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/Compiler.h"
  31 #include "llvm/Support/ErrorHandling.h"
  32
  33 using namespace llvm;
  34
  35 namespace {
  36
  37   /// LowerIntrinsics - This pass rewrites calls to the llvm.gcread or
  38   /// llvm.gcwrite intrinsics, replacing them with simple loads and stores as
  39   /// directed by the GCStrategy. It also performs automatic root initialization
  40   /// and custom intrinsic lowering.
  41   class VISIBILITY_HIDDEN LowerIntrinsics : public FunctionPass {
  42     static bool NeedsDefaultLoweringPass(const GCStrategy &C);
  43     static bool NeedsCustomLoweringPass(const GCStrategy &C);
  44     static bool CouldBecomeSafePoint(Instruction *I);
  45     bool PerformDefaultLowering(Function &F, GCStrategy &Coll);
  46     static bool InsertRootInitializers(Function &F,
  47                                        AllocaInst **Roots, unsigned Count);
  48
  49   public:
  50     static char ID;
  51
  52     LowerIntrinsics();
  53     const char *getPassName() const;
  54     void getAnalysisUsage(AnalysisUsage &AU) const;
  55
  56     bool doInitialization(Module &M);
  57     bool runOnFunction(Function &F);
  58   };
  59
  60
  61   /// MachineCodeAnalysis - This is a target-independent pass over the machine
  62   /// function representation to identify safe points for the garbage collector
  63   /// in the machine code. It inserts labels at safe points and populates a
  64   /// GCMetadata record for each function.
  65   class VISIBILITY_HIDDEN MachineCodeAnalysis : public MachineFunctionPass {
  66     const TargetMachine *TM;
  67     GCFunctionInfo *FI;
  68     MachineModuleInfo *MMI;
  69     const TargetInstrInfo *TII;
  70
  71     void FindSafePoints(MachineFunction &MF);
  72     void VisitCallPoint(MachineBasicBlock::iterator MI);
  73     unsigned InsertLabel(MachineBasicBlock &MBB,
  74                          MachineBasicBlock::iterator MI) const;
  75
  76     void FindStackOffsets(MachineFunction &MF);
  77
  78   public:
  79     static char ID;
  80
  81     MachineCodeAnalysis();
  82     const char *getPassName() const;
  83     void getAnalysisUsage(AnalysisUsage &AU) const;
  84
  85     bool runOnMachineFunction(MachineFunction &MF);
  86   };
  87
  88 }
  89
  90 // -----------------------------------------------------------------------------
  91
  92 GCStrategy::GCStrategy() :
  93   NeededSafePoints(0),
  94   CustomReadBarriers(false),
  95   CustomWriteBarriers(false),
  96   CustomRoots(false),
  97   InitRoots(true),
  98   UsesMetadata(false)
  99 {}
 100
 101 GCStrategy::~GCStrategy() {
 102   for (iterator I = begin(), E = end(); I != E; ++I)
 103     delete *I;
 104
 105   Functions.clear();
 106 }
 107
 108 bool GCStrategy::initializeCustomLowering(Module &M) { return false; }
 109
 110 bool GCStrategy::performCustomLowering(Function &F) {
 111   cerr << "gc " << getName() << " must override performCustomLowering.\n";
 112   llvm_unreachable(0);
 113   return 0;
 114 }
 115
 116 GCFunctionInfo *GCStrategy::insertFunctionInfo(const Function &F) {
 117   GCFunctionInfo *FI = new GCFunctionInfo(F, *this);
 118   Functions.push_back(FI);
 119   return FI;
 120 }
 121
 122 // -----------------------------------------------------------------------------
 123
 124 FunctionPass *llvm::createGCLoweringPass() {
 125   return new LowerIntrinsics();
 126 }
 127
 128 char LowerIntrinsics::ID = 0;
 129
 130 LowerIntrinsics::LowerIntrinsics()
 131   : FunctionPass(&ID) {}
 132
 133 const char *LowerIntrinsics::getPassName() const {
 134   return "Lower Garbage Collection Instructions";
 135 }
 136
 137 void LowerIntrinsics::getAnalysisUsage(AnalysisUsage &AU) const {
 138   FunctionPass::getAnalysisUsage(AU);
 139   AU.addRequired<GCModuleInfo>();
 140 }
 141
 142 /// doInitialization - If this module uses the GC intrinsics, find them now.
 143 bool LowerIntrinsics::doInitialization(Module &M) {
 144   // FIXME: This is rather antisocial in the context of a JIT since it performs
 145   //        work against the entire module. But this cannot be done at
 146   //        runFunction time (initializeCustomLowering likely needs to change
 147   //        the module).
 148   GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
 149   assert(MI && "LowerIntrinsics didn't require GCModuleInfo!?");
 150   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
 151     if (!I->isDeclaration() && I->hasGC())
 152       MI->getFunctionInfo(*I); // Instantiate the GC strategy.
 153
 154   bool MadeChange = false;
 155   for (GCModuleInfo::iterator I = MI->begin(), E = MI->end(); I != E; ++I)
 156     if (NeedsCustomLoweringPass(**I))
 157       if ((*I)->initializeCustomLowering(M))
 158         MadeChange = true;
 159
 160   return MadeChange;
 161 }
 162
 163 bool LowerIntrinsics::InsertRootInitializers(Function &F, AllocaInst **Roots,
 164                                                           unsigned Count) {
 165   // Scroll past alloca instructions.
 166   BasicBlock::iterator IP = F.getEntryBlock().begin();
 167   while (isa<AllocaInst>(IP)) ++IP;
 168
 169   // Search for initializers in the initial BB.
 170   SmallPtrSet<AllocaInst*,16> InitedRoots;
 171   for (; !CouldBecomeSafePoint(IP); ++IP)
 172     if (StoreInst *SI = dyn_cast<StoreInst>(IP))
 173       if (AllocaInst *AI =
 174           dyn_cast<AllocaInst>(SI->getOperand(1)->stripPointerCasts()))
 175         InitedRoots.insert(AI);
 176
 177   // Add root initializers.
 178   bool MadeChange = false;
 179
 180   for (AllocaInst **I = Roots, **E = Roots + Count; I != E; ++I)
 181     if (!InitedRoots.count(*I)) {
 182       new StoreInst(ConstantPointerNull::get(cast<PointerType>(
 183                       cast<PointerType>((*I)->getType())->getElementType())),
 184                     *I, IP);
 185       MadeChange = true;
 186     }
 187
 188   return MadeChange;
 189 }
 190
 191 bool LowerIntrinsics::NeedsDefaultLoweringPass(const GCStrategy &C) {
 192   // Default lowering is necessary only if read or write barriers have a default
 193   // action. The default for roots is no action.
 194   return !C.customWriteBarrier()
 195       || !C.customReadBarrier()
 196       || C.initializeRoots();
 197 }
 198
 199 bool LowerIntrinsics::NeedsCustomLoweringPass(const GCStrategy &C) {
 200   // Custom lowering is only necessary if enabled for some action.
 201   return C.customWriteBarrier()
 202       || C.customReadBarrier()
 203       || C.customRoots();
 204 }
 205
 206 /// CouldBecomeSafePoint - Predicate to conservatively determine whether the
 207 /// instruction could introduce a safe point.
 208 bool LowerIntrinsics::CouldBecomeSafePoint(Instruction *I) {
 209   // The natural definition of instructions which could introduce safe points
 210   // are:
 211   //
 212   //   - call, invoke (AfterCall, BeforeCall)
 213   //   - phis (Loops)
 214   //   - invoke, ret, unwind (Exit)
 215   //
 216   // However, instructions as seemingly inoccuous as arithmetic can become
 217   // libcalls upon lowering (e.g., div i64 on a 32-bit platform), so instead
 218   // it is necessary to take a conservative approach.
 219
 220   if (isa<AllocaInst>(I) || isa<GetElementPtrInst>(I) ||
 221       isa<StoreInst>(I) || isa<LoadInst>(I))
 222     return false;
 223
 224   // llvm.gcroot is safe because it doesn't do anything at runtime.
 225   if (CallInst *CI = dyn_cast<CallInst>(I))
 226     if (Function *F = CI->getCalledFunction())
 227       if (unsigned IID = F->getIntrinsicID())
 228         if (IID == Intrinsic::gcroot)
 229           return false;
 230
 231   return true;
 232 }
 233
 234 /// runOnFunction - Replace gcread/gcwrite intrinsics with loads and stores.
 235 /// Leave gcroot intrinsics; the code generator needs to see those.
 236 bool LowerIntrinsics::runOnFunction(Function &F) {
 237   // Quick exit for functions that do not use GC.
 238   if (!F.hasGC())
 239     return false;
 240
 241   GCFunctionInfo &FI = getAnalysis<GCModuleInfo>().getFunctionInfo(F);
 242   GCStrategy &S = FI.getStrategy();
 243
 244   bool MadeChange = false;
 245
 246   if (NeedsDefaultLoweringPass(S))
 247     MadeChange |= PerformDefaultLowering(F, S);
 248
 249   if (NeedsCustomLoweringPass(S))
 250     MadeChange |= S.performCustomLowering(F);
 251
 252   return MadeChange;
 253 }
 254
 255 bool LowerIntrinsics::PerformDefaultLowering(Function &F, GCStrategy &S) {
 256   bool LowerWr = !S.customWriteBarrier();
 257   bool LowerRd = !S.customReadBarrier();
 258   bool InitRoots = S.initializeRoots();
 259
 260   SmallVector<AllocaInst*,32> Roots;
 261
 262   bool MadeChange = false;
 263   for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
 264     for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
 265       if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) {
 266         Function *F = CI->getCalledFunction();
 267         switch (F->getIntrinsicID()) {
 268         case Intrinsic::gcwrite:
 269           if (LowerWr) {
 270             // Replace a write barrier with a simple store.
 271             Value *St = new StoreInst(CI->getOperand(1), CI->getOperand(3), CI);
 272             CI->replaceAllUsesWith(St);
 273             CI->eraseFromParent();
 274           }
 275           break;
 276         case Intrinsic::gcread:
 277           if (LowerRd) {
 278             // Replace a read barrier with a simple load.
 279             Value *Ld = new LoadInst(CI->getOperand(2), "", CI);
 280             Ld->takeName(CI);
 281             CI->replaceAllUsesWith(Ld);
 282             CI->eraseFromParent();
 283           }
 284           break;
 285         case Intrinsic::gcroot:
 286           if (InitRoots) {
 287             // Initialize the GC root, but do not delete the intrinsic. The
 288             // backend needs the intrinsic to flag the stack slot.
 289             Roots.push_back(cast<AllocaInst>(
 290                               CI->getOperand(1)->stripPointerCasts()));
 291           }
 292           break;
 293         default:
 294           continue;
 295         }
 296
 297         MadeChange = true;
 298       }
 299     }
 300   }
 301
 302   if (Roots.size())
 303     MadeChange |= InsertRootInitializers(F, Roots.begin(), Roots.size());
 304
 305   return MadeChange;
 306 }
 307
 308 // -----------------------------------------------------------------------------
 309
 310 FunctionPass *llvm::createGCMachineCodeAnalysisPass() {
 311   return new MachineCodeAnalysis();
 312 }
 313
 314 char MachineCodeAnalysis::ID = 0;
 315
 316 MachineCodeAnalysis::MachineCodeAnalysis()
 317   : MachineFunctionPass(&ID) {}
 318
 319 const char *MachineCodeAnalysis::getPassName() const {
 320   return "Analyze Machine Code For Garbage Collection";
 321 }
 322
 323 void MachineCodeAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
 324   MachineFunctionPass::getAnalysisUsage(AU);
 325   AU.setPreservesAll();
 326   AU.addRequired<MachineModuleInfo>();
 327   AU.addRequired<GCModuleInfo>();
 328 }
 329
 330 unsigned MachineCodeAnalysis::InsertLabel(MachineBasicBlock &MBB,
 331                                      MachineBasicBlock::iterator MI) const {
 332   unsigned Label = MMI->NextLabelID();
 333   // N.B. we assume that MI is *not* equal to the "end()" iterator.
 334   BuildMI(MBB, MI, MI->getDebugLoc(),
 335           TII->get(TargetInstrInfo::GC_LABEL)).addImm(Label);
 336   return Label;
 337 }
 338
 339 void MachineCodeAnalysis::VisitCallPoint(MachineBasicBlock::iterator CI) {
 340   // Find the return address (next instruction), too, so as to bracket the call
 341   // instruction.
 342   MachineBasicBlock::iterator RAI = CI;
 343   ++RAI;
 344
 345   if (FI->getStrategy().needsSafePoint(GC::PreCall))
 346     FI->addSafePoint(GC::PreCall, InsertLabel(*CI->getParent(), CI));
 347
 348   if (FI->getStrategy().needsSafePoint(GC::PostCall))
 349     FI->addSafePoint(GC::PostCall, InsertLabel(*CI->getParent(), RAI));
 350 }
 351
 352 void MachineCodeAnalysis::FindSafePoints(MachineFunction &MF) {
 353   for (MachineFunction::iterator BBI = MF.begin(),
 354                                  BBE = MF.end(); BBI != BBE; ++BBI)
 355     for (MachineBasicBlock::iterator MI = BBI->begin(),
 356                                      ME = BBI->end(); MI != ME; ++MI)
 357       if (MI->getDesc().isCall())
 358         VisitCallPoint(MI);
 359 }
 360
 361 void MachineCodeAnalysis::FindStackOffsets(MachineFunction &MF) {
 362   const TargetRegisterInfo *TRI = TM->getRegisterInfo();
 363   assert(TRI && "TargetRegisterInfo not available!");
 364
 365   for (GCFunctionInfo::roots_iterator RI = FI->roots_begin(),
 366                                       RE = FI->roots_end(); RI != RE; ++RI)
 367     RI->StackOffset = TRI->getFrameIndexOffset(MF, RI->Num);
 368 }
 369
 370 bool MachineCodeAnalysis::runOnMachineFunction(MachineFunction &MF) {
 371   // Quick exit for functions that do not use GC.
 372   if (!MF.getFunction()->hasGC())
 373     return false;
 374
 375   FI = &getAnalysis<GCModuleInfo>().getFunctionInfo(*MF.getFunction());
 376   if (!FI->getStrategy().needsSafePoints())
 377     return false;
 378
 379   TM = &MF.getTarget();
 380   MMI = &getAnalysis<MachineModuleInfo>();
 381   TII = TM->getInstrInfo();
 382
 383   // Find the size of the stack frame.
 384   FI->setFrameSize(MF.getFrameInfo()->getStackSize());
 385
 386   // Find all safe points.
 387   FindSafePoints(MF);
 388
 389   // Find the stack offsets for all roots.
 390   FindStackOffsets(MF);
 391
 392   return false;
 393 }