GCStrategy.cpp revision dce4a407a24b04eebc6a376f8e62b41aaa7b071f 
1//===-- GCStrategy.cpp - Garbage collection infrastructure -----------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file implements target- and collector-independent garbage collection
11// infrastructure.
12//
13// GCMachineCodeAnalysis identifies the GC safe points in the machine code.
14// Roots are identified in SelectionDAGISel.
15//
16//===----------------------------------------------------------------------===//
17
18#include "llvm/CodeGen/GCStrategy.h"
19#include "llvm/CodeGen/MachineFrameInfo.h"
20#include "llvm/CodeGen/MachineFunctionPass.h"
21#include "llvm/CodeGen/MachineInstrBuilder.h"
22#include "llvm/CodeGen/MachineModuleInfo.h"
23#include "llvm/CodeGen/Passes.h"
24#include "llvm/IR/Dominators.h"
25#include "llvm/IR/IntrinsicInst.h"
26#include "llvm/IR/Module.h"
27#include "llvm/Support/Debug.h"
28#include "llvm/Support/ErrorHandling.h"
29#include "llvm/Support/raw_ostream.h"
30#include "llvm/Target/TargetFrameLowering.h"
31#include "llvm/Target/TargetInstrInfo.h"
32#include "llvm/Target/TargetMachine.h"
33#include "llvm/Target/TargetRegisterInfo.h"
34
35using namespace llvm;
36
37namespace {
38
39  /// LowerIntrinsics - This pass rewrites calls to the llvm.gcread or
40  /// llvm.gcwrite intrinsics, replacing them with simple loads and stores as
41  /// directed by the GCStrategy. It also performs automatic root initialization
42  /// and custom intrinsic lowering.
43  class LowerIntrinsics : public FunctionPass {
44    static bool NeedsDefaultLoweringPass(const GCStrategy &C);
45    static bool NeedsCustomLoweringPass(const GCStrategy &C);
46    static bool CouldBecomeSafePoint(Instruction *I);
47    bool PerformDefaultLowering(Function &F, GCStrategy &Coll);
48    static bool InsertRootInitializers(Function &F,
49                                       AllocaInst **Roots, unsigned Count);
50
51  public:
52    static char ID;
53
54    LowerIntrinsics();
55    const char *getPassName() const override;
56    void getAnalysisUsage(AnalysisUsage &AU) const override;
57
58    bool doInitialization(Module &M) override;
59    bool runOnFunction(Function &F) override;
60  };
61
62
63  /// GCMachineCodeAnalysis - This is a target-independent pass over the machine
64  /// function representation to identify safe points for the garbage collector
65  /// in the machine code. It inserts labels at safe points and populates a
66  /// GCMetadata record for each function.
67  class GCMachineCodeAnalysis : public MachineFunctionPass {
68    const TargetMachine *TM;
69    GCFunctionInfo *FI;
70    MachineModuleInfo *MMI;
71    const TargetInstrInfo *TII;
72
73    void FindSafePoints(MachineFunction &MF);
74    void VisitCallPoint(MachineBasicBlock::iterator MI);
75    MCSymbol *InsertLabel(MachineBasicBlock &MBB,
76                          MachineBasicBlock::iterator MI,
77                          DebugLoc DL) const;
78
79    void FindStackOffsets(MachineFunction &MF);
80
81  public:
82    static char ID;
83
84    GCMachineCodeAnalysis();
85    void getAnalysisUsage(AnalysisUsage &AU) const override;
86
87    bool runOnMachineFunction(MachineFunction &MF) override;
88  };
89
90}
91
92// -----------------------------------------------------------------------------
93
94GCStrategy::GCStrategy() :
95  NeededSafePoints(0),
96  CustomReadBarriers(false),
97  CustomWriteBarriers(false),
98  CustomRoots(false),
99  CustomSafePoints(false),
100  InitRoots(true),
101  UsesMetadata(false)
102{}
103
104bool GCStrategy::initializeCustomLowering(Module &M) { return false; }
105
106bool GCStrategy::performCustomLowering(Function &F) {
107  dbgs() << "gc " << getName() << " must override performCustomLowering.\n";
108  llvm_unreachable("must override performCustomLowering");
109}
110
111
112bool GCStrategy::findCustomSafePoints(GCFunctionInfo& FI, MachineFunction &F) {
113  dbgs() << "gc " << getName() << " must override findCustomSafePoints.\n";
114  llvm_unreachable(nullptr);
115}
116
117
118GCFunctionInfo *GCStrategy::insertFunctionInfo(const Function &F) {
119  Functions.push_back(make_unique<GCFunctionInfo>(F, *this));
120  return Functions.back().get();
121}
122
123// -----------------------------------------------------------------------------
124
125INITIALIZE_PASS_BEGIN(LowerIntrinsics, "gc-lowering", "GC Lowering",
126                      false, false)
127INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
128INITIALIZE_PASS_END(LowerIntrinsics, "gc-lowering", "GC Lowering", false, false)
129
130FunctionPass *llvm::createGCLoweringPass() {
131  return new LowerIntrinsics();
132}
133
134char LowerIntrinsics::ID = 0;
135
136LowerIntrinsics::LowerIntrinsics()
137  : FunctionPass(ID) {
138    initializeLowerIntrinsicsPass(*PassRegistry::getPassRegistry());
139  }
140
141const char *LowerIntrinsics::getPassName() const {
142  return "Lower Garbage Collection Instructions";
143}
144
145void LowerIntrinsics::getAnalysisUsage(AnalysisUsage &AU) const {
146  FunctionPass::getAnalysisUsage(AU);
147  AU.addRequired<GCModuleInfo>();
148  AU.addPreserved<DominatorTreeWrapperPass>();
149}
150
151/// doInitialization - If this module uses the GC intrinsics, find them now.
152bool LowerIntrinsics::doInitialization(Module &M) {
153  // FIXME: This is rather antisocial in the context of a JIT since it performs
154  //        work against the entire module. But this cannot be done at
155  //        runFunction time (initializeCustomLowering likely needs to change
156  //        the module).
157  GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
158  assert(MI && "LowerIntrinsics didn't require GCModuleInfo!?");
159  for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
160    if (!I->isDeclaration() && I->hasGC())
161      MI->getFunctionInfo(*I); // Instantiate the GC strategy.
162
163  bool MadeChange = false;
164  for (GCModuleInfo::iterator I = MI->begin(), E = MI->end(); I != E; ++I)
165    if (NeedsCustomLoweringPass(**I))
166      if ((*I)->initializeCustomLowering(M))
167        MadeChange = true;
168
169  return MadeChange;
170}
171
172bool LowerIntrinsics::InsertRootInitializers(Function &F, AllocaInst **Roots,
173                                                          unsigned Count) {
174  // Scroll past alloca instructions.
175  BasicBlock::iterator IP = F.getEntryBlock().begin();
176  while (isa<AllocaInst>(IP)) ++IP;
177
178  // Search for initializers in the initial BB.
179  SmallPtrSet<AllocaInst*,16> InitedRoots;
180  for (; !CouldBecomeSafePoint(IP); ++IP)
181    if (StoreInst *SI = dyn_cast<StoreInst>(IP))
182      if (AllocaInst *AI =
183          dyn_cast<AllocaInst>(SI->getOperand(1)->stripPointerCasts()))
184        InitedRoots.insert(AI);
185
186  // Add root initializers.
187  bool MadeChange = false;
188
189  for (AllocaInst **I = Roots, **E = Roots + Count; I != E; ++I)
190    if (!InitedRoots.count(*I)) {
191      StoreInst* SI = new StoreInst(ConstantPointerNull::get(cast<PointerType>(
192                        cast<PointerType>((*I)->getType())->getElementType())),
193                        *I);
194      SI->insertAfter(*I);
195      MadeChange = true;
196    }
197
198  return MadeChange;
199}
200
201bool LowerIntrinsics::NeedsDefaultLoweringPass(const GCStrategy &C) {
202  // Default lowering is necessary only if read or write barriers have a default
203  // action. The default for roots is no action.
204  return !C.customWriteBarrier()
205      || !C.customReadBarrier()
206      || C.initializeRoots();
207}
208
209bool LowerIntrinsics::NeedsCustomLoweringPass(const GCStrategy &C) {
210  // Custom lowering is only necessary if enabled for some action.
211  return C.customWriteBarrier()
212      || C.customReadBarrier()
213      || C.customRoots();
214}
215
216/// CouldBecomeSafePoint - Predicate to conservatively determine whether the
217/// instruction could introduce a safe point.
218bool LowerIntrinsics::CouldBecomeSafePoint(Instruction *I) {
219  // The natural definition of instructions which could introduce safe points
220  // are:
221  //
222  //   - call, invoke (AfterCall, BeforeCall)
223  //   - phis (Loops)
224  //   - invoke, ret, unwind (Exit)
225  //
226  // However, instructions as seemingly inoccuous as arithmetic can become
227  // libcalls upon lowering (e.g., div i64 on a 32-bit platform), so instead
228  // it is necessary to take a conservative approach.
229
230  if (isa<AllocaInst>(I) || isa<GetElementPtrInst>(I) ||
231      isa<StoreInst>(I) || isa<LoadInst>(I))
232    return false;
233
234  // llvm.gcroot is safe because it doesn't do anything at runtime.
235  if (CallInst *CI = dyn_cast<CallInst>(I))
236    if (Function *F = CI->getCalledFunction())
237      if (unsigned IID = F->getIntrinsicID())
238        if (IID == Intrinsic::gcroot)
239          return false;
240
241  return true;
242}
243
244/// runOnFunction - Replace gcread/gcwrite intrinsics with loads and stores.
245/// Leave gcroot intrinsics; the code generator needs to see those.
246bool LowerIntrinsics::runOnFunction(Function &F) {
247  // Quick exit for functions that do not use GC.
248  if (!F.hasGC())
249    return false;
250
251  GCFunctionInfo &FI = getAnalysis<GCModuleInfo>().getFunctionInfo(F);
252  GCStrategy &S = FI.getStrategy();
253
254  bool MadeChange = false;
255
256  if (NeedsDefaultLoweringPass(S))
257    MadeChange |= PerformDefaultLowering(F, S);
258
259  bool UseCustomLoweringPass = NeedsCustomLoweringPass(S);
260  if (UseCustomLoweringPass)
261    MadeChange |= S.performCustomLowering(F);
262
263  // Custom lowering may modify the CFG, so dominators must be recomputed.
264  if (UseCustomLoweringPass) {
265    if (DominatorTreeWrapperPass *DTWP =
266            getAnalysisIfAvailable<DominatorTreeWrapperPass>())
267      DTWP->getDomTree().recalculate(F);
268  }
269
270  return MadeChange;
271}
272
273bool LowerIntrinsics::PerformDefaultLowering(Function &F, GCStrategy &S) {
274  bool LowerWr = !S.customWriteBarrier();
275  bool LowerRd = !S.customReadBarrier();
276  bool InitRoots = S.initializeRoots();
277
278  SmallVector<AllocaInst*, 32> Roots;
279
280  bool MadeChange = false;
281  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
282    for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
283      if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) {
284        Function *F = CI->getCalledFunction();
285        switch (F->getIntrinsicID()) {
286        case Intrinsic::gcwrite:
287          if (LowerWr) {
288            // Replace a write barrier with a simple store.
289            Value *St = new StoreInst(CI->getArgOperand(0),
290                                      CI->getArgOperand(2), CI);
291            CI->replaceAllUsesWith(St);
292            CI->eraseFromParent();
293          }
294          break;
295        case Intrinsic::gcread:
296          if (LowerRd) {
297            // Replace a read barrier with a simple load.
298            Value *Ld = new LoadInst(CI->getArgOperand(1), "", CI);
299            Ld->takeName(CI);
300            CI->replaceAllUsesWith(Ld);
301            CI->eraseFromParent();
302          }
303          break;
304        case Intrinsic::gcroot:
305          if (InitRoots) {
306            // Initialize the GC root, but do not delete the intrinsic. The
307            // backend needs the intrinsic to flag the stack slot.
308            Roots.push_back(cast<AllocaInst>(
309                              CI->getArgOperand(0)->stripPointerCasts()));
310          }
311          break;
312        default:
313          continue;
314        }
315
316        MadeChange = true;
317      }
318    }
319  }
320
321  if (Roots.size())
322    MadeChange |= InsertRootInitializers(F, Roots.begin(), Roots.size());
323
324  return MadeChange;
325}
326
327// -----------------------------------------------------------------------------
328
329char GCMachineCodeAnalysis::ID = 0;
330char &llvm::GCMachineCodeAnalysisID = GCMachineCodeAnalysis::ID;
331
332INITIALIZE_PASS(GCMachineCodeAnalysis, "gc-analysis",
333                "Analyze Machine Code For Garbage Collection", false, false)
334
335GCMachineCodeAnalysis::GCMachineCodeAnalysis()
336  : MachineFunctionPass(ID) {}
337
338void GCMachineCodeAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
339  MachineFunctionPass::getAnalysisUsage(AU);
340  AU.setPreservesAll();
341  AU.addRequired<MachineModuleInfo>();
342  AU.addRequired<GCModuleInfo>();
343}
344
345MCSymbol *GCMachineCodeAnalysis::InsertLabel(MachineBasicBlock &MBB,
346                                             MachineBasicBlock::iterator MI,
347                                             DebugLoc DL) const {
348  MCSymbol *Label = MBB.getParent()->getContext().CreateTempSymbol();
349  BuildMI(MBB, MI, DL, TII->get(TargetOpcode::GC_LABEL)).addSym(Label);
350  return Label;
351}
352
353void GCMachineCodeAnalysis::VisitCallPoint(MachineBasicBlock::iterator CI) {
354  // Find the return address (next instruction), too, so as to bracket the call
355  // instruction.
356  MachineBasicBlock::iterator RAI = CI;
357  ++RAI;
358
359  if (FI->getStrategy().needsSafePoint(GC::PreCall)) {
360    MCSymbol* Label = InsertLabel(*CI->getParent(), CI, CI->getDebugLoc());
361    FI->addSafePoint(GC::PreCall, Label, CI->getDebugLoc());
362  }
363
364  if (FI->getStrategy().needsSafePoint(GC::PostCall)) {
365    MCSymbol* Label = InsertLabel(*CI->getParent(), RAI, CI->getDebugLoc());
366    FI->addSafePoint(GC::PostCall, Label, CI->getDebugLoc());
367  }
368}
369
370void GCMachineCodeAnalysis::FindSafePoints(MachineFunction &MF) {
371  for (MachineFunction::iterator BBI = MF.begin(),
372                                 BBE = MF.end(); BBI != BBE; ++BBI)
373    for (MachineBasicBlock::iterator MI = BBI->begin(),
374                                     ME = BBI->end(); MI != ME; ++MI)
375      if (MI->isCall())
376        VisitCallPoint(MI);
377}
378
379void GCMachineCodeAnalysis::FindStackOffsets(MachineFunction &MF) {
380  const TargetFrameLowering *TFI = TM->getFrameLowering();
381  assert(TFI && "TargetRegisterInfo not available!");
382
383  for (GCFunctionInfo::roots_iterator RI = FI->roots_begin();
384       RI != FI->roots_end();) {
385    // If the root references a dead object, no need to keep it.
386    if (MF.getFrameInfo()->isDeadObjectIndex(RI->Num)) {
387      RI = FI->removeStackRoot(RI);
388    } else {
389      RI->StackOffset = TFI->getFrameIndexOffset(MF, RI->Num);
390      ++RI;
391    }
392  }
393}
394
395bool GCMachineCodeAnalysis::runOnMachineFunction(MachineFunction &MF) {
396  // Quick exit for functions that do not use GC.
397  if (!MF.getFunction()->hasGC())
398    return false;
399
400  FI = &getAnalysis<GCModuleInfo>().getFunctionInfo(*MF.getFunction());
401  if (!FI->getStrategy().needsSafePoints())
402    return false;
403
404  TM = &MF.getTarget();
405  MMI = &getAnalysis<MachineModuleInfo>();
406  TII = TM->getInstrInfo();
407
408  // Find the size of the stack frame.
409  FI->setFrameSize(MF.getFrameInfo()->getStackSize());
410
411  // Find all safe points.
412  if (FI->getStrategy().customSafePoints()) {
413    FI->getStrategy().findCustomSafePoints(*FI, MF);
414  } else {
415    FindSafePoints(MF);
416  }
417
418  // Find the stack offsets for all roots.
419  FindStackOffsets(MF);
420
421  return false;
422}
423