GCStrategy.cpp revision 3b26693d33f77dc8c1ad358c3008c1b6e38c7be4 
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/Compiler.h"
31
32using namespace llvm;
33
34namespace {
35
36  /// LowerIntrinsics - This pass rewrites calls to the llvm.gcread or
37  /// llvm.gcwrite intrinsics, replacing them with simple loads and stores as
38  /// directed by the GCStrategy. It also performs automatic root initialization
39  /// and custom intrinsic lowering.
40  class VISIBILITY_HIDDEN LowerIntrinsics : public FunctionPass {
41    static bool NeedsDefaultLoweringPass(const GCStrategy &C);
42    static bool NeedsCustomLoweringPass(const GCStrategy &C);
43    static bool CouldBecomeSafePoint(Instruction *I);
44    bool PerformDefaultLowering(Function &F, GCStrategy &Coll);
45    static bool InsertRootInitializers(Function &F,
46                                       AllocaInst **Roots, unsigned Count);
47
48  public:
49    static char ID;
50
51    LowerIntrinsics();
52    const char *getPassName() const;
53    void getAnalysisUsage(AnalysisUsage &AU) const;
54
55    bool doInitialization(Module &M);
56    bool runOnFunction(Function &F);
57  };
58
59
60  /// MachineCodeAnalysis - This is a target-independent pass over the machine
61  /// function representation to identify safe points for the garbage collector
62  /// in the machine code. It inserts labels at safe points and populates a
63  /// GCMetadata record for each function.
64  class VISIBILITY_HIDDEN MachineCodeAnalysis : public MachineFunctionPass {
65    const TargetMachine *TM;
66    GCFunctionInfo *FI;
67    MachineModuleInfo *MMI;
68    const TargetInstrInfo *TII;
69
70    void FindSafePoints(MachineFunction &MF);
71    void VisitCallPoint(MachineBasicBlock::iterator MI);
72    unsigned InsertLabel(MachineBasicBlock &MBB,
73                         MachineBasicBlock::iterator MI) const;
74
75    void FindStackOffsets(MachineFunction &MF);
76
77  public:
78    static char ID;
79
80    MachineCodeAnalysis();
81    const char *getPassName() const;
82    void getAnalysisUsage(AnalysisUsage &AU) const;
83
84    bool runOnMachineFunction(MachineFunction &MF);
85  };
86
87}
88
89// -----------------------------------------------------------------------------
90
91GCStrategy::GCStrategy() :
92  NeededSafePoints(0),
93  CustomReadBarriers(false),
94  CustomWriteBarriers(false),
95  CustomRoots(false),
96  InitRoots(true),
97  UsesMetadata(false)
98{}
99
100GCStrategy::~GCStrategy() {
101  for (iterator I = begin(), E = end(); I != E; ++I)
102    delete *I;
103
104  Functions.clear();
105}
106
107bool GCStrategy::initializeCustomLowering(Module &M) { return false; }
108
109bool GCStrategy::performCustomLowering(Function &F) {
110  cerr << "gc " << getName() << " must override performCustomLowering.\n";
111  abort();
112  return 0;
113}
114
115GCFunctionInfo *GCStrategy::insertFunctionInfo(const Function &F) {
116  GCFunctionInfo *FI = new GCFunctionInfo(F, *this);
117  Functions.push_back(FI);
118  return FI;
119}
120
121// -----------------------------------------------------------------------------
122
123FunctionPass *llvm::createGCLoweringPass() {
124  return new LowerIntrinsics();
125}
126
127char LowerIntrinsics::ID = 0;
128
129LowerIntrinsics::LowerIntrinsics()
130  : FunctionPass((intptr_t)&ID) {}
131
132const char *LowerIntrinsics::getPassName() const {
133  return "Lower Garbage Collection Instructions";
134}
135
136void LowerIntrinsics::getAnalysisUsage(AnalysisUsage &AU) const {
137  FunctionPass::getAnalysisUsage(AU);
138  AU.addRequired<GCModuleInfo>();
139}
140
141/// doInitialization - If this module uses the GC intrinsics, find them now.
142bool LowerIntrinsics::doInitialization(Module &M) {
143  // FIXME: This is rather antisocial in the context of a JIT since it performs
144  //        work against the entire module. But this cannot be done at
145  //        runFunction time (initializeCustomLowering likely needs to change
146  //        the module).
147  GCModuleInfo *MI = getAnalysisToUpdate<GCModuleInfo>();
148  assert(MI && "LowerIntrinsics didn't require GCModuleInfo!?");
149  for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
150    if (!I->isDeclaration() && I->hasGC())
151      MI->getFunctionInfo(*I); // Instantiate the GC strategy.
152
153  bool MadeChange = false;
154  for (GCModuleInfo::iterator I = MI->begin(), E = MI->end(); I != E; ++I)
155    if (NeedsCustomLoweringPass(**I))
156      if ((*I)->initializeCustomLowering(M))
157        MadeChange = true;
158
159  return MadeChange;
160}
161
162bool LowerIntrinsics::InsertRootInitializers(Function &F, AllocaInst **Roots,
163                                                          unsigned Count) {
164  // Scroll past alloca instructions.
165  BasicBlock::iterator IP = F.getEntryBlock().begin();
166  while (isa<AllocaInst>(IP)) ++IP;
167
168  // Search for initializers in the initial BB.
169  SmallPtrSet<AllocaInst*,16> InitedRoots;
170  for (; !CouldBecomeSafePoint(IP); ++IP)
171    if (StoreInst *SI = dyn_cast<StoreInst>(IP))
172      if (AllocaInst *AI =
173          dyn_cast<AllocaInst>(SI->getOperand(1)->stripPointerCasts()))
174        InitedRoots.insert(AI);
175
176  // Add root initializers.
177  bool MadeChange = false;
178
179  for (AllocaInst **I = Roots, **E = Roots + Count; I != E; ++I)
180    if (!InitedRoots.count(*I)) {
181      new StoreInst(ConstantPointerNull::get(cast<PointerType>(
182                      cast<PointerType>((*I)->getType())->getElementType())),
183                    *I, IP);
184      MadeChange = true;
185    }
186
187  return MadeChange;
188}
189
190bool LowerIntrinsics::NeedsDefaultLoweringPass(const GCStrategy &C) {
191  // Default lowering is necessary only if read or write barriers have a default
192  // action. The default for roots is no action.
193  return !C.customWriteBarrier()
194      || !C.customReadBarrier()
195      || C.initializeRoots();
196}
197
198bool LowerIntrinsics::NeedsCustomLoweringPass(const GCStrategy &C) {
199  // Custom lowering is only necessary if enabled for some action.
200  return C.customWriteBarrier()
201      || C.customReadBarrier()
202      || C.customRoots();
203}
204
205/// CouldBecomeSafePoint - Predicate to conservatively determine whether the
206/// instruction could introduce a safe point.
207bool LowerIntrinsics::CouldBecomeSafePoint(Instruction *I) {
208  // The natural definition of instructions which could introduce safe points
209  // are:
210  //
211  //   - call, invoke (AfterCall, BeforeCall)
212  //   - phis (Loops)
213  //   - invoke, ret, unwind (Exit)
214  //
215  // However, instructions as seemingly inoccuous as arithmetic can become
216  // libcalls upon lowering (e.g., div i64 on a 32-bit platform), so instead
217  // it is necessary to take a conservative approach.
218
219  if (isa<AllocaInst>(I) || isa<GetElementPtrInst>(I) ||
220      isa<StoreInst>(I) || isa<LoadInst>(I))
221    return false;
222
223  // llvm.gcroot is safe because it doesn't do anything at runtime.
224  if (CallInst *CI = dyn_cast<CallInst>(I))
225    if (Function *F = CI->getCalledFunction())
226      if (unsigned IID = F->getIntrinsicID())
227        if (IID == Intrinsic::gcroot)
228          return false;
229
230  return true;
231}
232
233/// runOnFunction - Replace gcread/gcwrite intrinsics with loads and stores.
234/// Leave gcroot intrinsics; the code generator needs to see those.
235bool LowerIntrinsics::runOnFunction(Function &F) {
236  // Quick exit for functions that do not use GC.
237  if (!F.hasGC())
238    return false;
239
240  GCFunctionInfo &FI = getAnalysis<GCModuleInfo>().getFunctionInfo(F);
241  GCStrategy &S = FI.getStrategy();
242
243  bool MadeChange = false;
244
245  if (NeedsDefaultLoweringPass(S))
246    MadeChange |= PerformDefaultLowering(F, S);
247
248  if (NeedsCustomLoweringPass(S))
249    MadeChange |= S.performCustomLowering(F);
250
251  return MadeChange;
252}
253
254bool LowerIntrinsics::PerformDefaultLowering(Function &F, GCStrategy &S) {
255  bool LowerWr = !S.customWriteBarrier();
256  bool LowerRd = !S.customReadBarrier();
257  bool InitRoots = S.initializeRoots();
258
259  SmallVector<AllocaInst*,32> Roots;
260
261  bool MadeChange = false;
262  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
263    for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
264      if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) {
265        Function *F = CI->getCalledFunction();
266        switch (F->getIntrinsicID()) {
267        case Intrinsic::gcwrite:
268          if (LowerWr) {
269            // Replace a write barrier with a simple store.
270            Value *St = new StoreInst(CI->getOperand(1), CI->getOperand(3), CI);
271            CI->replaceAllUsesWith(St);
272            CI->eraseFromParent();
273          }
274          break;
275        case Intrinsic::gcread:
276          if (LowerRd) {
277            // Replace a read barrier with a simple load.
278            Value *Ld = new LoadInst(CI->getOperand(2), "", CI);
279            Ld->takeName(CI);
280            CI->replaceAllUsesWith(Ld);
281            CI->eraseFromParent();
282          }
283          break;
284        case Intrinsic::gcroot:
285          if (InitRoots) {
286            // Initialize the GC root, but do not delete the intrinsic. The
287            // backend needs the intrinsic to flag the stack slot.
288            Roots.push_back(cast<AllocaInst>(
289                              CI->getOperand(1)->stripPointerCasts()));
290          }
291          break;
292        default:
293          continue;
294        }
295
296        MadeChange = true;
297      }
298    }
299  }
300
301  if (Roots.size())
302    MadeChange |= InsertRootInitializers(F, Roots.begin(), Roots.size());
303
304  return MadeChange;
305}
306
307// -----------------------------------------------------------------------------
308
309FunctionPass *llvm::createGCMachineCodeAnalysisPass() {
310  return new MachineCodeAnalysis();
311}
312
313char MachineCodeAnalysis::ID = 0;
314
315MachineCodeAnalysis::MachineCodeAnalysis()
316  : MachineFunctionPass(intptr_t(&ID)) {}
317
318const char *MachineCodeAnalysis::getPassName() const {
319  return "Analyze Machine Code For Garbage Collection";
320}
321
322void MachineCodeAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
323  MachineFunctionPass::getAnalysisUsage(AU);
324  AU.setPreservesAll();
325  AU.addRequired<MachineModuleInfo>();
326  AU.addRequired<GCModuleInfo>();
327}
328
329unsigned MachineCodeAnalysis::InsertLabel(MachineBasicBlock &MBB,
330                                     MachineBasicBlock::iterator MI) const {
331  unsigned Label = MMI->NextLabelID();
332  BuildMI(MBB, MI, TII->get(TargetInstrInfo::GC_LABEL)).addImm(Label);
333  return Label;
334}
335
336void MachineCodeAnalysis::VisitCallPoint(MachineBasicBlock::iterator CI) {
337  // Find the return address (next instruction), too, so as to bracket the call
338  // instruction.
339  MachineBasicBlock::iterator RAI = CI;
340  ++RAI;
341
342  if (FI->getStrategy().needsSafePoint(GC::PreCall))
343    FI->addSafePoint(GC::PreCall, InsertLabel(*CI->getParent(), CI));
344
345  if (FI->getStrategy().needsSafePoint(GC::PostCall))
346    FI->addSafePoint(GC::PostCall, InsertLabel(*CI->getParent(), RAI));
347}
348
349void MachineCodeAnalysis::FindSafePoints(MachineFunction &MF) {
350  for (MachineFunction::iterator BBI = MF.begin(),
351                                 BBE = MF.end(); BBI != BBE; ++BBI)
352    for (MachineBasicBlock::iterator MI = BBI->begin(),
353                                     ME = BBI->end(); MI != ME; ++MI)
354      if (MI->getDesc().isCall())
355        VisitCallPoint(MI);
356}
357
358void MachineCodeAnalysis::FindStackOffsets(MachineFunction &MF) {
359  const TargetRegisterInfo *TRI = TM->getRegisterInfo();
360  assert(TRI && "TargetRegisterInfo not available!");
361
362  for (GCFunctionInfo::roots_iterator RI = FI->roots_begin(),
363                                      RE = FI->roots_end(); RI != RE; ++RI)
364    RI->StackOffset = TRI->getFrameIndexOffset(MF, RI->Num);
365}
366
367bool MachineCodeAnalysis::runOnMachineFunction(MachineFunction &MF) {
368  // Quick exit for functions that do not use GC.
369  if (!MF.getFunction()->hasGC())
370    return false;
371
372  FI = &getAnalysis<GCModuleInfo>().getFunctionInfo(*MF.getFunction());
373  if (!FI->getStrategy().needsSafePoints())
374    return false;
375
376  TM = &MF.getTarget();
377  MMI = &getAnalysis<MachineModuleInfo>();
378  TII = TM->getInstrInfo();
379
380  // Find the size of the stack frame.
381  FI->setFrameSize(MF.getFrameInfo()->getStackSize());
382
383  // Find all safe points.
384  FindSafePoints(MF);
385
386  // Find the stack offsets for all roots.
387  FindStackOffsets(MF);
388
389  return false;
390}
391