1 //===-- ShrinkWrap.cpp - Compute safe point for prolog/epilog insertion ---===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass looks for safe point where the prologue and epilogue can be
12 // The safe point for the prologue (resp. epilogue) is called Save
14 // A point is safe for prologue (resp. epilogue) if and only if
15 // it 1) dominates (resp. post-dominates) all the frame related operations and
16 // between 2) two executions of the Save (resp. Restore) point there is an
17 // execution of the Restore (resp. Save) point.
19 // For instance, the following points are safe:
20 // for (int i = 0; i < 10; ++i) {
25 // Indeed, the execution looks like Save -> Restore -> Save -> Restore ...
26 // And the following points are not:
27 // for (int i = 0; i < 10; ++i) {
31 // for (int i = 0; i < 10; ++i) {
35 // Indeed, the execution looks like Save -> Save -> ... -> Restore -> Restore.
37 // This pass also ensures that the safe points are 3) cheaper than the regular
38 // entry and exits blocks.
40 // Property #1 is ensured via the use of MachineDominatorTree and
41 // MachinePostDominatorTree.
42 // Property #2 is ensured via property #1 and MachineLoopInfo, i.e., both
43 // points must be in the same loop.
44 // Property #3 is ensured via the MachineBlockFrequencyInfo.
46 // If this pass found points matching all these properties, then
47 // MachineFrameInfo is updated with this information.
48 //===----------------------------------------------------------------------===//
49 #include "llvm/ADT/BitVector.h"
50 #include "llvm/ADT/PostOrderIterator.h"
51 #include "llvm/ADT/SetVector.h"
52 #include "llvm/ADT/Statistic.h"
53 // To check for profitability.
54 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
55 // For property #1 for Save.
56 #include "llvm/CodeGen/MachineDominators.h"
57 #include "llvm/CodeGen/MachineFunctionPass.h"
58 // To record the result of the analysis.
59 #include "llvm/CodeGen/MachineFrameInfo.h"
61 #include "llvm/CodeGen/MachineLoopInfo.h"
62 // For property #1 for Restore.
63 #include "llvm/CodeGen/MachinePostDominators.h"
64 #include "llvm/CodeGen/Passes.h"
65 // To know about callee-saved.
66 #include "llvm/CodeGen/RegisterClassInfo.h"
67 #include "llvm/CodeGen/RegisterScavenging.h"
68 #include "llvm/MC/MCAsmInfo.h"
69 #include "llvm/Support/Debug.h"
70 // To query the target about frame lowering.
71 #include "llvm/Target/TargetFrameLowering.h"
72 // To know about frame setup operation.
73 #include "llvm/Target/TargetInstrInfo.h"
74 #include "llvm/Target/TargetMachine.h"
75 // To access TargetInstrInfo.
76 #include "llvm/Target/TargetSubtargetInfo.h"
78 #define DEBUG_TYPE "shrink-wrap"
82 STATISTIC(NumFunc, "Number of functions");
83 STATISTIC(NumCandidates, "Number of shrink-wrapping candidates");
84 STATISTIC(NumCandidatesDropped,
85 "Number of shrink-wrapping candidates dropped because of frequency");
87 static cl::opt<cl::boolOrDefault>
88 EnableShrinkWrapOpt("enable-shrink-wrap", cl::Hidden,
89 cl::desc("enable the shrink-wrapping pass"));
92 /// \brief Class to determine where the safe point to insert the
93 /// prologue and epilogue are.
94 /// Unlike the paper from Fred C. Chow, PLDI'88, that introduces the
95 /// shrink-wrapping term for prologue/epilogue placement, this pass
96 /// does not rely on expensive data-flow analysis. Instead we use the
97 /// dominance properties and loop information to decide which point
98 /// are safe for such insertion.
99 class ShrinkWrap : public MachineFunctionPass {
100 /// Hold callee-saved information.
101 RegisterClassInfo RCI;
102 MachineDominatorTree *MDT;
103 MachinePostDominatorTree *MPDT;
104 /// Current safe point found for the prologue.
105 /// The prologue will be inserted before the first instruction
106 /// in this basic block.
107 MachineBasicBlock *Save;
108 /// Current safe point found for the epilogue.
109 /// The epilogue will be inserted before the first terminator instruction
110 /// in this basic block.
111 MachineBasicBlock *Restore;
112 /// Hold the information of the basic block frequency.
113 /// Use to check the profitability of the new points.
114 MachineBlockFrequencyInfo *MBFI;
115 /// Hold the loop information. Used to determine if Save and Restore
116 /// are in the same loop.
117 MachineLoopInfo *MLI;
118 /// Frequency of the Entry block.
120 /// Current opcode for frame setup.
121 unsigned FrameSetupOpcode;
122 /// Current opcode for frame destroy.
123 unsigned FrameDestroyOpcode;
125 const MachineBasicBlock *Entry;
126 typedef SmallSetVector<unsigned, 16> SetOfRegs;
127 /// Registers that need to be saved for the current function.
128 mutable SetOfRegs CurrentCSRs;
129 /// Current MachineFunction.
130 MachineFunction *MachineFunc;
132 /// \brief Check if \p MI uses or defines a callee-saved register or
133 /// a frame index. If this is the case, this means \p MI must happen
134 /// after Save and before Restore.
135 bool useOrDefCSROrFI(const MachineInstr &MI, RegScavenger *RS) const;
137 const SetOfRegs &getCurrentCSRs(RegScavenger *RS) const {
138 if (CurrentCSRs.empty()) {
140 const TargetFrameLowering *TFI =
141 MachineFunc->getSubtarget().getFrameLowering();
143 TFI->determineCalleeSaves(*MachineFunc, SavedRegs, RS);
145 for (int Reg = SavedRegs.find_first(); Reg != -1;
146 Reg = SavedRegs.find_next(Reg))
147 CurrentCSRs.insert((unsigned)Reg);
152 /// \brief Update the Save and Restore points such that \p MBB is in
153 /// the region that is dominated by Save and post-dominated by Restore
154 /// and Save and Restore still match the safe point definition.
155 /// Such point may not exist and Save and/or Restore may be null after
157 void updateSaveRestorePoints(MachineBasicBlock &MBB, RegScavenger *RS);
159 /// \brief Initialize the pass for \p MF.
160 void init(MachineFunction &MF) {
161 RCI.runOnMachineFunction(MF);
162 MDT = &getAnalysis<MachineDominatorTree>();
163 MPDT = &getAnalysis<MachinePostDominatorTree>();
166 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
167 MLI = &getAnalysis<MachineLoopInfo>();
168 EntryFreq = MBFI->getEntryFreq();
169 const TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
170 FrameSetupOpcode = TII.getCallFrameSetupOpcode();
171 FrameDestroyOpcode = TII.getCallFrameDestroyOpcode();
179 /// Check whether or not Save and Restore points are still interesting for
181 bool ArePointsInteresting() const { return Save != Entry && Save && Restore; }
183 /// \brief Check if shrink wrapping is enabled for this target and function.
184 static bool isShrinkWrapEnabled(const MachineFunction &MF);
189 ShrinkWrap() : MachineFunctionPass(ID) {
190 initializeShrinkWrapPass(*PassRegistry::getPassRegistry());
193 void getAnalysisUsage(AnalysisUsage &AU) const override {
194 AU.setPreservesAll();
195 AU.addRequired<MachineBlockFrequencyInfo>();
196 AU.addRequired<MachineDominatorTree>();
197 AU.addRequired<MachinePostDominatorTree>();
198 AU.addRequired<MachineLoopInfo>();
199 MachineFunctionPass::getAnalysisUsage(AU);
202 StringRef getPassName() const override { return "Shrink Wrapping analysis"; }
204 /// \brief Perform the shrink-wrapping analysis and update
205 /// the MachineFrameInfo attached to \p MF with the results.
206 bool runOnMachineFunction(MachineFunction &MF) override;
208 } // End anonymous namespace.
210 char ShrinkWrap::ID = 0;
211 char &llvm::ShrinkWrapID = ShrinkWrap::ID;
213 INITIALIZE_PASS_BEGIN(ShrinkWrap, "shrink-wrap", "Shrink Wrap Pass", false,
215 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
216 INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
217 INITIALIZE_PASS_DEPENDENCY(MachinePostDominatorTree)
218 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
219 INITIALIZE_PASS_END(ShrinkWrap, "shrink-wrap", "Shrink Wrap Pass", false, false)
221 bool ShrinkWrap::useOrDefCSROrFI(const MachineInstr &MI,
222 RegScavenger *RS) const {
223 if (MI.getOpcode() == FrameSetupOpcode ||
224 MI.getOpcode() == FrameDestroyOpcode) {
225 DEBUG(dbgs() << "Frame instruction: " << MI << '\n');
228 for (const MachineOperand &MO : MI.operands()) {
229 bool UseOrDefCSR = false;
231 unsigned PhysReg = MO.getReg();
234 assert(TargetRegisterInfo::isPhysicalRegister(PhysReg) &&
235 "Unallocated register?!");
236 UseOrDefCSR = RCI.getLastCalleeSavedAlias(PhysReg);
237 } else if (MO.isRegMask()) {
238 // Check if this regmask clobbers any of the CSRs.
239 for (unsigned Reg : getCurrentCSRs(RS)) {
240 if (MO.clobbersPhysReg(Reg)) {
246 if (UseOrDefCSR || MO.isFI()) {
247 DEBUG(dbgs() << "Use or define CSR(" << UseOrDefCSR << ") or FI("
248 << MO.isFI() << "): " << MI << '\n');
255 /// \brief Helper function to find the immediate (post) dominator.
256 template <typename ListOfBBs, typename DominanceAnalysis>
257 static MachineBasicBlock *FindIDom(MachineBasicBlock &Block, ListOfBBs BBs,
258 DominanceAnalysis &Dom) {
259 MachineBasicBlock *IDom = &Block;
260 for (MachineBasicBlock *BB : BBs) {
261 IDom = Dom.findNearestCommonDominator(IDom, BB);
270 void ShrinkWrap::updateSaveRestorePoints(MachineBasicBlock &MBB,
272 // Get rid of the easy cases first.
276 Save = MDT->findNearestCommonDominator(Save, &MBB);
279 DEBUG(dbgs() << "Found a block that is not reachable from Entry\n");
286 Restore = MPDT->findNearestCommonDominator(Restore, &MBB);
288 // Make sure we would be able to insert the restore code before the
290 if (Restore == &MBB) {
291 for (const MachineInstr &Terminator : MBB.terminators()) {
292 if (!useOrDefCSROrFI(Terminator, RS))
294 // One of the terminator needs to happen before the restore point.
295 if (MBB.succ_empty()) {
299 // Look for a restore point that post-dominates all the successors.
300 // The immediate post-dominator is what we are looking for.
301 Restore = FindIDom<>(*Restore, Restore->successors(), *MPDT);
307 DEBUG(dbgs() << "Restore point needs to be spanned on several blocks\n");
311 // Make sure Save and Restore are suitable for shrink-wrapping:
312 // 1. all path from Save needs to lead to Restore before exiting.
313 // 2. all path to Restore needs to go through Save from Entry.
314 // We achieve that by making sure that:
315 // A. Save dominates Restore.
316 // B. Restore post-dominates Save.
317 // C. Save and Restore are in the same loop.
318 bool SaveDominatesRestore = false;
319 bool RestorePostDominatesSave = false;
320 while (Save && Restore &&
321 (!(SaveDominatesRestore = MDT->dominates(Save, Restore)) ||
322 !(RestorePostDominatesSave = MPDT->dominates(Restore, Save)) ||
323 // Post-dominance is not enough in loops to ensure that all uses/defs
324 // are after the prologue and before the epilogue at runtime.
333 // All the uses/defs of CSRs are dominated by Save and post-dominated
334 // by Restore. However, the CSRs uses are still reachable after
335 // Restore and before Save are executed.
337 // For now, just push the restore/save points outside of loops.
338 // FIXME: Refine the criteria to still find interesting cases
340 MLI->getLoopFor(Save) || MLI->getLoopFor(Restore))) {
342 if (!SaveDominatesRestore) {
343 Save = MDT->findNearestCommonDominator(Save, Restore);
347 if (!RestorePostDominatesSave)
348 Restore = MPDT->findNearestCommonDominator(Restore, Save);
351 if (Save && Restore &&
352 (MLI->getLoopFor(Save) || MLI->getLoopFor(Restore))) {
353 if (MLI->getLoopDepth(Save) > MLI->getLoopDepth(Restore)) {
354 // Push Save outside of this loop if immediate dominator is different
355 // from save block. If immediate dominator is not different, bail out.
356 Save = FindIDom<>(*Save, Save->predecessors(), *MDT);
360 // If the loop does not exit, there is no point in looking
361 // for a post-dominator outside the loop.
362 SmallVector<MachineBasicBlock*, 4> ExitBlocks;
363 MLI->getLoopFor(Restore)->getExitingBlocks(ExitBlocks);
364 // Push Restore outside of this loop.
365 // Look for the immediate post-dominator of the loop exits.
366 MachineBasicBlock *IPdom = Restore;
367 for (MachineBasicBlock *LoopExitBB: ExitBlocks) {
368 IPdom = FindIDom<>(*IPdom, LoopExitBB->successors(), *MPDT);
372 // If the immediate post-dominator is not in a less nested loop,
373 // then we are stuck in a program with an infinite loop.
374 // In that case, we will not find a safe point, hence, bail out.
375 if (IPdom && MLI->getLoopDepth(IPdom) < MLI->getLoopDepth(Restore))
386 /// Check whether the edge (\p SrcBB, \p DestBB) is a backedge according to MLI.
387 /// I.e., check if it exists a loop that contains SrcBB and where DestBB is the
389 static bool isProperBackedge(const MachineLoopInfo &MLI,
390 const MachineBasicBlock *SrcBB,
391 const MachineBasicBlock *DestBB) {
392 for (const MachineLoop *Loop = MLI.getLoopFor(SrcBB); Loop;
393 Loop = Loop->getParentLoop()) {
394 if (Loop->getHeader() == DestBB)
400 /// Check if the CFG of \p MF is irreducible.
401 static bool isIrreducibleCFG(const MachineFunction &MF,
402 const MachineLoopInfo &MLI) {
403 const MachineBasicBlock *Entry = &*MF.begin();
404 ReversePostOrderTraversal<const MachineBasicBlock *> RPOT(Entry);
405 BitVector VisitedBB(MF.getNumBlockIDs());
406 for (const MachineBasicBlock *MBB : RPOT) {
407 VisitedBB.set(MBB->getNumber());
408 for (const MachineBasicBlock *SuccBB : MBB->successors()) {
409 if (!VisitedBB.test(SuccBB->getNumber()))
411 // We already visited SuccBB, thus MBB->SuccBB must be a backedge.
412 // Check that the head matches what we have in the loop information.
413 // Otherwise, we have an irreducible graph.
414 if (!isProperBackedge(MLI, MBB, SuccBB))
421 bool ShrinkWrap::runOnMachineFunction(MachineFunction &MF) {
422 if (MF.empty() || !isShrinkWrapEnabled(MF))
425 DEBUG(dbgs() << "**** Analysing " << MF.getName() << '\n');
429 if (isIrreducibleCFG(MF, *MLI)) {
430 // If MF is irreducible, a block may be in a loop without
431 // MachineLoopInfo reporting it. I.e., we may use the
432 // post-dominance property in loops, which lead to incorrect
433 // results. Moreover, we may miss that the prologue and
434 // epilogue are not in the same loop, leading to unbalanced
435 // construction/deconstruction of the stack frame.
436 DEBUG(dbgs() << "Irreducible CFGs are not supported yet\n");
440 const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
441 std::unique_ptr<RegScavenger> RS(
442 TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : nullptr);
444 for (MachineBasicBlock &MBB : MF) {
445 DEBUG(dbgs() << "Look into: " << MBB.getNumber() << ' ' << MBB.getName()
448 if (MBB.isEHFuncletEntry()) {
449 DEBUG(dbgs() << "EH Funclets are not supported yet.\n");
453 for (const MachineInstr &MI : MBB) {
454 if (!useOrDefCSROrFI(MI, RS.get()))
456 // Save (resp. restore) point must dominate (resp. post dominate)
457 // MI. Look for the proper basic block for those.
458 updateSaveRestorePoints(MBB, RS.get());
459 // If we are at a point where we cannot improve the placement of
460 // save/restore instructions, just give up.
461 if (!ArePointsInteresting()) {
462 DEBUG(dbgs() << "No Shrink wrap candidate found\n");
465 // No need to look for other instructions, this basic block
466 // will already be part of the handled region.
470 if (!ArePointsInteresting()) {
471 // If the points are not interesting at this point, then they must be null
472 // because it means we did not encounter any frame/CSR related code.
473 // Otherwise, we would have returned from the previous loop.
474 assert(!Save && !Restore && "We miss a shrink-wrap opportunity?!");
475 DEBUG(dbgs() << "Nothing to shrink-wrap\n");
479 DEBUG(dbgs() << "\n ** Results **\nFrequency of the Entry: " << EntryFreq
482 const TargetFrameLowering *TFI = MF.getSubtarget().getFrameLowering();
484 DEBUG(dbgs() << "Shrink wrap candidates (#, Name, Freq):\nSave: "
485 << Save->getNumber() << ' ' << Save->getName() << ' '
486 << MBFI->getBlockFreq(Save).getFrequency() << "\nRestore: "
487 << Restore->getNumber() << ' ' << Restore->getName() << ' '
488 << MBFI->getBlockFreq(Restore).getFrequency() << '\n');
490 bool IsSaveCheap, TargetCanUseSaveAsPrologue = false;
491 if (((IsSaveCheap = EntryFreq >= MBFI->getBlockFreq(Save).getFrequency()) &&
492 EntryFreq >= MBFI->getBlockFreq(Restore).getFrequency()) &&
493 ((TargetCanUseSaveAsPrologue = TFI->canUseAsPrologue(*Save)) &&
494 TFI->canUseAsEpilogue(*Restore)))
496 DEBUG(dbgs() << "New points are too expensive or invalid for the target\n");
497 MachineBasicBlock *NewBB;
498 if (!IsSaveCheap || !TargetCanUseSaveAsPrologue) {
499 Save = FindIDom<>(*Save, Save->predecessors(), *MDT);
504 // Restore is expensive.
505 Restore = FindIDom<>(*Restore, Restore->successors(), *MPDT);
510 updateSaveRestorePoints(*NewBB, RS.get());
511 } while (Save && Restore);
513 if (!ArePointsInteresting()) {
514 ++NumCandidatesDropped;
518 DEBUG(dbgs() << "Final shrink wrap candidates:\nSave: " << Save->getNumber()
519 << ' ' << Save->getName() << "\nRestore: "
520 << Restore->getNumber() << ' ' << Restore->getName() << '\n');
522 MachineFrameInfo &MFI = MF.getFrameInfo();
523 MFI.setSavePoint(Save);
524 MFI.setRestorePoint(Restore);
529 bool ShrinkWrap::isShrinkWrapEnabled(const MachineFunction &MF) {
530 const TargetFrameLowering *TFI = MF.getSubtarget().getFrameLowering();
532 switch (EnableShrinkWrapOpt) {
534 return TFI->enableShrinkWrapping(MF) &&
535 // Windows with CFI has some limitations that make it impossible
536 // to use shrink-wrapping.
537 !MF.getTarget().getMCAsmInfo()->usesWindowsCFI() &&
538 // Sanitizers look at the value of the stack at the location
539 // of the crash. Since a crash can happen anywhere, the
540 // frame must be lowered before anything else happen for the
541 // sanitizers to be able to get a correct stack frame.
542 !(MF.getFunction()->hasFnAttribute(Attribute::SanitizeAddress) ||
543 MF.getFunction()->hasFnAttribute(Attribute::SanitizeThread) ||
544 MF.getFunction()->hasFnAttribute(Attribute::SanitizeMemory));
545 // If EnableShrinkWrap is set, it takes precedence on whatever the
546 // target sets. The rational is that we assume we want to test
547 // something related to shrink-wrapping.
553 llvm_unreachable("Invalid shrink-wrapping state");