GCStrategy.cpp revision b86413602e2d1d0d510016044092949168ac4d3f 
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// MachineCodeAnalysis identifies the GC safe points in the machine code. Roots
14// are identified in SelectionDAGISel.
15//
16//===----------------------------------------------------------------------===//
17
18#include "llvm/CodeGen/GCStrategy.h"
19#include "llvm/CodeGen/Passes.h"
20#include "llvm/IntrinsicInst.h"
21#include "llvm/Module.h"
22#include "llvm/CodeGen/MachineFrameInfo.h"
23#include "llvm/CodeGen/MachineFunctionPass.h"
24#include "llvm/CodeGen/MachineInstrBuilder.h"
25#include "llvm/CodeGen/MachineModuleInfo.h"
26#include "llvm/Target/TargetFrameInfo.h"
27#include "llvm/Target/TargetInstrInfo.h"
28#include "llvm/Target/TargetMachine.h"
29#include "llvm/Target/TargetRegisterInfo.h"
30#include "llvm/Support/Debug.h"
31#include "llvm/Support/ErrorHandling.h"
32#include "llvm/Support/raw_ostream.h"
33
34using namespace llvm;
35
36namespace {
37
38  /// LowerIntrinsics - This pass rewrites calls to the llvm.gcread or
39  /// llvm.gcwrite intrinsics, replacing them with simple loads and stores as
40  /// directed by the GCStrategy. It also performs automatic root initialization
41  /// and custom intrinsic lowering.
42  class LowerIntrinsics : public FunctionPass {
43    static bool NeedsDefaultLoweringPass(const GCStrategy &C);
44    static bool NeedsCustomLoweringPass(const GCStrategy &C);
45    static bool CouldBecomeSafePoint(Instruction *I);
46    bool PerformDefaultLowering(Function &F, GCStrategy &Coll);
47    static bool InsertRootInitializers(Function &F,
48                                       AllocaInst **Roots, unsigned Count);
49
50  public:
51    static char ID;
52
53    LowerIntrinsics();
54    const char *getPassName() const;
55    void getAnalysisUsage(AnalysisUsage &AU) const;
56
57    bool doInitialization(Module &M);
58    bool runOnFunction(Function &F);
59  };
60
61
62  /// MachineCodeAnalysis - This is a target-independent pass over the machine
63  /// function representation to identify safe points for the garbage collector
64  /// in the machine code. It inserts labels at safe points and populates a
65  /// GCMetadata record for each function.
66  class MachineCodeAnalysis : public MachineFunctionPass {
67    const TargetMachine *TM;
68    GCFunctionInfo *FI;
69    MachineModuleInfo *MMI;
70    const TargetInstrInfo *TII;
71
72    void FindSafePoints(MachineFunction &MF);
73    void VisitCallPoint(MachineBasicBlock::iterator MI);
74    MCSymbol *InsertLabel(MachineBasicBlock &MBB,
75                          MachineBasicBlock::iterator MI,
76                          DebugLoc DL) const;
77
78    void FindStackOffsets(MachineFunction &MF);
79
80  public:
81    static char ID;
82
83    MachineCodeAnalysis();
84    const char *getPassName() const;
85    void getAnalysisUsage(AnalysisUsage &AU) const;
86
87    bool runOnMachineFunction(MachineFunction &MF);
88  };
89
90}
91
92// -----------------------------------------------------------------------------
93
94GCStrategy::GCStrategy() :
95  NeededSafePoints(0),
96  CustomReadBarriers(false),
97  CustomWriteBarriers(false),
98  CustomRoots(false),
99  InitRoots(true),
100  UsesMetadata(false)
101{}
102
103GCStrategy::~GCStrategy() {
104  for (iterator I = begin(), E = end(); I != E; ++I)
105    delete *I;
106
107  Functions.clear();
108}
109
110bool GCStrategy::initializeCustomLowering(Module &M) { return false; }
111
112bool GCStrategy::performCustomLowering(Function &F) {
113  dbgs() << "gc " << getName() << " must override performCustomLowering.\n";
114  llvm_unreachable(0);
115  return 0;
116}
117
118GCFunctionInfo *GCStrategy::insertFunctionInfo(const Function &F) {
119  GCFunctionInfo *FI = new GCFunctionInfo(F, *this);
120  Functions.push_back(FI);
121  return FI;
122}
123
124// -----------------------------------------------------------------------------
125
126FunctionPass *llvm::createGCLoweringPass() {
127  return new LowerIntrinsics();
128}
129
130char LowerIntrinsics::ID = 0;
131
132LowerIntrinsics::LowerIntrinsics()
133  : FunctionPass(&ID) {}
134
135const char *LowerIntrinsics::getPassName() const {
136  return "Lower Garbage Collection Instructions";
137}
138
139void LowerIntrinsics::getAnalysisUsage(AnalysisUsage &AU) const {
140  FunctionPass::getAnalysisUsage(AU);
141  AU.addRequired<GCModuleInfo>();
142}
143
144/// doInitialization - If this module uses the GC intrinsics, find them now.
145bool LowerIntrinsics::doInitialization(Module &M) {
146  // FIXME: This is rather antisocial in the context of a JIT since it performs
147  //        work against the entire module. But this cannot be done at
148  //        runFunction time (initializeCustomLowering likely needs to change
149  //        the module).
150  GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
151  assert(MI && "LowerIntrinsics didn't require GCModuleInfo!?");
152  for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
153    if (!I->isDeclaration() && I->hasGC())
154      MI->getFunctionInfo(*I); // Instantiate the GC strategy.
155
156  bool MadeChange = false;
157  for (GCModuleInfo::iterator I = MI->begin(), E = MI->end(); I != E; ++I)
158    if (NeedsCustomLoweringPass(**I))
159      if ((*I)->initializeCustomLowering(M))
160        MadeChange = true;
161
162  return MadeChange;
163}
164
165bool LowerIntrinsics::InsertRootInitializers(Function &F, AllocaInst **Roots,
166                                                          unsigned Count) {
167  // Scroll past alloca instructions.
168  BasicBlock::iterator IP = F.getEntryBlock().begin();
169  while (isa<AllocaInst>(IP)) ++IP;
170
171  // Search for initializers in the initial BB.
172  SmallPtrSet<AllocaInst*,16> InitedRoots;
173  for (; !CouldBecomeSafePoint(IP); ++IP)
174    if (StoreInst *SI = dyn_cast<StoreInst>(IP))
175      if (AllocaInst *AI =
176          dyn_cast<AllocaInst>(SI->getOperand(1)->stripPointerCasts()))
177        InitedRoots.insert(AI);
178
179  // Add root initializers.
180  bool MadeChange = false;
181
182  for (AllocaInst **I = Roots, **E = Roots + Count; I != E; ++I)
183    if (!InitedRoots.count(*I)) {
184      StoreInst* SI = new StoreInst(ConstantPointerNull::get(cast<PointerType>(
185                        cast<PointerType>((*I)->getType())->getElementType())),
186                        *I);
187      SI->insertAfter(*I);
188      MadeChange = true;
189    }
190
191  return MadeChange;
192}
193
194bool LowerIntrinsics::NeedsDefaultLoweringPass(const GCStrategy &C) {
195  // Default lowering is necessary only if read or write barriers have a default
196  // action. The default for roots is no action.
197  return !C.customWriteBarrier()
198      || !C.customReadBarrier()
199      || C.initializeRoots();
200}
201
202bool LowerIntrinsics::NeedsCustomLoweringPass(const GCStrategy &C) {
203  // Custom lowering is only necessary if enabled for some action.
204  return C.customWriteBarrier()
205      || C.customReadBarrier()
206      || C.customRoots();
207}
208
209/// CouldBecomeSafePoint - Predicate to conservatively determine whether the
210/// instruction could introduce a safe point.
211bool LowerIntrinsics::CouldBecomeSafePoint(Instruction *I) {
212  // The natural definition of instructions which could introduce safe points
213  // are:
214  //
215  //   - call, invoke (AfterCall, BeforeCall)
216  //   - phis (Loops)
217  //   - invoke, ret, unwind (Exit)
218  //
219  // However, instructions as seemingly inoccuous as arithmetic can become
220  // libcalls upon lowering (e.g., div i64 on a 32-bit platform), so instead
221  // it is necessary to take a conservative approach.
222
223  if (isa<AllocaInst>(I) || isa<GetElementPtrInst>(I) ||
224      isa<StoreInst>(I) || isa<LoadInst>(I))
225    return false;
226
227  // llvm.gcroot is safe because it doesn't do anything at runtime.
228  if (CallInst *CI = dyn_cast<CallInst>(I))
229    if (Function *F = CI->getCalledFunction())
230      if (unsigned IID = F->getIntrinsicID())
231        if (IID == Intrinsic::gcroot)
232          return false;
233
234  return true;
235}
236
237/// runOnFunction - Replace gcread/gcwrite intrinsics with loads and stores.
238/// Leave gcroot intrinsics; the code generator needs to see those.
239bool LowerIntrinsics::runOnFunction(Function &F) {
240  // Quick exit for functions that do not use GC.
241  if (!F.hasGC())
242    return false;
243
244  GCFunctionInfo &FI = getAnalysis<GCModuleInfo>().getFunctionInfo(F);
245  GCStrategy &S = FI.getStrategy();
246
247  bool MadeChange = false;
248
249  if (NeedsDefaultLoweringPass(S))
250    MadeChange |= PerformDefaultLowering(F, S);
251
252  if (NeedsCustomLoweringPass(S))
253    MadeChange |= S.performCustomLowering(F);
254
255  return MadeChange;
256}
257
258bool LowerIntrinsics::PerformDefaultLowering(Function &F, GCStrategy &S) {
259  bool LowerWr = !S.customWriteBarrier();
260  bool LowerRd = !S.customReadBarrier();
261  bool InitRoots = S.initializeRoots();
262
263  SmallVector<AllocaInst*,32> Roots;
264
265  bool MadeChange = false;
266  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
267    for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
268      if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) {
269        Function *F = CI->getCalledFunction();
270        switch (F->getIntrinsicID()) {
271        case Intrinsic::gcwrite:
272          if (LowerWr) {
273            // Replace a write barrier with a simple store.
274            Value *St = new StoreInst(CI->getOperand(0), CI->getOperand(2), CI);
275            CI->replaceAllUsesWith(St);
276            CI->eraseFromParent();
277          }
278          break;
279        case Intrinsic::gcread:
280          if (LowerRd) {
281            // Replace a read barrier with a simple load.
282            Value *Ld = new LoadInst(CI->getOperand(1), "", CI);
283            Ld->takeName(CI);
284            CI->replaceAllUsesWith(Ld);
285            CI->eraseFromParent();
286          }
287          break;
288        case Intrinsic::gcroot:
289          if (InitRoots) {
290            // Initialize the GC root, but do not delete the intrinsic. The
291            // backend needs the intrinsic to flag the stack slot.
292            Roots.push_back(cast<AllocaInst>(
293                              CI->getOperand(0)->stripPointerCasts()));
294          }
295          break;
296        default:
297          continue;
298        }
299
300        MadeChange = true;
301      }
302    }
303  }
304
305  if (Roots.size())
306    MadeChange |= InsertRootInitializers(F, Roots.begin(), Roots.size());
307
308  return MadeChange;
309}
310
311// -----------------------------------------------------------------------------
312
313FunctionPass *llvm::createGCMachineCodeAnalysisPass() {
314  return new MachineCodeAnalysis();
315}
316
317char MachineCodeAnalysis::ID = 0;
318
319MachineCodeAnalysis::MachineCodeAnalysis()
320  : MachineFunctionPass(&ID) {}
321
322const char *MachineCodeAnalysis::getPassName() const {
323  return "Analyze Machine Code For Garbage Collection";
324}
325
326void MachineCodeAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
327  MachineFunctionPass::getAnalysisUsage(AU);
328  AU.setPreservesAll();
329  AU.addRequired<MachineModuleInfo>();
330  AU.addRequired<GCModuleInfo>();
331}
332
333MCSymbol *MachineCodeAnalysis::InsertLabel(MachineBasicBlock &MBB,
334                                           MachineBasicBlock::iterator MI,
335                                           DebugLoc DL) const {
336  MCSymbol *Label = MBB.getParent()->getContext().CreateTempSymbol();
337  BuildMI(MBB, MI, DL, TII->get(TargetOpcode::GC_LABEL)).addSym(Label);
338  return Label;
339}
340
341void MachineCodeAnalysis::VisitCallPoint(MachineBasicBlock::iterator CI) {
342  // Find the return address (next instruction), too, so as to bracket the call
343  // instruction.
344  MachineBasicBlock::iterator RAI = CI;
345  ++RAI;
346
347  if (FI->getStrategy().needsSafePoint(GC::PreCall))
348    FI->addSafePoint(GC::PreCall, InsertLabel(*CI->getParent(), CI,
349                                              CI->getDebugLoc()));
350
351  if (FI->getStrategy().needsSafePoint(GC::PostCall))
352    FI->addSafePoint(GC::PostCall, InsertLabel(*CI->getParent(), RAI,
353                                               CI->getDebugLoc()));
354}
355
356void MachineCodeAnalysis::FindSafePoints(MachineFunction &MF) {
357  for (MachineFunction::iterator BBI = MF.begin(),
358                                 BBE = MF.end(); BBI != BBE; ++BBI)
359    for (MachineBasicBlock::iterator MI = BBI->begin(),
360                                     ME = BBI->end(); MI != ME; ++MI)
361      if (MI->getDesc().isCall())
362        VisitCallPoint(MI);
363}
364
365void MachineCodeAnalysis::FindStackOffsets(MachineFunction &MF) {
366  const TargetRegisterInfo *TRI = TM->getRegisterInfo();
367  assert(TRI && "TargetRegisterInfo not available!");
368
369  for (GCFunctionInfo::roots_iterator RI = FI->roots_begin(),
370                                      RE = FI->roots_end(); RI != RE; ++RI)
371    RI->StackOffset = TRI->getFrameIndexOffset(MF, RI->Num);
372}
373
374bool MachineCodeAnalysis::runOnMachineFunction(MachineFunction &MF) {
375  // Quick exit for functions that do not use GC.
376  if (!MF.getFunction()->hasGC())
377    return false;
378
379  FI = &getAnalysis<GCModuleInfo>().getFunctionInfo(*MF.getFunction());
380  if (!FI->getStrategy().needsSafePoints())
381    return false;
382
383  TM = &MF.getTarget();
384  MMI = &getAnalysis<MachineModuleInfo>();
385  TII = TM->getInstrInfo();
386
387  // Find the size of the stack frame.
388  FI->setFrameSize(MF.getFrameInfo()->getStackSize());
389
390  // Find all safe points.
391  FindSafePoints(MF);
392
393  // Find the stack offsets for all roots.
394  FindStackOffsets(MF);
395
396  return false;
397}
398