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/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
  34 using namespace llvm;
  35
  36 namespace {
  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
  94 GCStrategy::GCStrategy() :
  95   NeededSafePoints(0),
  96   CustomReadBarriers(false),
  97   CustomWriteBarriers(false),
  98   CustomRoots(false),
  99   InitRoots(true),
 100   UsesMetadata(false)
 101 {}
 102
 103 GCStrategy::~GCStrategy() {
 104   for (iterator I = begin(), E = end(); I != E; ++I)
 105     delete *I;
 106
 107   Functions.clear();
 108 }
 109
 110 bool GCStrategy::initializeCustomLowering(Module &M) { return false; }
 111
 112 bool GCStrategy::performCustomLowering(Function &F) {
 113   dbgs() << "gc " << getName() << " must override performCustomLowering.\n";
 114   llvm_unreachable(0);
 115   return 0;
 116 }
 117
 118 GCFunctionInfo *GCStrategy::insertFunctionInfo(const Function &F) {
 119   GCFunctionInfo *FI = new GCFunctionInfo(F, *this);
 120   Functions.push_back(FI);
 121   return FI;
 122 }
 123
 124 // -----------------------------------------------------------------------------
 125
 126 FunctionPass *llvm::createGCLoweringPass() {
 127   return new LowerIntrinsics();
 128 }
 129
 130 char LowerIntrinsics::ID = 0;
 131
 132 LowerIntrinsics::LowerIntrinsics()
 133   : FunctionPass(&ID) {}
 134
 135 const char *LowerIntrinsics::getPassName() const {
 136   return "Lower Garbage Collection Instructions";
 137 }
 138
 139 void 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.
 145 bool 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
 165 bool 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       StoreInst* SI = new StoreInst(ConstantPointerNull::get(cast<PointerType>(
 185                         cast<PointerType>((*I)->getType())->getElementType())),
 186                         *I);
 187       SI->insertAfter(*I);
 188       MadeChange = true;
 189     }
 190
 191   return MadeChange;
 192 }
 193
 194 bool LowerIntrinsics::NeedsDefaultLoweringPass(const GCStrategy &C) {
 195   // Default lowering is necessary only if read or write barriers have a default
 196   // action. The default for roots is no action.
 197   return !C.customWriteBarrier()
 198       || !C.customReadBarrier()
 199       || C.initializeRoots();
 200 }
 201
 202 bool LowerIntrinsics::NeedsCustomLoweringPass(const GCStrategy &C) {
 203   // Custom lowering is only necessary if enabled for some action.
 204   return C.customWriteBarrier()
 205       || C.customReadBarrier()
 206       || C.customRoots();
 207 }
 208
 209 /// CouldBecomeSafePoint - Predicate to conservatively determine whether the
 210 /// instruction could introduce a safe point.
 211 bool LowerIntrinsics::CouldBecomeSafePoint(Instruction *I) {
 212   // The natural definition of instructions which could introduce safe points
 213   // are:
 214   //
 215   //   - call, invoke (AfterCall, BeforeCall)
 216   //   - phis (Loops)
 217   //   - invoke, ret, unwind (Exit)
 218   //
 219   // However, instructions as seemingly inoccuous as arithmetic can become
 220   // libcalls upon lowering (e.g., div i64 on a 32-bit platform), so instead
 221   // it is necessary to take a conservative approach.
 222
 223   if (isa<AllocaInst>(I) || isa<GetElementPtrInst>(I) ||
 224       isa<StoreInst>(I) || isa<LoadInst>(I))
 225     return false;
 226
 227   // llvm.gcroot is safe because it doesn't do anything at runtime.
 228   if (CallInst *CI = dyn_cast<CallInst>(I))
 229     if (Function *F = CI->getCalledFunction())
 230       if (unsigned IID = F->getIntrinsicID())
 231         if (IID == Intrinsic::gcroot)
 232           return false;
 233
 234   return true;
 235 }
 236
 237 /// runOnFunction - Replace gcread/gcwrite intrinsics with loads and stores.
 238 /// Leave gcroot intrinsics; the code generator needs to see those.
 239 bool LowerIntrinsics::runOnFunction(Function &F) {
 240   // Quick exit for functions that do not use GC.
 241   if (!F.hasGC())
 242     return false;
 243
 244   GCFunctionInfo &FI = getAnalysis<GCModuleInfo>().getFunctionInfo(F);
 245   GCStrategy &S = FI.getStrategy();
 246
 247   bool MadeChange = false;
 248
 249   if (NeedsDefaultLoweringPass(S))
 250     MadeChange |= PerformDefaultLowering(F, S);
 251
 252   if (NeedsCustomLoweringPass(S))
 253     MadeChange |= S.performCustomLowering(F);
 254
 255   return MadeChange;
 256 }
 257
 258 bool LowerIntrinsics::PerformDefaultLowering(Function &F, GCStrategy &S) {
 259   bool LowerWr = !S.customWriteBarrier();
 260   bool LowerRd = !S.customReadBarrier();
 261   bool InitRoots = S.initializeRoots();
 262
 263   SmallVector<AllocaInst*,32> Roots;
 264
 265   bool MadeChange = false;
 266   for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
 267     for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
 268       if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) {
 269         Function *F = CI->getCalledFunction();
 270         switch (F->getIntrinsicID()) {
 271         case Intrinsic::gcwrite:
 272           if (LowerWr) {
 273             // Replace a write barrier with a simple store.
 274             Value *St = new StoreInst(CI->getOperand(1), CI->getOperand(3), CI);
 275             CI->replaceAllUsesWith(St);
 276             CI->eraseFromParent();
 277           }
 278           break;
 279         case Intrinsic::gcread:
 280           if (LowerRd) {
 281             // Replace a read barrier with a simple load.
 282             Value *Ld = new LoadInst(CI->getOperand(2), "", CI);
 283             Ld->takeName(CI);
 284             CI->replaceAllUsesWith(Ld);
 285             CI->eraseFromParent();
 286           }
 287           break;
 288         case Intrinsic::gcroot:
 289           if (InitRoots) {
 290             // Initialize the GC root, but do not delete the intrinsic. The
 291             // backend needs the intrinsic to flag the stack slot.
 292             Roots.push_back(cast<AllocaInst>(
 293                               CI->getOperand(1)->stripPointerCasts()));
 294           }
 295           break;
 296         default:
 297           continue;
 298         }
 299
 300         MadeChange = true;
 301       }
 302     }
 303   }
 304
 305   if (Roots.size())
 306     MadeChange |= InsertRootInitializers(F, Roots.begin(), Roots.size());
 307
 308   return MadeChange;
 309 }
 310
 311 // -----------------------------------------------------------------------------
 312
 313 FunctionPass *llvm::createGCMachineCodeAnalysisPass() {
 314   return new MachineCodeAnalysis();
 315 }
 316
 317 char MachineCodeAnalysis::ID = 0;
 318
 319 MachineCodeAnalysis::MachineCodeAnalysis()
 320   : MachineFunctionPass(&ID) {}
 321
 322 const char *MachineCodeAnalysis::getPassName() const {
 323   return "Analyze Machine Code For Garbage Collection";
 324 }
 325
 326 void MachineCodeAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
 327   MachineFunctionPass::getAnalysisUsage(AU);
 328   AU.setPreservesAll();
 329   AU.addRequired<MachineModuleInfo>();
 330   AU.addRequired<GCModuleInfo>();
 331 }
 332
 333 MCSymbol *MachineCodeAnalysis::InsertLabel(MachineBasicBlock &MBB,
 334                                            MachineBasicBlock::iterator MI,
 335                                            DebugLoc DL) const {
 336   MCSymbol *Label = MBB.getParent()->getContext().CreateTempSymbol();
 337   BuildMI(MBB, MI, DL, TII->get(TargetOpcode::GC_LABEL)).addSym(Label);
 338   return Label;
 339 }
 340
 341 void MachineCodeAnalysis::VisitCallPoint(MachineBasicBlock::iterator CI) {
 342   // Find the return address (next instruction), too, so as to bracket the call
 343   // instruction.
 344   MachineBasicBlock::iterator RAI = CI;
 345   ++RAI;
 346
 347   if (FI->getStrategy().needsSafePoint(GC::PreCall))
 348     FI->addSafePoint(GC::PreCall, InsertLabel(*CI->getParent(), CI,
 349                                               CI->getDebugLoc()));
 350
 351   if (FI->getStrategy().needsSafePoint(GC::PostCall))
 352     FI->addSafePoint(GC::PostCall, InsertLabel(*CI->getParent(), RAI,
 353                                                CI->getDebugLoc()));
 354 }
 355
 356 void MachineCodeAnalysis::FindSafePoints(MachineFunction &MF) {
 357   for (MachineFunction::iterator BBI = MF.begin(),
 358                                  BBE = MF.end(); BBI != BBE; ++BBI)
 359     for (MachineBasicBlock::iterator MI = BBI->begin(),
 360                                      ME = BBI->end(); MI != ME; ++MI)
 361       if (MI->getDesc().isCall())
 362         VisitCallPoint(MI);
 363 }
 364
 365 void MachineCodeAnalysis::FindStackOffsets(MachineFunction &MF) {
 366   const TargetRegisterInfo *TRI = TM->getRegisterInfo();
 367   assert(TRI && "TargetRegisterInfo not available!");
 368
 369   for (GCFunctionInfo::roots_iterator RI = FI->roots_begin(),
 370                                       RE = FI->roots_end(); RI != RE; ++RI)
 371     RI->StackOffset = TRI->getFrameIndexOffset(MF, RI->Num);
 372 }
 373
 374 bool MachineCodeAnalysis::runOnMachineFunction(MachineFunction &MF) {
 375   // Quick exit for functions that do not use GC.
 376   if (!MF.getFunction()->hasGC())
 377     return false;
 378
 379   FI = &getAnalysis<GCModuleInfo>().getFunctionInfo(*MF.getFunction());
 380   if (!FI->getStrategy().needsSafePoints())
 381     return false;
 382
 383   TM = &MF.getTarget();
 384   MMI = &getAnalysis<MachineModuleInfo>();
 385   TII = TM->getInstrInfo();
 386
 387   // Find the size of the stack frame.
 388   FI->setFrameSize(MF.getFrameInfo()->getStackSize());
 389
 390   // Find all safe points.
 391   FindSafePoints(MF);
 392
 393   // Find the stack offsets for all roots.
 394   FindStackOffsets(MF);
 395
 396   return false;
 397 }