace
289
290char ShrinkWrapLegacy::ID = 0;
291
292char &llvm::ShrinkWrapID = ShrinkWrapLegacy::ID;
293
294INITIALIZE_PASS_BEGIN(ShrinkWrapLegacy, DEBUG_TYPE, "Shrink Wrap Pass", false,
295 false)
296INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfoWrapperPass)
297INITIALIZE_PASS_DEPENDENCY(MachineDominatorTreeWrapperPass)
298INITIALIZE_PASS_DEPENDENCY(MachinePostDominatorTreeWrapperPass)
299INITIALIZE_PASS_DEPENDENCY(MachineLoopInfoWrapperPass)
300INITIALIZE_PASS_DEPENDENCY(MachineOptimizationRemarkEmitterPass)
301INITIALIZE_PASS_END(ShrinkWrapLegacy, DEBUG_TYPE, "Shrink Wrap Pass", false,
302 false)
303
304bool ShrinkWrapImpl::useOrDefCSROrFI(const MachineInstr &MI, RegScavenger *RS,
305 bool StackAddressUsed) const {
306 /// Check if \p Op is known to access an address not on the function's stack .
307 /// At the moment, accesses where the underlying object is a global, function
308 /// argument, or jump table are considered non-stack accesses. Note that the
309 /// caller's stack may get accessed when passing an argument via the stack,
310 /// but not the stack of the current function.
311 ///
312 auto IsKnownNonStackPtr = [](MachineMemOperand *Op) {
313 if (Op->getValue()) {
314 const Value *UO = getUnderlyingObject(V: Op->getValue());
315 if (!UO)
316 return false;
317 if (auto *Arg = dyn_cast<Argument>(Val: UO))
318 return !Arg->hasPassPointeeByValueCopyAttr();
319 return isa<GlobalValue>(Val: UO);
320 }
321 if (const PseudoSourceValue *PSV = Op->getPseudoValue())
322 return PSV->isJumpTable();
323 return false;
324 };
325 // Load/store operations may access the stack indirectly when we previously
326 // computed an address to a stack location.
327 if (StackAddressUsed && MI.mayLoadOrStore() &&
328 (MI.isCall() || MI.hasUnmodeledSideEffects() || MI.memoperands_empty() ||
329 !all_of(Range: MI.memoperands(), P: IsKnownNonStackPtr)))
330 return true;
331
332 if (MI.getOpcode() == FrameSetupOpcode ||
333 MI.getOpcode() == FrameDestroyOpcode) {
334 LLVM_DEBUG(dbgs() << "Frame instruction: " << MI << '\n');
335 return true;
336 }
337 const MachineFunction *MF = MI.getParent()->getParent();
338 const TargetRegisterInfo *TRI = MF->getSubtarget().getRegisterInfo();
339 for (const MachineOperand &MO : MI.operands()) {
340 bool UseOrDefCSR = false;
341 if (MO.isReg()) {
342 // Ignore instructions like DBG_VALUE which don't read/def the register.
343 if (!MO.isDef() && !MO.readsReg())
344 continue;
345 Register PhysReg = MO.getReg();
346 if (!PhysReg)
347 continue;
348 assert(PhysReg.isPhysical() && "Unallocated register?!");
349 // The stack pointer is not normally described as a callee-saved register
350 // in calling convention definitions, so we need to watch for it
351 // separately. An SP mentioned by a call instruction, we can ignore,
352 // though, as it's harmless and we do not want to effectively disable tail
353 // calls by forcing the restore point to post-dominate them.
354 // PPC's LR is also not normally described as a callee-saved register in
355 // calling convention definitions, so we need to watch for it, too. An LR
356 // mentioned implicitly by a return (or "branch to link register")
357 // instruction we can ignore, otherwise we may pessimize shrinkwrapping.
358 // PPC's Frame pointer (FP) is also not described as a callee-saved
359 // register. Until the FP is assigned a Physical Register PPC's FP needs
360 // to be checked separately.
361 UseOrDefCSR = (!MI.isCall() && PhysReg == SP) ||
362 RCI.getLastCalleeSavedAlias(PhysReg) ||
363 (!MI.isReturn() &&
364 TRI->isNonallocatableRegisterCalleeSave(Reg: PhysReg)) ||
365 TRI->isVirtualFrameRegister(Reg: PhysReg);
366 } else if (MO.isRegMask()) {
367 // Check if this regmask clobbers any of the CSRs.
368 for (unsigned Reg : getCurrentCSRs(RS)) {
369 if (MO.clobbersPhysReg(PhysReg: Reg)) {
370 UseOrDefCSR = true;
371 break;
372 }
373 }
374 }
375 // Skip FrameIndex operands in DBG_VALUE instructions.
376 if (UseOrDefCSR || (MO.isFI() && !MI.isDebugValue())) {
377 LLVM_DEBUG(dbgs() << "Use or define CSR(" << UseOrDefCSR << ") or FI("
378 << MO.isFI() << "): " << MI << '\n');
379 return true;
380 }
381 }
382 return false;
383}
384
385/// Helper function to find the immediate (post) dominator.
386template <typename ListOfBBs, typename DominanceAnalysis>
387static MachineBasicBlock *FindIDom(MachineBasicBlock &Block, ListOfBBs BBs,
388 DominanceAnalysis &Dom, bool Strict = true) {
389 MachineBasicBlock *IDom = Dom.findNearestCommonDominator(iterator_range(BBs));
390 if (Strict && IDom == &Block)
391 return nullptr;
392 return IDom;
393}
394
395static bool isAnalyzableBB(const TargetInstrInfo &TII,
396 MachineBasicBlock &Entry) {
397 // Check if the block is analyzable.
398 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
399 SmallVector<MachineOperand, 4> Cond;
400 return !TII.analyzeBranch(MBB&: Entry, TBB, FBB, Cond);
401}
402
403/// Determines if any predecessor of MBB is on the path from block that has use
404/// or def of CSRs/FI to MBB.
405/// ReachableByDirty: All blocks reachable from block that has use or def of
406/// CSR/FI.
407static bool
408hasDirtyPred(const DenseSet<const MachineBasicBlock *> &ReachableByDirty,
409 const MachineBasicBlock &MBB) {
410 for (const MachineBasicBlock *PredBB : MBB.predecessors())
411 if (ReachableByDirty.count(V: PredBB))
412 return true;
413 return false;
414}
415
416/// Derives the list of all the basic blocks reachable from MBB.
417static void markAllReachable(DenseSet<const MachineBasicBlock *> &Visited,
418 const MachineBasicBlock &MBB) {
419 SmallVector<MachineBasicBlock *, 4> Worklist(MBB.successors());
420 Visited.insert(V: &MBB);
421 while (!Worklist.empty()) {
422 MachineBasicBlock *SuccMBB = Worklist.pop_back_val();
423 if (!Visited.insert(V: SuccMBB).second)
424 continue;
425 Worklist.append(in_start: SuccMBB->succ_begin(), in_end: SuccMBB->succ_end());
426 }
427}
428
429/// Collect blocks reachable by use or def of CSRs/FI.
430static void collectBlocksReachableByDirty(
431 const DenseSet<const MachineBasicBlock *> &DirtyBBs,
432 DenseSet<const MachineBasicBlock *> &ReachableByDirty) {
433 for (const MachineBasicBlock *MBB : DirtyBBs) {
434 if (ReachableByDirty.count(V: MBB))
435 continue;
436 // Mark all offsprings as reachable.
437 markAllReachable(Visited&: ReachableByDirty, MBB: *MBB);
438 }
439}
440
441/// \return true if there is a clean path from SavePoint to the original
442/// Restore.
443static bool
444isSaveReachableThroughClean(const MachineBasicBlock *SavePoint,
445 ArrayRef<MachineBasicBlock *> CleanPreds) {
446 DenseSet<const MachineBasicBlock *> Visited;
447 SmallVector<MachineBasicBlock *, 4> Worklist(CleanPreds);
448 while (!Worklist.empty()) {
449 MachineBasicBlock *CleanBB = Worklist.pop_back_val();
450 if (CleanBB == SavePoint)
451 return true;
452 if (!Visited.insert(V: CleanBB).second || !CleanBB->pred_size())
453 continue;
454 Worklist.append(in_start: CleanBB->pred_begin(), in_end: CleanBB->pred_end());
455 }
456 return false;
457}
458
459/// This function updates the branches post restore point split.
460///
461/// Restore point has been split.
462/// Old restore point: MBB
463/// New restore point: NMBB
464/// Any basic block(say BBToUpdate) which had a fallthrough to MBB
465/// previously should
466/// 1. Fallthrough to NMBB iff NMBB is inserted immediately above MBB in the
467/// block layout OR
468/// 2. Branch unconditionally to NMBB iff NMBB is inserted at any other place.
469static void updateTerminator(MachineBasicBlock *BBToUpdate,
470 MachineBasicBlock *NMBB,
471 const TargetInstrInfo *TII) {
472 DebugLoc DL = BBToUpdate->findBranchDebugLoc();
473 // if NMBB isn't the new layout successor for BBToUpdate, insert unconditional
474 // branch to it
475 if (!BBToUpdate->isLayoutSuccessor(MBB: NMBB))
476 TII->insertUnconditionalBranch(MBB&: *BBToUpdate, DestBB: NMBB, DL);
477}
478
479/// This function splits the restore point and returns new restore point/BB.
480///
481/// DirtyPreds: Predessors of \p MBB that are ReachableByDirty
482///
483/// Decision has been made to split the restore point.
484/// old restore point: \p MBB
485/// new restore point: \p NMBB
486/// This function makes the necessary block layout changes so that
487/// 1. \p NMBB points to \p MBB unconditionally
488/// 2. All dirtyPreds that previously pointed to \p MBB point to \p NMBB
489static MachineBasicBlock *
490tryToSplitRestore(MachineBasicBlock *MBB,
491 ArrayRef<MachineBasicBlock *> DirtyPreds,
492 const TargetInstrInfo *TII) {
493 MachineFunction *MF = MBB->getParent();
494
495 // get the list of DirtyPreds who have a fallthrough to MBB
496 // before the block layout change. This is just to ensure that if the NMBB is
497 // inserted after MBB, then we create unconditional branch from
498 // DirtyPred/CleanPred to NMBB
499 SmallPtrSet<MachineBasicBlock *, 8> MBBFallthrough;
500 for (MachineBasicBlock *BB : DirtyPreds)
501 if (BB->getFallThrough(JumpToFallThrough: false) == MBB)
502 MBBFallthrough.insert(Ptr: BB);
503
504 MachineBasicBlock *NMBB = MF->CreateMachineBasicBlock();
505 // Insert this block at the end of the function. Inserting in between may
506 // interfere with control flow optimizer decisions.
507 MF->insert(MBBI: MF->end(), MBB: NMBB);
508
509 for (const MachineBasicBlock::RegisterMaskPair &LI : MBB->liveins())
510 NMBB->addLiveIn(PhysReg: LI.PhysReg);
511
512 TII->insertUnconditionalBranch(MBB&: *NMBB, DestBB: MBB, DL: DebugLoc());
513
514 // After splitting, all predecessors of the restore point should be dirty
515 // blocks.
516 for (MachineBasicBlock *SuccBB : DirtyPreds)
517 SuccBB->ReplaceUsesOfBlockWith(Old: MBB, New: NMBB);
518
519 NMBB->addSuccessor(Succ: MBB);
520
521 for (MachineBasicBlock *BBToUpdate : MBBFallthrough)
522 updateTerminator(BBToUpdate, NMBB, TII);
523
524 return NMBB;
525}
526
527/// This function undoes the restore point split done earlier.
528///
529/// DirtyPreds: All predecessors of \p NMBB that are ReachableByDirty.
530///
531/// Restore point was split and the change needs to be unrolled. Make necessary
532/// changes to reset restore point from \p NMBB to \p MBB.
533static void rollbackRestoreSplit(MachineFunction &MF, MachineBasicBlock *NMBB,
534 MachineBasicBlock *MBB,
535 ArrayRef<MachineBasicBlock *> DirtyPreds,
536 const TargetInstrInfo *TII) {
537 // For a BB, if NMBB is fallthrough in the current layout, then in the new
538 // layout a. BB should fallthrough to MBB OR b. BB should undconditionally
539 // branch to MBB
540 SmallPtrSet<MachineBasicBlock *, 8> NMBBFallthrough;
541 for (MachineBasicBlock *BB : DirtyPreds)
542 if (BB->getFallThrough(JumpToFallThrough: false) == NMBB)
543 NMBBFallthrough.insert(Ptr: BB);
544
545 NMBB->removeSuccessor(Succ: MBB);
546 for (MachineBasicBlock *SuccBB : DirtyPreds)
547 SuccBB->ReplaceUsesOfBlockWith(Old: NMBB, New: MBB);
548
549 NMBB->erase(I: NMBB->begin(), E: NMBB->end());
550 NMBB->eraseFromParent();
551
552 for (MachineBasicBlock *BBToUpdate : NMBBFallthrough)
553 updateTerminator(BBToUpdate, NMBB: MBB, TII);
554}
555
556// A block is deemed fit for restore point split iff there exist
557// 1. DirtyPreds - preds of CurRestore reachable from use or def of CSR/FI
558// 2. CleanPreds - preds of CurRestore that arent DirtyPreds
559bool ShrinkWrapImpl::checkIfRestoreSplittable(
560 const MachineBasicBlock *CurRestore,
561 const DenseSet<const MachineBasicBlock *> &ReachableByDirty,
562 SmallVectorImpl<MachineBasicBlock *> &DirtyPreds,
563 SmallVectorImpl<MachineBasicBlock *> &CleanPreds,
564 const TargetInstrInfo *TII, RegScavenger *RS) {
565 for (const MachineInstr &MI : *CurRestore)
566 if (useOrDefCSROrFI(MI, RS, /*StackAddressUsed=*/true))
567 return false;
568
569 for (MachineBasicBlock *PredBB : CurRestore->predecessors()) {
570 if (!isAnalyzableBB(TII: *TII, Entry&: *PredBB))
571 return false;
572
573 if (ReachableByDirty.count(V: PredBB))
574 DirtyPreds.push_back(Elt: PredBB);
575 else
576 CleanPreds.push_back(Elt: PredBB);
577 }
578
579 return !(CleanPreds.empty() || DirtyPreds.empty());
580}
581
582bool ShrinkWrapImpl::postShrinkWrapping(bool HasCandidate, MachineFunction &MF,
583 RegScavenger *RS) {
584 if (!EnablePostShrinkWrapOpt)
585 return false;
586
587 MachineBasicBlock *InitSave = nullptr;
588 MachineBasicBlock *InitRestore = nullptr;
589
590 if (HasCandidate) {
591 InitSave = Save;
592 InitRestore = Restore;
593 } else {
594 InitRestore = nullptr;
595 InitSave = &MF.front();
596 for (MachineBasicBlock &MBB : MF) {
597 if (MBB.isEHFuncletEntry())
598 return false;
599 if (MBB.isReturnBlock()) {
600 // Do not support multiple restore points.
601 if (InitRestore)
602 return false;
603 InitRestore = &MBB;
604 }
605 }
606 }
607
608 if (!InitSave || !InitRestore || InitRestore == InitSave ||
609 !MDT->dominates(A: InitSave, B: InitRestore) ||
610 !MPDT->dominates(A: InitRestore, B: InitSave))
611 return false;
612
613 // Bail out of the optimization if any of the basic block is target of
614 // INLINEASM_BR instruction
615 for (MachineBasicBlock &MBB : MF)
616 if (MBB.isInlineAsmBrIndirectTarget())
617 return false;
618
619 DenseSet<const MachineBasicBlock *> DirtyBBs;
620 for (MachineBasicBlock &MBB : MF) {
621 if (MBB.isEHPad()) {
622 DirtyBBs.insert(V: &MBB);
623 continue;
624 }
625 for (const MachineInstr &MI : MBB)
626 if (useOrDefCSROrFI(MI, RS, /*StackAddressUsed=*/true)) {
627 DirtyBBs.insert(V: &MBB);
628 break;
629 }
630 }
631
632 // Find blocks reachable from the use or def of CSRs/FI.
633 DenseSet<const MachineBasicBlock *> ReachableByDirty;
634 collectBlocksReachableByDirty(DirtyBBs, ReachableByDirty);
635
636 const TargetInstrInfo *TII = MF.getSubtarget().getInstrInfo();
637 SmallVector<MachineBasicBlock *, 2> DirtyPreds;
638 SmallVector<MachineBasicBlock *, 2> CleanPreds;
639 if (!checkIfRestoreSplittable(CurRestore: InitRestore, ReachableByDirty, DirtyPreds,
640 CleanPreds, TII, RS))
641 return false;
642
643 // Trying to reach out to the new save point which dominates all dirty blocks.
644 MachineBasicBlock *NewSave =
645 FindIDom<>(Block&: **DirtyPreds.begin(), BBs: DirtyPreds, Dom&: *MDT, Strict: false);
646
647 while (NewSave && (hasDirtyPred(ReachableByDirty, MBB: *NewSave) ||
648 EntryFreq < MBFI->getBlockFreq(MBB: NewSave) ||
649 /*Entry freq has been observed more than a loop block in
650 some cases*/
651 MLI->getLoopFor(BB: NewSave)))
652 NewSave = FindIDom<>(Block&: **NewSave->pred_begin(), BBs: NewSave->predecessors(), Dom&: *MDT,
653 Strict: false);
654
655 const TargetFrameLowering *TFI = MF.getSubtarget().getFrameLowering();
656 if (!NewSave || NewSave == InitSave ||
657 isSaveReachableThroughClean(SavePoint: NewSave, CleanPreds) ||
658 !TFI->canUseAsPrologue(MBB: *NewSave))
659 return false;
660
661 // Now we know that splitting a restore point can isolate the restore point
662 // from clean blocks and doing so can shrink the save point.
663 MachineBasicBlock *NewRestore =
664 tryToSplitRestore(MBB: InitRestore, DirtyPreds, TII);
665
666 // Make sure if the new restore point is valid as an epilogue, depending on
667 // targets.
668 if (!TFI->canUseAsEpilogue(MBB: *NewRestore)) {
669 rollbackRestoreSplit(MF, NMBB: NewRestore, MBB: InitRestore, DirtyPreds, TII);
670 return false;
671 }
672
673 Save = NewSave;
674 Restore = NewRestore;
675
676 MDT->recalculate(Func&: MF);
677 MPDT->recalculate(Func&: MF);
678
679 assert((MDT->dominates(Save, Restore) && MPDT->dominates(Restore, Save)) &&
680 "Incorrect save or restore point due to dominance relations");
681 assert((!MLI->getLoopFor(Save) && !MLI->getLoopFor(Restore)) &&
682 "Unexpected save or restore point in a loop");
683 assert((EntryFreq >= MBFI->getBlockFreq(Save) &&
684 EntryFreq >= MBFI->getBlockFreq(Restore)) &&
685 "Incorrect save or restore point based on block frequency");
686 return true;
687}
688
689void ShrinkWrapImpl::updateSaveRestorePoints(MachineBasicBlock &MBB,
690 RegScavenger *RS) {
691 // Get rid of the easy cases first.
692 if (!Save)
693 Save = &MBB;
694 else
695 Save = MDT->findNearestCommonDominator(A: Save, B: &MBB);
696 assert(Save);
697
698 if (!Restore)
699 Restore = &MBB;
700 else if (MPDT->getNode(BB: &MBB)) // If the block is not in the post dom tree, it
701 // means the block never returns. If that's the
702 // case, we don't want to call
703 // `findNearestCommonDominator`, which will
704 // return `Restore`.
705 Restore = MPDT->findNearestCommonDominator(A: Restore, B: &MBB);
706 else
707 Restore = nullptr; // Abort, we can't find a restore point in this case.
708
709 // Make sure we would be able to insert the restore code before the
710 // terminator.
711 if (Restore == &MBB) {
712 for (const MachineInstr &Terminator : MBB.terminators()) {
713 if (!useOrDefCSROrFI(MI: Terminator, RS, /*StackAddressUsed=*/true))
714 continue;
715 // One of the terminator needs to happen before the restore point.
716 if (MBB.succ_empty()) {
717 Restore = nullptr; // Abort, we can't find a restore point in this case.
718 break;
719 }
720 // Look for a restore point that post-dominates all the successors.
721 // The immediate post-dominator is what we are looking for.
722 Restore = FindIDom<>(Block&: *Restore, BBs: Restore->successors(), Dom&: *MPDT);
723 break;
724 }
725 }
726
727 if (!Restore) {
728 LLVM_DEBUG(
729 dbgs() << "Restore point needs to be spanned on several blocks\n");
730 return;
731 }
732
733 // Make sure Save and Restore are suitable for shrink-wrapping:
734 // 1. all path from Save needs to lead to Restore before exiting.
735 // 2. all path to Restore needs to go through Save from Entry.
736 // We achieve that by making sure that:
737 // A. Save dominates Restore.
738 // B. Restore post-dominates Save.
739 // C. Save and Restore are in the same loop.
740 bool SaveDominatesRestore = false;
741 bool RestorePostDominatesSave = false;
742 while (Restore &&
743 (!(SaveDominatesRestore = MDT->dominates(A: Save, B: Restore)) ||
744 !(RestorePostDominatesSave = MPDT->dominates(A: Restore, B: Save)) ||
745 // Post-dominance is not enough in loops to ensure that all uses/defs
746 // are after the prologue and before the epilogue at runtime.
747 // E.g.,
748 // while(1) {
749 // Save
750 // Restore
751 // if (...)
752 // break;
753 // use/def CSRs
754 // }
755 // All the uses/defs of CSRs are dominated by Save and post-dominated
756 // by Restore. However, the CSRs uses are still reachable after
757 // Restore and before Save are executed.
758 //
759 // For now, just push the restore/save points outside of loops.
760 // FIXME: Refine the criteria to still find interesting cases
761 // for loops.
762 MLI->getLoopFor(BB: Save) || MLI->getLoopFor(BB: Restore))) {
763 // Fix (A).
764 if (!SaveDominatesRestore) {
765 Save = MDT->findNearestCommonDominator(A: Save, B: Restore);
766 continue;
767 }
768 // Fix (B).
769 if (!RestorePostDominatesSave)
770 Restore = MPDT->findNearestCommonDominator(A: Restore, B: Save);
771
772 // Fix (C).
773 if (Restore && (MLI->getLoopFor(BB: Save) || MLI->getLoopFor(BB: Restore))) {
774 if (MLI->getLoopDepth(BB: Save) > MLI->getLoopDepth(BB: Restore)) {
775 // Push Save outside of this loop if immediate dominator is different
776 // from save block. If immediate dominator is not different, bail out.
777 Save = FindIDom<>(Block&: *Save, BBs: Save->predecessors(), Dom&: *MDT);
778 if (!Save)
779 break;
780 } else {
781 // If the loop does not exit, there is no point in looking
782 // for a post-dominator outside the loop.
783 SmallVector<MachineBasicBlock*, 4> ExitBlocks;
784 MLI->getLoopFor(BB: Restore)->getExitingBlocks(ExitingBlocks&: ExitBlocks);
785 // Push Restore outside of this loop.
786 // Look for the immediate post-dominator of the loop exits.
787 MachineBasicBlock *IPdom = Restore;
788 for (MachineBasicBlock *LoopExitBB: ExitBlocks) {
789 IPdom = FindIDom<>(Block&: *IPdom, BBs: LoopExitBB->successors(), Dom&: *MPDT);
790 if (!IPdom)
791 break;
792 }
793 // If the immediate post-dominator is not in a less nested loop,
794 // then we are stuck in a program with an infinite loop.
795 // In that case, we will not find a safe point, hence, bail out.
796 if (IPdom && MLI->getLoopDepth(BB: IPdom) < MLI->getLoopDepth(BB: Restore))
797 Restore = IPdom;
798 else {
799 Restore = nullptr;
800 break;
801 }
802 }
803 }
804 }
805}
806
807static bool giveUpWithRemarks(MachineOptimizationRemarkEmitter *ORE,
808 StringRef RemarkName, StringRef RemarkMessage,
809 const DiagnosticLocation &Loc,
810 const MachineBasicBlock *MBB) {
811 ORE->emit(RemarkBuilder: [&]() {
812 return MachineOptimizationRemarkMissed(DEBUG_TYPE, RemarkName, Loc, MBB)
813 << RemarkMessage;
814 });
815
816 LLVM_DEBUG(dbgs() << RemarkMessage << '\n');
817 return false;
818}
819
820bool ShrinkWrapImpl::performShrinkWrapping(
821 const ReversePostOrderTraversal<MachineBasicBlock *> &RPOT,
822 RegScavenger *RS) {
823 for (MachineBasicBlock *MBB : RPOT) {
824 LLVM_DEBUG(dbgs() << "Look into: " << printMBBReference(*MBB) << '\n');
825
826 if (MBB->isEHFuncletEntry())
827 return giveUpWithRemarks(ORE, RemarkName: "UnsupportedEHFunclets",
828 RemarkMessage: "EH Funclets are not supported yet.",
829 Loc: MBB->front().getDebugLoc(), MBB);
830
831 if (MBB->isEHPad() || MBB->isInlineAsmBrIndirectTarget()) {
832 // Push the prologue and epilogue outside of the region that may throw (or
833 // jump out via inlineasm_br), by making sure that all the landing pads
834 // are at least at the boundary of the save and restore points. The
835 // problem is that a basic block can jump out from the middle in these
836 // cases, which we do not handle.
837 updateSaveRestorePoints(MBB&: *MBB, RS);
838 if (!ArePointsInteresting()) {
839 LLVM_DEBUG(dbgs() << "EHPad/inlineasm_br prevents shrink-wrapping\n");
840 return false;
841 }
842 continue;
843 }
844
845 bool StackAddressUsed = false;
846 // Check if we found any stack accesses in the predecessors. We are not
847 // doing a full dataflow analysis here to keep things simple but just
848 // rely on a reverse portorder traversal (RPOT) to guarantee predecessors
849 // are already processed except for loops (and accept the conservative
850 // result for loops).
851 for (const MachineBasicBlock *Pred : MBB->predecessors()) {
852 if (StackAddressUsedBlockInfo.test(Idx: Pred->getNumber())) {
853 StackAddressUsed = true;
854 break;
855 }
856 }
857
858 for (const MachineInstr &MI : *MBB) {
859 if (useOrDefCSROrFI(MI, RS, StackAddressUsed)) {
860 // Save (resp. restore) point must dominate (resp. post dominate)
861 // MI. Look for the proper basic block for those.
862 updateSaveRestorePoints(MBB&: *MBB, RS);
863 // If we are at a point where we cannot improve the placement of
864 // save/restore instructions, just give up.
865 if (!ArePointsInteresting()) {
866 LLVM_DEBUG(dbgs() << "No Shrink wrap candidate found\n");
867 return false;
868 }
869 // No need to look for other instructions, this basic block
870 // will already be part of the handled region.
871 StackAddressUsed = true;
872 break;
873 }
874 }
875 StackAddressUsedBlockInfo[MBB->getNumber()] = StackAddressUsed;
876 }
877 if (!ArePointsInteresting()) {
878 // If the points are not interesting at this point, then they must be null
879 // because it means we did not encounter any frame/CSR related code.
880 // Otherwise, we would have returned from the previous loop.
881 assert(!Save && !Restore && "We miss a shrink-wrap opportunity?!");
882 LLVM_DEBUG(dbgs() << "Nothing to shrink-wrap\n");
883 return false;
884 }
885
886 LLVM_DEBUG(dbgs() << "\n ** Results **\nFrequency of the Entry: "
887 << EntryFreq.getFrequency() << '\n');
888
889 const TargetFrameLowering *TFI =
890 MachineFunc->getSubtarget().getFrameLowering();
891 do {
892 LLVM_DEBUG(dbgs() << "Shrink wrap candidates (#, Name, Freq):\nSave: "
893 << printMBBReference(*Save) << ' '
894 << printBlockFreq(*MBFI, *Save)
895 << "\nRestore: " << printMBBReference(*Restore) << ' '
896 << printBlockFreq(*MBFI, *Restore) << '\n');
897
898 bool IsSaveCheap, TargetCanUseSaveAsPrologue = false;
899 if (((IsSaveCheap = EntryFreq >= MBFI->getBlockFreq(MBB: Save)) &&
900 EntryFreq >= MBFI->getBlockFreq(MBB: Restore)) &&
901 ((TargetCanUseSaveAsPrologue = TFI->canUseAsPrologue(MBB: *Save)) &&
902 TFI->canUseAsEpilogue(MBB: *Restore)))
903 break;
904 LLVM_DEBUG(
905 dbgs() << "New points are too expensive or invalid for the target\n");
906 MachineBasicBlock *NewBB;
907 if (!IsSaveCheap || !TargetCanUseSaveAsPrologue) {
908 Save = FindIDom<>(Block&: *Save, BBs: Save->predecessors(), Dom&: *MDT);
909 if (!Save)
910 break;
911 NewBB = Save;
912 } else {
913 // Restore is expensive.
914 Restore = FindIDom<>(Block&: *Restore, BBs: Restore->successors(), Dom&: *MPDT);
915 if (!Restore)
916 break;
917 NewBB = Restore;
918 }
919 updateSaveRestorePoints(MBB&: *NewBB, RS);
920 } while (Save && Restore);
921
922 if (!ArePointsInteresting()) {
923 ++NumCandidatesDropped;
924 return false;
925 }
926 return true;
927}
928
929bool ShrinkWrapImpl::run(MachineFunction &MF) {
930 LLVM_DEBUG(dbgs() << "**** Analysing " << MF.getName() << '\n');
931
932 init(MF);
933
934 ReversePostOrderTraversal<MachineBasicBlock *> RPOT(&*MF.begin());
935 if (containsIrreducibleCFG<MachineBasicBlock *>(RPOTraversal&: RPOT, LI: *MLI)) {
936 // If MF is irreducible, a block may be in a loop without
937 // MachineLoopInfo reporting it. I.e., we may use the
938 // post-dominance property in loops, which lead to incorrect
939 // results. Moreover, we may miss that the prologue and
940 // epilogue are not in the same loop, leading to unbalanced
941 // construction/deconstruction of the stack frame.
942 return giveUpWithRemarks(ORE, RemarkName: "UnsupportedIrreducibleCFG",
943 RemarkMessage: "Irreducible CFGs are not supported yet.",
944 Loc: MF.getFunction().getSubprogram(), MBB: &MF.front());
945 }
946
947 const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
948 std::unique_ptr<RegScavenger> RS(
949 TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : nullptr);
950
951 bool Changed = false;
952
953 // Initially, conservatively assume that stack addresses can be used in each
954 // basic block and change the state only for those basic blocks for which we
955 // were able to prove the opposite.
956 StackAddressUsedBlockInfo.resize(N: MF.getNumBlockIDs(), t: true);
957 bool HasCandidate = performShrinkWrapping(RPOT, RS: RS.get());
958 StackAddressUsedBlockInfo.clear();
959 Changed = postShrinkWrapping(HasCandidate, MF, RS: RS.get());
960 if (!HasCandidate && !Changed)
961 return false;
962 if (!ArePointsInteresting())
963 return Changed;
964
965 LLVM_DEBUG(dbgs() << "Final shrink wrap candidates:\nSave: "
966 << printMBBReference(*Save) << ' '
967 << "\nRestore: " << printMBBReference(*Restore) << '\n');
968
969 MachineFrameInfo &MFI = MF.getFrameInfo();
970 MFI.setSavePoint(Save);
971 MFI.setRestorePoint(Restore);
972 ++NumCandidates;
973 return Changed;
974}
975
976bool ShrinkWrapLegacy::runOnMachineFunction(MachineFunction &MF) {
977 if (skipFunction(F: MF.getFunction()) || MF.empty() ||
978 !ShrinkWrapImpl::isShrinkWrapEnabled(MF))
979 return false;
980
981 MachineDominatorTree *MDT =
982 &getAnalysis<MachineDominatorTreeWrapperPass>().getDomTree();
983 MachinePostDominatorTree *MPDT =
984 &getAnalysis<MachinePostDominatorTreeWrapperPass>().getPostDomTree();
985 MachineBlockFrequencyInfo *MBFI =
986 &getAnalysis<MachineBlockFrequencyInfoWrapperPass>().getMBFI();
987 MachineLoopInfo *MLI = &getAnalysis<MachineLoopInfoWrapperPass>().getLI();
988 MachineOptimizationRemarkEmitter *ORE =
989 &getAnalysis<MachineOptimizationRemarkEmitterPass>().getORE();
990
991 return ShrinkWrapImpl(MDT, MPDT, MBFI, MLI, ORE).run(MF);
992}
993
994PreservedAnalyses ShrinkWrapPass::run(MachineFunction &MF,
995 MachineFunctionAnalysisManager &MFAM) {
996 MFPropsModifier _(*this, MF);
997 if (MF.empty() || !ShrinkWrapImpl::isShrinkWrapEnabled(MF))
998 return PreservedAnalyses::all();
999
1000 MachineDominatorTree &MDT = MFAM.getResult<MachineDominatorTreeAnalysis>(IR&: MF);
1001 MachinePostDominatorTree &MPDT =
1002 MFAM.getResult<MachinePostDominatorTreeAnalysis>(IR&: MF);
1003 MachineBlockFrequencyInfo &MBFI =
1004 MFAM.getResult<MachineBlockFrequencyAnalysis>(IR&: MF);
1005 MachineLoopInfo &MLI = MFAM.getResult<MachineLoopAnalysis>(IR&: MF);
1006 MachineOptimizationRemarkEmitter &ORE =
1007 MFAM.getResult<MachineOptimizationRemarkEmitterAnalysis>(IR&: MF);
1008
1009 ShrinkWrapImpl(&MDT, &MPDT, &MBFI, &MLI, &ORE).run(MF);
1010 return PreservedAnalyses::all();
1011}
1012
1013bool ShrinkWrapImpl::isShrinkWrapEnabled(const MachineFunction &MF) {
1014 const TargetFrameLowering *TFI = MF.getSubtarget().getFrameLowering();
1015
1016 switch (EnableShrinkWrapOpt) {
1017 case cl::BOU_UNSET:
1018 return TFI->enableShrinkWrapping(MF) &&
1019 // Windows with CFI has some limitations that make it impossible
1020 // to use shrink-wrapping.
1021 !MF.getTarget().getMCAsmInfo()->usesWindowsCFI() &&
1022 // Sanitizers look at the value of the stack at the location
1023 // of the crash. Since a crash can happen anywhere, the
1024 // frame must be lowered before anything else happen for the
1025 // sanitizers to be able to get a correct stack frame.
1026 !(MF.getFunction().hasFnAttribute(Kind: Attribute::SanitizeAddress) ||
1027 MF.getFunction().hasFnAttribute(Kind: Attribute::SanitizeThread) ||
1028 MF.getFunction().hasFnAttribute(Kind: Attribute::SanitizeMemory) ||
1029 MF.getFunction().hasFnAttribute(Kind: Attribute::SanitizeType) ||
1030 MF.getFunction().hasFnAttribute(Kind: Attribute::SanitizeHWAddress));
1031 // If EnableShrinkWrap is set, it takes precedence on whatever the
1032 // target sets. The rational is that we assume we want to test
1033 // something related to shrink-wrapping.
1034 case cl::BOU_TRUE:
1035 return true;
1036 case cl::BOU_FALSE:
1037 return false;
1038 }
1039 llvm_unreachable("Invalid shrink-wrapping state");
1040}
1041