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 35 using namespace llvm; 36 37 namespace { 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 94 GCStrategy::GCStrategy() : 95 NeededSafePoints(0), 96 CustomReadBarriers(false), 97 CustomWriteBarriers(false), 98 CustomRoots(false), 99 CustomSafePoints(false), 100 InitRoots(true), 101 UsesMetadata(false) 102 {} 103 104 bool GCStrategy::initializeCustomLowering(Module &M) { return false; } 105 106 bool GCStrategy::performCustomLowering(Function &F) { 107 dbgs() << "gc " << getName() << " must override performCustomLowering.\n"; 108 llvm_unreachable("must override performCustomLowering"); 109 } 110 111 112 bool GCStrategy::findCustomSafePoints(GCFunctionInfo& FI, MachineFunction &F) { 113 dbgs() << "gc " << getName() << " must override findCustomSafePoints.\n"; 114 llvm_unreachable(nullptr); 115 } 116 117 118 GCFunctionInfo *GCStrategy::insertFunctionInfo(const Function &F) { 119 Functions.push_back(make_unique<GCFunctionInfo>(F, *this)); 120 return Functions.back().get(); 121 } 122 123 // ----------------------------------------------------------------------------- 124 125 INITIALIZE_PASS_BEGIN(LowerIntrinsics, "gc-lowering", "GC Lowering", 126 false, false) 127 INITIALIZE_PASS_DEPENDENCY(GCModuleInfo) 128 INITIALIZE_PASS_END(LowerIntrinsics, "gc-lowering", "GC Lowering", false, false) 129 130 FunctionPass *llvm::createGCLoweringPass() { 131 return new LowerIntrinsics(); 132 } 133 134 char LowerIntrinsics::ID = 0; 135 136 LowerIntrinsics::LowerIntrinsics() 137 : FunctionPass(ID) { 138 initializeLowerIntrinsicsPass(*PassRegistry::getPassRegistry()); 139 } 140 141 const char *LowerIntrinsics::getPassName() const { 142 return "Lower Garbage Collection Instructions"; 143 } 144 145 void 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. 152 bool 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 } 171 172 bool 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 } 200 201 bool 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 } 208 209 bool 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. 218 bool 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. 246 bool 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 } 272 273 bool 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 // ----------------------------------------------------------------------------- 328 329 char GCMachineCodeAnalysis::ID = 0; 330 char &llvm::GCMachineCodeAnalysisID = GCMachineCodeAnalysis::ID; 331 332 INITIALIZE_PASS(GCMachineCodeAnalysis, "gc-analysis", 333 "Analyze Machine Code For Garbage Collection", false, false) 334 335 GCMachineCodeAnalysis::GCMachineCodeAnalysis() 336 : MachineFunctionPass(ID) {} 337 338 void GCMachineCodeAnalysis::getAnalysisUsage(AnalysisUsage &AU) const { 339 MachineFunctionPass::getAnalysisUsage(AU); 340 AU.setPreservesAll(); 341 AU.addRequired<MachineModuleInfo>(); 342 AU.addRequired<GCModuleInfo>(); 343 } 344 345 MCSymbol *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 } 352 353 void 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 } 369 370 void 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 } 378 379 void 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 } 394 395 bool 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