lib/CodeGen/GCStrategy.cpp

   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/Analysis/Dominators.h"
  23 #include "llvm/CodeGen/MachineFrameInfo.h"
  24 #include "llvm/CodeGen/MachineFunctionPass.h"
  25 #include "llvm/CodeGen/MachineInstrBuilder.h"
  26 #include "llvm/CodeGen/MachineModuleInfo.h"
  27 #include "llvm/Target/TargetFrameLowering.h"
  28 #include "llvm/Target/TargetInstrInfo.h"
  29 #include "llvm/Target/TargetMachine.h"
  30 #include "llvm/Target/TargetRegisterInfo.h"
  31 #include "llvm/Support/Debug.h"
  32 #include "llvm/Support/ErrorHandling.h"
  33 #include "llvm/Support/raw_ostream.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;
  56     void getAnalysisUsage(AnalysisUsage &AU) const;
  57
  58     bool doInitialization(Module &M);
  59     bool runOnFunction(Function &F);
  60   };
  61
  62
  63   /// MachineCodeAnalysis - 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 MachineCodeAnalysis : 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     MachineCodeAnalysis();
  85     const char *getPassName() const;
  86     void getAnalysisUsage(AnalysisUsage &AU) const;
  87
  88     bool runOnMachineFunction(MachineFunction &MF);
  89   };
  90
  91 }
  92
  93 // -----------------------------------------------------------------------------
  94
  95 GCStrategy::GCStrategy() :
  96   NeededSafePoints(0),
  97   CustomReadBarriers(false),
  98   CustomWriteBarriers(false),
  99   CustomRoots(false),
 100   InitRoots(true),
 101   UsesMetadata(false)
 102 {}
 103
 104 GCStrategy::~GCStrategy() {
 105   for (iterator I = begin(), E = end(); I != E; ++I)
 106     delete *I;
 107
 108   Functions.clear();
 109 }
 110
 111 bool GCStrategy::initializeCustomLowering(Module &M) { return false; }
 112
 113 bool GCStrategy::performCustomLowering(Function &F) {
 114   dbgs() << "gc " << getName() << " must override performCustomLowering.\n";
 115   llvm_unreachable(0);
 116   return 0;
 117 }
 118
 119 GCFunctionInfo *GCStrategy::insertFunctionInfo(const Function &F) {
 120   GCFunctionInfo *FI = new GCFunctionInfo(F, *this);
 121   Functions.push_back(FI);
 122   return FI;
 123 }
 124
 125 // -----------------------------------------------------------------------------
 126
 127 INITIALIZE_PASS_BEGIN(LowerIntrinsics, "gc-lowering", "GC Lowering",
 128                       false, false)
 129 INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
 130 INITIALIZE_PASS_END(LowerIntrinsics, "gc-lowering", "GC Lowering", false, false)
 131
 132 FunctionPass *llvm::createGCLoweringPass() {
 133   return new LowerIntrinsics();
 134 }
 135
 136 char LowerIntrinsics::ID = 0;
 137
 138 LowerIntrinsics::LowerIntrinsics()
 139   : FunctionPass(ID) {
 140     initializeLowerIntrinsicsPass(*PassRegistry::getPassRegistry());
 141   }
 142
 143 const char *LowerIntrinsics::getPassName() const {
 144   return "Lower Garbage Collection Instructions";
 145 }
 146
 147 void LowerIntrinsics::getAnalysisUsage(AnalysisUsage &AU) const {
 148   FunctionPass::getAnalysisUsage(AU);
 149   AU.addRequired<GCModuleInfo>();
 150   AU.addPreserved<DominatorTree>();
 151 }
 152
 153 /// doInitialization - If this module uses the GC intrinsics, find them now.
 154 bool LowerIntrinsics::doInitialization(Module &M) {
 155   // FIXME: This is rather antisocial in the context of a JIT since it performs
 156   //        work against the entire module. But this cannot be done at
 157   //        runFunction time (initializeCustomLowering likely needs to change
 158   //        the module).
 159   GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
 160   assert(MI && "LowerIntrinsics didn't require GCModuleInfo!?");
 161   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
 162     if (!I->isDeclaration() && I->hasGC())
 163       MI->getFunctionInfo(*I); // Instantiate the GC strategy.
 164
 165   bool MadeChange = false;
 166   for (GCModuleInfo::iterator I = MI->begin(), E = MI->end(); I != E; ++I)
 167     if (NeedsCustomLoweringPass(**I))
 168       if ((*I)->initializeCustomLowering(M))
 169         MadeChange = true;
 170
 171   return MadeChange;
 172 }
 173
 174 bool LowerIntrinsics::InsertRootInitializers(Function &F, AllocaInst **Roots,
 175                                                           unsigned Count) {
 176   // Scroll past alloca instructions.
 177   BasicBlock::iterator IP = F.getEntryBlock().begin();
 178   while (isa<AllocaInst>(IP)) ++IP;
 179
 180   // Search for initializers in the initial BB.
 181   SmallPtrSet<AllocaInst*,16> InitedRoots;
 182   for (; !CouldBecomeSafePoint(IP); ++IP)
 183     if (StoreInst *SI = dyn_cast<StoreInst>(IP))
 184       if (AllocaInst *AI =
 185           dyn_cast<AllocaInst>(SI->getOperand(1)->stripPointerCasts()))
 186         InitedRoots.insert(AI);
 187
 188   // Add root initializers.
 189   bool MadeChange = false;
 190
 191   for (AllocaInst **I = Roots, **E = Roots + Count; I != E; ++I)
 192     if (!InitedRoots.count(*I)) {
 193       StoreInst* SI = new StoreInst(ConstantPointerNull::get(cast<PointerType>(
 194                         cast<PointerType>((*I)->getType())->getElementType())),
 195                         *I);
 196       SI->insertAfter(*I);
 197       MadeChange = true;
 198     }
 199
 200   return MadeChange;
 201 }
 202
 203 bool LowerIntrinsics::NeedsDefaultLoweringPass(const GCStrategy &C) {
 204   // Default lowering is necessary only if read or write barriers have a default
 205   // action. The default for roots is no action.
 206   return !C.customWriteBarrier()
 207       || !C.customReadBarrier()
 208       || C.initializeRoots();
 209 }
 210
 211 bool LowerIntrinsics::NeedsCustomLoweringPass(const GCStrategy &C) {
 212   // Custom lowering is only necessary if enabled for some action.
 213   return C.customWriteBarrier()
 214       || C.customReadBarrier()
 215       || C.customRoots();
 216 }
 217
 218 /// CouldBecomeSafePoint - Predicate to conservatively determine whether the
 219 /// instruction could introduce a safe point.
 220 bool LowerIntrinsics::CouldBecomeSafePoint(Instruction *I) {
 221   // The natural definition of instructions which could introduce safe points
 222   // are:
 223   //
 224   //   - call, invoke (AfterCall, BeforeCall)
 225   //   - phis (Loops)
 226   //   - invoke, ret, unwind (Exit)
 227   //
 228   // However, instructions as seemingly inoccuous as arithmetic can become
 229   // libcalls upon lowering (e.g., div i64 on a 32-bit platform), so instead
 230   // it is necessary to take a conservative approach.
 231
 232   if (isa<AllocaInst>(I) || isa<GetElementPtrInst>(I) ||
 233       isa<StoreInst>(I) || isa<LoadInst>(I))
 234     return false;
 235
 236   // llvm.gcroot is safe because it doesn't do anything at runtime.
 237   if (CallInst *CI = dyn_cast<CallInst>(I))
 238     if (Function *F = CI->getCalledFunction())
 239       if (unsigned IID = F->getIntrinsicID())
 240         if (IID == Intrinsic::gcroot)
 241           return false;
 242
 243   return true;
 244 }
 245
 246 /// runOnFunction - Replace gcread/gcwrite intrinsics with loads and stores.
 247 /// Leave gcroot intrinsics; the code generator needs to see those.
 248 bool LowerIntrinsics::runOnFunction(Function &F) {
 249   // Quick exit for functions that do not use GC.
 250   if (!F.hasGC())
 251     return false;
 252
 253   GCFunctionInfo &FI = getAnalysis<GCModuleInfo>().getFunctionInfo(F);
 254   GCStrategy &S = FI.getStrategy();
 255
 256   bool MadeChange = false;
 257
 258   if (NeedsDefaultLoweringPass(S))
 259     MadeChange |= PerformDefaultLowering(F, S);
 260
 261   bool UseCustomLoweringPass = NeedsCustomLoweringPass(S);
 262   if (UseCustomLoweringPass)
 263     MadeChange |= S.performCustomLowering(F);
 264
 265   // Custom lowering may modify the CFG, so dominators must be recomputed.
 266   if (UseCustomLoweringPass) {
 267     if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>())
 268       DT->DT->recalculate(F);
 269   }
 270
 271   return MadeChange;
 272 }
 273
 274 bool LowerIntrinsics::PerformDefaultLowering(Function &F, GCStrategy &S) {
 275   bool LowerWr = !S.customWriteBarrier();
 276   bool LowerRd = !S.customReadBarrier();
 277   bool InitRoots = S.initializeRoots();
 278
 279   SmallVector<AllocaInst*, 32> Roots;
 280
 281   bool MadeChange = false;
 282   for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
 283     for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
 284       if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) {
 285         Function *F = CI->getCalledFunction();
 286         switch (F->getIntrinsicID()) {
 287         case Intrinsic::gcwrite:
 288           if (LowerWr) {
 289             // Replace a write barrier with a simple store.
 290             Value *St = new StoreInst(CI->getArgOperand(0),
 291                                       CI->getArgOperand(2), CI);
 292             CI->replaceAllUsesWith(St);
 293             CI->eraseFromParent();
 294           }
 295           break;
 296         case Intrinsic::gcread:
 297           if (LowerRd) {
 298             // Replace a read barrier with a simple load.
 299             Value *Ld = new LoadInst(CI->getArgOperand(1), "", CI);
 300             Ld->takeName(CI);
 301             CI->replaceAllUsesWith(Ld);
 302             CI->eraseFromParent();
 303           }
 304           break;
 305         case Intrinsic::gcroot:
 306           if (InitRoots) {
 307             // Initialize the GC root, but do not delete the intrinsic. The
 308             // backend needs the intrinsic to flag the stack slot.
 309             Roots.push_back(cast<AllocaInst>(
 310                               CI->getArgOperand(0)->stripPointerCasts()));
 311           }
 312           break;
 313         default:
 314           continue;
 315         }
 316
 317         MadeChange = true;
 318       }
 319     }
 320   }
 321
 322   if (Roots.size())
 323     MadeChange |= InsertRootInitializers(F, Roots.begin(), Roots.size());
 324
 325   return MadeChange;
 326 }
 327
 328 // -----------------------------------------------------------------------------
 329
 330 FunctionPass *llvm::createGCMachineCodeAnalysisPass() {
 331   return new MachineCodeAnalysis();
 332 }
 333
 334 char MachineCodeAnalysis::ID = 0;
 335
 336 MachineCodeAnalysis::MachineCodeAnalysis()
 337   : MachineFunctionPass(ID) {}
 338
 339 const char *MachineCodeAnalysis::getPassName() const {
 340   return "Analyze Machine Code For Garbage Collection";
 341 }
 342
 343 void MachineCodeAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
 344   MachineFunctionPass::getAnalysisUsage(AU);
 345   AU.setPreservesAll();
 346   AU.addRequired<MachineModuleInfo>();
 347   AU.addRequired<GCModuleInfo>();
 348 }
 349
 350 MCSymbol *MachineCodeAnalysis::InsertLabel(MachineBasicBlock &MBB,
 351                                            MachineBasicBlock::iterator MI,
 352                                            DebugLoc DL) const {
 353   MCSymbol *Label = MBB.getParent()->getContext().CreateTempSymbol();
 354   BuildMI(MBB, MI, DL, TII->get(TargetOpcode::GC_LABEL)).addSym(Label);
 355   return Label;
 356 }
 357
 358 void MachineCodeAnalysis::VisitCallPoint(MachineBasicBlock::iterator CI) {
 359   // Find the return address (next instruction), too, so as to bracket the call
 360   // instruction.
 361   MachineBasicBlock::iterator RAI = CI;
 362   ++RAI;
 363
 364   if (FI->getStrategy().needsSafePoint(GC::PreCall)) {
 365     MCSymbol* Label = InsertLabel(*CI->getParent(), CI, CI->getDebugLoc());
 366     FI->addSafePoint(GC::PreCall, Label, CI->getDebugLoc());
 367   }
 368
 369   if (FI->getStrategy().needsSafePoint(GC::PostCall)) {
 370     MCSymbol* Label = InsertLabel(*CI->getParent(), RAI, CI->getDebugLoc());
 371     FI->addSafePoint(GC::PostCall, Label, CI->getDebugLoc());
 372   }
 373 }
 374
 375 void MachineCodeAnalysis::FindSafePoints(MachineFunction &MF) {
 376   for (MachineFunction::iterator BBI = MF.begin(),
 377                                  BBE = MF.end(); BBI != BBE; ++BBI)
 378     for (MachineBasicBlock::iterator MI = BBI->begin(),
 379                                      ME = BBI->end(); MI != ME; ++MI)
 380       if (MI->getDesc().isCall())
 381         VisitCallPoint(MI);
 382 }
 383
 384 void MachineCodeAnalysis::FindStackOffsets(MachineFunction &MF) {
 385   const TargetFrameLowering *TFI = TM->getFrameLowering();
 386   assert(TFI && "TargetRegisterInfo not available!");
 387
 388   for (GCFunctionInfo::roots_iterator RI = FI->roots_begin(),
 389                                       RE = FI->roots_end(); RI != RE; ++RI)
 390     RI->StackOffset = TFI->getFrameIndexOffset(MF, RI->Num);
 391 }
 392
 393 bool MachineCodeAnalysis::runOnMachineFunction(MachineFunction &MF) {
 394   // Quick exit for functions that do not use GC.
 395   if (!MF.getFunction()->hasGC())
 396     return false;
 397
 398   FI = &getAnalysis<GCModuleInfo>().getFunctionInfo(*MF.getFunction());
 399   if (!FI->getStrategy().needsSafePoints())
 400     return false;
 401
 402   TM = &MF.getTarget();
 403   MMI = &getAnalysis<MachineModuleInfo>();
 404   TII = TM->getInstrInfo();
 405
 406   // Find the size of the stack frame.
 407   FI->setFrameSize(MF.getFrameInfo()->getStackSize());
 408
 409   // Find all safe points.
 410   FindSafePoints(MF);
 411
 412   // Find the stack offsets for all roots.
 413   FindStackOffsets(MF);
 414
 415   return false;
 416 }