GCStrategy.cpp revision 45cfe545ec8177262dabc70580ce05feaa1c3880 
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#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 VISIBILITY_HIDDEN 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 VISIBILITY_HIDDEN 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    unsigned InsertLabel(MachineBasicBlock &MBB,
75                         MachineBasicBlock::iterator MI) const;
76
77    void FindStackOffsets(MachineFunction &MF);
78
79  public:
80    static char ID;
81
82    MachineCodeAnalysis();
83    const char *getPassName() const;
84    void getAnalysisUsage(AnalysisUsage &AU) const;
85
86    bool runOnMachineFunction(MachineFunction &MF);
87  };
88
89}
90
91// -----------------------------------------------------------------------------
92
93GCStrategy::GCStrategy() :
94  NeededSafePoints(0),
95  CustomReadBarriers(false),
96  CustomWriteBarriers(false),
97  CustomRoots(false),
98  InitRoots(true),
99  UsesMetadata(false)
100{}
101
102GCStrategy::~GCStrategy() {
103  for (iterator I = begin(), E = end(); I != E; ++I)
104    delete *I;
105
106  Functions.clear();
107}
108
109bool GCStrategy::initializeCustomLowering(Module &M) { return false; }
110
111bool GCStrategy::performCustomLowering(Function &F) {
112  errs() << "gc " << getName() << " must override performCustomLowering.\n";
113  llvm_unreachable(0);
114  return 0;
115}
116
117GCFunctionInfo *GCStrategy::insertFunctionInfo(const Function &F) {
118  GCFunctionInfo *FI = new GCFunctionInfo(F, *this);
119  Functions.push_back(FI);
120  return FI;
121}
122
123// -----------------------------------------------------------------------------
124
125FunctionPass *llvm::createGCLoweringPass() {
126  return new LowerIntrinsics();
127}
128
129char LowerIntrinsics::ID = 0;
130
131LowerIntrinsics::LowerIntrinsics()
132  : FunctionPass(&ID) {}
133
134const char *LowerIntrinsics::getPassName() const {
135  return "Lower Garbage Collection Instructions";
136}
137
138void LowerIntrinsics::getAnalysisUsage(AnalysisUsage &AU) const {
139  FunctionPass::getAnalysisUsage(AU);
140  AU.addRequired<GCModuleInfo>();
141}
142
143/// doInitialization - If this module uses the GC intrinsics, find them now.
144bool LowerIntrinsics::doInitialization(Module &M) {
145  // FIXME: This is rather antisocial in the context of a JIT since it performs
146  //        work against the entire module. But this cannot be done at
147  //        runFunction time (initializeCustomLowering likely needs to change
148  //        the module).
149  GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
150  assert(MI && "LowerIntrinsics didn't require GCModuleInfo!?");
151  for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
152    if (!I->isDeclaration() && I->hasGC())
153      MI->getFunctionInfo(*I); // Instantiate the GC strategy.
154
155  bool MadeChange = false;
156  for (GCModuleInfo::iterator I = MI->begin(), E = MI->end(); I != E; ++I)
157    if (NeedsCustomLoweringPass(**I))
158      if ((*I)->initializeCustomLowering(M))
159        MadeChange = true;
160
161  return MadeChange;
162}
163
164bool LowerIntrinsics::InsertRootInitializers(Function &F, AllocaInst **Roots,
165                                                          unsigned Count) {
166  // Scroll past alloca instructions.
167  BasicBlock::iterator IP = F.getEntryBlock().begin();
168  while (isa<AllocaInst>(IP)) ++IP;
169
170  // Search for initializers in the initial BB.
171  SmallPtrSet<AllocaInst*,16> InitedRoots;
172  for (; !CouldBecomeSafePoint(IP); ++IP)
173    if (StoreInst *SI = dyn_cast<StoreInst>(IP))
174      if (AllocaInst *AI =
175          dyn_cast<AllocaInst>(SI->getOperand(1)->stripPointerCasts()))
176        InitedRoots.insert(AI);
177
178  // Add root initializers.
179  bool MadeChange = false;
180
181  for (AllocaInst **I = Roots, **E = Roots + Count; I != E; ++I)
182    if (!InitedRoots.count(*I)) {
183      new StoreInst(ConstantPointerNull::get(cast<PointerType>(
184                      cast<PointerType>((*I)->getType())->getElementType())),
185                    *I, IP);
186      MadeChange = true;
187    }
188
189  return MadeChange;
190}
191
192bool LowerIntrinsics::NeedsDefaultLoweringPass(const GCStrategy &C) {
193  // Default lowering is necessary only if read or write barriers have a default
194  // action. The default for roots is no action.
195  return !C.customWriteBarrier()
196      || !C.customReadBarrier()
197      || C.initializeRoots();
198}
199
200bool LowerIntrinsics::NeedsCustomLoweringPass(const GCStrategy &C) {
201  // Custom lowering is only necessary if enabled for some action.
202  return C.customWriteBarrier()
203      || C.customReadBarrier()
204      || C.customRoots();
205}
206
207/// CouldBecomeSafePoint - Predicate to conservatively determine whether the
208/// instruction could introduce a safe point.
209bool LowerIntrinsics::CouldBecomeSafePoint(Instruction *I) {
210  // The natural definition of instructions which could introduce safe points
211  // are:
212  //
213  //   - call, invoke (AfterCall, BeforeCall)
214  //   - phis (Loops)
215  //   - invoke, ret, unwind (Exit)
216  //
217  // However, instructions as seemingly inoccuous as arithmetic can become
218  // libcalls upon lowering (e.g., div i64 on a 32-bit platform), so instead
219  // it is necessary to take a conservative approach.
220
221  if (isa<AllocaInst>(I) || isa<GetElementPtrInst>(I) ||
222      isa<StoreInst>(I) || isa<LoadInst>(I))
223    return false;
224
225  // llvm.gcroot is safe because it doesn't do anything at runtime.
226  if (CallInst *CI = dyn_cast<CallInst>(I))
227    if (Function *F = CI->getCalledFunction())
228      if (unsigned IID = F->getIntrinsicID())
229        if (IID == Intrinsic::gcroot)
230          return false;
231
232  return true;
233}
234
235/// runOnFunction - Replace gcread/gcwrite intrinsics with loads and stores.
236/// Leave gcroot intrinsics; the code generator needs to see those.
237bool LowerIntrinsics::runOnFunction(Function &F) {
238  // Quick exit for functions that do not use GC.
239  if (!F.hasGC())
240    return false;
241
242  GCFunctionInfo &FI = getAnalysis<GCModuleInfo>().getFunctionInfo(F);
243  GCStrategy &S = FI.getStrategy();
244
245  bool MadeChange = false;
246
247  if (NeedsDefaultLoweringPass(S))
248    MadeChange |= PerformDefaultLowering(F, S);
249
250  if (NeedsCustomLoweringPass(S))
251    MadeChange |= S.performCustomLowering(F);
252
253  return MadeChange;
254}
255
256bool LowerIntrinsics::PerformDefaultLowering(Function &F, GCStrategy &S) {
257  bool LowerWr = !S.customWriteBarrier();
258  bool LowerRd = !S.customReadBarrier();
259  bool InitRoots = S.initializeRoots();
260
261  SmallVector<AllocaInst*,32> Roots;
262
263  bool MadeChange = false;
264  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
265    for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
266      if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) {
267        Function *F = CI->getCalledFunction();
268        switch (F->getIntrinsicID()) {
269        case Intrinsic::gcwrite:
270          if (LowerWr) {
271            // Replace a write barrier with a simple store.
272            Value *St = new StoreInst(CI->getOperand(1), CI->getOperand(3), CI);
273            CI->replaceAllUsesWith(St);
274            CI->eraseFromParent();
275          }
276          break;
277        case Intrinsic::gcread:
278          if (LowerRd) {
279            // Replace a read barrier with a simple load.
280            Value *Ld = new LoadInst(CI->getOperand(2), "", CI);
281            Ld->takeName(CI);
282            CI->replaceAllUsesWith(Ld);
283            CI->eraseFromParent();
284          }
285          break;
286        case Intrinsic::gcroot:
287          if (InitRoots) {
288            // Initialize the GC root, but do not delete the intrinsic. The
289            // backend needs the intrinsic to flag the stack slot.
290            Roots.push_back(cast<AllocaInst>(
291                              CI->getOperand(1)->stripPointerCasts()));
292          }
293          break;
294        default:
295          continue;
296        }
297
298        MadeChange = true;
299      }
300    }
301  }
302
303  if (Roots.size())
304    MadeChange |= InsertRootInitializers(F, Roots.begin(), Roots.size());
305
306  return MadeChange;
307}
308
309// -----------------------------------------------------------------------------
310
311FunctionPass *llvm::createGCMachineCodeAnalysisPass() {
312  return new MachineCodeAnalysis();
313}
314
315char MachineCodeAnalysis::ID = 0;
316
317MachineCodeAnalysis::MachineCodeAnalysis()
318  : MachineFunctionPass(&ID) {}
319
320const char *MachineCodeAnalysis::getPassName() const {
321  return "Analyze Machine Code For Garbage Collection";
322}
323
324void MachineCodeAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
325  MachineFunctionPass::getAnalysisUsage(AU);
326  AU.setPreservesAll();
327  AU.addRequired<MachineModuleInfo>();
328  AU.addRequired<GCModuleInfo>();
329}
330
331unsigned MachineCodeAnalysis::InsertLabel(MachineBasicBlock &MBB,
332                                     MachineBasicBlock::iterator MI) const {
333  unsigned Label = MMI->NextLabelID();
334  // N.B. we assume that MI is *not* equal to the "end()" iterator.
335  BuildMI(MBB, MI, MI->getDebugLoc(),
336          TII->get(TargetInstrInfo::GC_LABEL)).addImm(Label);
337  return Label;
338}
339
340void MachineCodeAnalysis::VisitCallPoint(MachineBasicBlock::iterator CI) {
341  // Find the return address (next instruction), too, so as to bracket the call
342  // instruction.
343  MachineBasicBlock::iterator RAI = CI;
344  ++RAI;
345
346  if (FI->getStrategy().needsSafePoint(GC::PreCall))
347    FI->addSafePoint(GC::PreCall, InsertLabel(*CI->getParent(), CI));
348
349  if (FI->getStrategy().needsSafePoint(GC::PostCall))
350    FI->addSafePoint(GC::PostCall, InsertLabel(*CI->getParent(), RAI));
351}
352
353void MachineCodeAnalysis::FindSafePoints(MachineFunction &MF) {
354  for (MachineFunction::iterator BBI = MF.begin(),
355                                 BBE = MF.end(); BBI != BBE; ++BBI)
356    for (MachineBasicBlock::iterator MI = BBI->begin(),
357                                     ME = BBI->end(); MI != ME; ++MI)
358      if (MI->getDesc().isCall())
359        VisitCallPoint(MI);
360}
361
362void MachineCodeAnalysis::FindStackOffsets(MachineFunction &MF) {
363  const TargetRegisterInfo *TRI = TM->getRegisterInfo();
364  assert(TRI && "TargetRegisterInfo not available!");
365
366  for (GCFunctionInfo::roots_iterator RI = FI->roots_begin(),
367                                      RE = FI->roots_end(); RI != RE; ++RI)
368    RI->StackOffset = TRI->getFrameIndexOffset(MF, RI->Num);
369}
370
371bool MachineCodeAnalysis::runOnMachineFunction(MachineFunction &MF) {
372  // Quick exit for functions that do not use GC.
373  if (!MF.getFunction()->hasGC())
374    return false;
375
376  FI = &getAnalysis<GCModuleInfo>().getFunctionInfo(*MF.getFunction());
377  if (!FI->getStrategy().needsSafePoints())
378    return false;
379
380  TM = &MF.getTarget();
381  MMI = &getAnalysis<MachineModuleInfo>();
382  TII = TM->getInstrInfo();
383
384  // Find the size of the stack frame.
385  FI->setFrameSize(MF.getFrameInfo()->getStackSize());
386
387  // Find all safe points.
388  FindSafePoints(MF);
389
390  // Find the stack offsets for all roots.
391  FindStackOffsets(MF);
392
393  return false;
394}
395