GCStrategy.cpp revision 6803e53ec9e1ed1cc4221eed1b93c672346dc26e 
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      new StoreInst(ConstantPointerNull::get(cast<PointerType>(
185                      cast<PointerType>((*I)->getType())->getElementType())),
186                    *I, IP);
187      MadeChange = true;
188    }
189
190  return MadeChange;
191}
192
193bool LowerIntrinsics::NeedsDefaultLoweringPass(const GCStrategy &C) {
194  // Default lowering is necessary only if read or write barriers have a default
195  // action. The default for roots is no action.
196  return !C.customWriteBarrier()
197      || !C.customReadBarrier()
198      || C.initializeRoots();
199}
200
201bool LowerIntrinsics::NeedsCustomLoweringPass(const GCStrategy &C) {
202  // Custom lowering is only necessary if enabled for some action.
203  return C.customWriteBarrier()
204      || C.customReadBarrier()
205      || C.customRoots();
206}
207
208/// CouldBecomeSafePoint - Predicate to conservatively determine whether the
209/// instruction could introduce a safe point.
210bool LowerIntrinsics::CouldBecomeSafePoint(Instruction *I) {
211  // The natural definition of instructions which could introduce safe points
212  // are:
213  //
214  //   - call, invoke (AfterCall, BeforeCall)
215  //   - phis (Loops)
216  //   - invoke, ret, unwind (Exit)
217  //
218  // However, instructions as seemingly inoccuous as arithmetic can become
219  // libcalls upon lowering (e.g., div i64 on a 32-bit platform), so instead
220  // it is necessary to take a conservative approach.
221
222  if (isa<AllocaInst>(I) || isa<GetElementPtrInst>(I) ||
223      isa<StoreInst>(I) || isa<LoadInst>(I))
224    return false;
225
226  // llvm.gcroot is safe because it doesn't do anything at runtime.
227  if (CallInst *CI = dyn_cast<CallInst>(I))
228    if (Function *F = CI->getCalledFunction())
229      if (unsigned IID = F->getIntrinsicID())
230        if (IID == Intrinsic::gcroot)
231          return false;
232
233  return true;
234}
235
236/// runOnFunction - Replace gcread/gcwrite intrinsics with loads and stores.
237/// Leave gcroot intrinsics; the code generator needs to see those.
238bool LowerIntrinsics::runOnFunction(Function &F) {
239  // Quick exit for functions that do not use GC.
240  if (!F.hasGC())
241    return false;
242
243  GCFunctionInfo &FI = getAnalysis<GCModuleInfo>().getFunctionInfo(F);
244  GCStrategy &S = FI.getStrategy();
245
246  bool MadeChange = false;
247
248  if (NeedsDefaultLoweringPass(S))
249    MadeChange |= PerformDefaultLowering(F, S);
250
251  if (NeedsCustomLoweringPass(S))
252    MadeChange |= S.performCustomLowering(F);
253
254  return MadeChange;
255}
256
257bool LowerIntrinsics::PerformDefaultLowering(Function &F, GCStrategy &S) {
258  bool LowerWr = !S.customWriteBarrier();
259  bool LowerRd = !S.customReadBarrier();
260  bool InitRoots = S.initializeRoots();
261
262  SmallVector<AllocaInst*,32> Roots;
263
264  bool MadeChange = false;
265  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
266    for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
267      if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) {
268        Function *F = CI->getCalledFunction();
269        switch (F->getIntrinsicID()) {
270        case Intrinsic::gcwrite:
271          if (LowerWr) {
272            // Replace a write barrier with a simple store.
273            Value *St = new StoreInst(CI->getOperand(1), CI->getOperand(3), CI);
274            CI->replaceAllUsesWith(St);
275            CI->eraseFromParent();
276          }
277          break;
278        case Intrinsic::gcread:
279          if (LowerRd) {
280            // Replace a read barrier with a simple load.
281            Value *Ld = new LoadInst(CI->getOperand(2), "", CI);
282            Ld->takeName(CI);
283            CI->replaceAllUsesWith(Ld);
284            CI->eraseFromParent();
285          }
286          break;
287        case Intrinsic::gcroot:
288          if (InitRoots) {
289            // Initialize the GC root, but do not delete the intrinsic. The
290            // backend needs the intrinsic to flag the stack slot.
291            Roots.push_back(cast<AllocaInst>(
292                              CI->getOperand(1)->stripPointerCasts()));
293          }
294          break;
295        default:
296          continue;
297        }
298
299        MadeChange = true;
300      }
301    }
302  }
303
304  if (Roots.size())
305    MadeChange |= InsertRootInitializers(F, Roots.begin(), Roots.size());
306
307  return MadeChange;
308}
309
310// -----------------------------------------------------------------------------
311
312FunctionPass *llvm::createGCMachineCodeAnalysisPass() {
313  return new MachineCodeAnalysis();
314}
315
316char MachineCodeAnalysis::ID = 0;
317
318MachineCodeAnalysis::MachineCodeAnalysis()
319  : MachineFunctionPass(&ID) {}
320
321const char *MachineCodeAnalysis::getPassName() const {
322  return "Analyze Machine Code For Garbage Collection";
323}
324
325void MachineCodeAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
326  MachineFunctionPass::getAnalysisUsage(AU);
327  AU.setPreservesAll();
328  AU.addRequired<MachineModuleInfo>();
329  AU.addRequired<GCModuleInfo>();
330}
331
332MCSymbol *MachineCodeAnalysis::InsertLabel(MachineBasicBlock &MBB,
333                                           MachineBasicBlock::iterator MI,
334                                           DebugLoc DL) const {
335  MCSymbol *Label = MBB.getParent()->getContext().CreateTempSymbol();
336  BuildMI(MBB, MI, DL, TII->get(TargetOpcode::GC_LABEL)).addSym(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                                              CI->getDebugLoc()));
349
350  if (FI->getStrategy().needsSafePoint(GC::PostCall))
351    FI->addSafePoint(GC::PostCall, InsertLabel(*CI->getParent(), RAI,
352                                               CI->getDebugLoc()));
353}
354
355void MachineCodeAnalysis::FindSafePoints(MachineFunction &MF) {
356  for (MachineFunction::iterator BBI = MF.begin(),
357                                 BBE = MF.end(); BBI != BBE; ++BBI)
358    for (MachineBasicBlock::iterator MI = BBI->begin(),
359                                     ME = BBI->end(); MI != ME; ++MI)
360      if (MI->getDesc().isCall())
361        VisitCallPoint(MI);
362}
363
364void MachineCodeAnalysis::FindStackOffsets(MachineFunction &MF) {
365  const TargetRegisterInfo *TRI = TM->getRegisterInfo();
366  assert(TRI && "TargetRegisterInfo not available!");
367
368  for (GCFunctionInfo::roots_iterator RI = FI->roots_begin(),
369                                      RE = FI->roots_end(); RI != RE; ++RI)
370    RI->StackOffset = TRI->getFrameIndexOffset(MF, RI->Num);
371}
372
373bool MachineCodeAnalysis::runOnMachineFunction(MachineFunction &MF) {
374  // Quick exit for functions that do not use GC.
375  if (!MF.getFunction()->hasGC())
376    return false;
377
378  FI = &getAnalysis<GCModuleInfo>().getFunctionInfo(*MF.getFunction());
379  if (!FI->getStrategy().needsSafePoints())
380    return false;
381
382  TM = &MF.getTarget();
383  MMI = &getAnalysis<MachineModuleInfo>();
384  TII = TM->getInstrInfo();
385
386  // Find the size of the stack frame.
387  FI->setFrameSize(MF.getFrameInfo()->getStackSize());
388
389  // Find all safe points.
390  FindSafePoints(MF);
391
392  // Find the stack offsets for all roots.
393  FindStackOffsets(MF);
394
395  return false;
396}
397