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 #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 VISIBILITY_HIDDEN 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 VISIBILITY_HIDDEN 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     unsigned InsertLabel(MachineBasicBlock &MBB,
  75                          MachineBasicBlock::iterator MI) const;
  76
  77     void FindStackOffsets(MachineFunction &MF);
  78
  79   public:
  80     static char ID;
  81
  82     MachineCodeAnalysis();
  83     const char *getPassName() const;
  84     void getAnalysisUsage(AnalysisUsage &AU) const;
  85
  86     bool runOnMachineFunction(MachineFunction &MF);
  87   };
  88
  89 }
  90
  91 // -----------------------------------------------------------------------------
  92
  93 GCStrategy::GCStrategy() :
  94   NeededSafePoints(0),
  95   CustomReadBarriers(false),
  96   CustomWriteBarriers(false),
  97   CustomRoots(false),
  98   InitRoots(true),
  99   UsesMetadata(false)
 100 {}
 101
 102 GCStrategy::~GCStrategy() {
 103   for (iterator I = begin(), E = end(); I != E; ++I)
 104     delete *I;
 105
 106   Functions.clear();
 107 }
 108
 109 bool GCStrategy::initializeCustomLowering(Module &M) { return false; }
 110
 111 bool GCStrategy::performCustomLowering(Function &F) {
 112   errs() << "gc " << getName() << " must override performCustomLowering.\n";
 113   llvm_unreachable(0);
 114   return 0;
 115 }
 116
 117 GCFunctionInfo *GCStrategy::insertFunctionInfo(const Function &F) {
 118   GCFunctionInfo *FI = new GCFunctionInfo(F, *this);
 119   Functions.push_back(FI);
 120   return FI;
 121 }
 122
 123 // -----------------------------------------------------------------------------
 124
 125 FunctionPass *llvm::createGCLoweringPass() {
 126   return new LowerIntrinsics();
 127 }
 128
 129 char LowerIntrinsics::ID = 0;
 130
 131 LowerIntrinsics::LowerIntrinsics()
 132   : FunctionPass(&ID) {}
 133
 134 const char *LowerIntrinsics::getPassName() const {
 135   return "Lower Garbage Collection Instructions";
 136 }
 137
 138 void LowerIntrinsics::getAnalysisUsage(AnalysisUsage &AU) const {
 139   FunctionPass::getAnalysisUsage(AU);
 140   AU.addRequired<GCModuleInfo>();
 141 }
 142
 143 /// doInitialization - If this module uses the GC intrinsics, find them now.
 144 bool LowerIntrinsics::doInitialization(Module &M) {
 145   // FIXME: This is rather antisocial in the context of a JIT since it performs
 146   //        work against the entire module. But this cannot be done at
 147   //        runFunction time (initializeCustomLowering likely needs to change
 148   //        the module).
 149   GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
 150   assert(MI && "LowerIntrinsics didn't require GCModuleInfo!?");
 151   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
 152     if (!I->isDeclaration() && I->hasGC())
 153       MI->getFunctionInfo(*I); // Instantiate the GC strategy.
 154
 155   bool MadeChange = false;
 156   for (GCModuleInfo::iterator I = MI->begin(), E = MI->end(); I != E; ++I)
 157     if (NeedsCustomLoweringPass(**I))
 158       if ((*I)->initializeCustomLowering(M))
 159         MadeChange = true;
 160
 161   return MadeChange;
 162 }
 163
 164 bool LowerIntrinsics::InsertRootInitializers(Function &F, AllocaInst **Roots,
 165                                                           unsigned Count) {
 166   // Scroll past alloca instructions.
 167   BasicBlock::iterator IP = F.getEntryBlock().begin();
 168   while (isa<AllocaInst>(IP)) ++IP;
 169
 170   // Search for initializers in the initial BB.
 171   SmallPtrSet<AllocaInst*,16> InitedRoots;
 172   for (; !CouldBecomeSafePoint(IP); ++IP)
 173     if (StoreInst *SI = dyn_cast<StoreInst>(IP))
 174       if (AllocaInst *AI =
 175           dyn_cast<AllocaInst>(SI->getOperand(1)->stripPointerCasts()))
 176         InitedRoots.insert(AI);
 177
 178   // Add root initializers.
 179   bool MadeChange = false;
 180
 181   for (AllocaInst **I = Roots, **E = Roots + Count; I != E; ++I)
 182     if (!InitedRoots.count(*I)) {
 183       new StoreInst(ConstantPointerNull::get(cast<PointerType>(
 184                       cast<PointerType>((*I)->getType())->getElementType())),
 185                     *I, IP);
 186       MadeChange = true;
 187     }
 188
 189   return MadeChange;
 190 }
 191
 192 bool LowerIntrinsics::NeedsDefaultLoweringPass(const GCStrategy &C) {
 193   // Default lowering is necessary only if read or write barriers have a default
 194   // action. The default for roots is no action.
 195   return !C.customWriteBarrier()
 196       || !C.customReadBarrier()
 197       || C.initializeRoots();
 198 }
 199
 200 bool LowerIntrinsics::NeedsCustomLoweringPass(const GCStrategy &C) {
 201   // Custom lowering is only necessary if enabled for some action.
 202   return C.customWriteBarrier()
 203       || C.customReadBarrier()
 204       || C.customRoots();
 205 }
 206
 207 /// CouldBecomeSafePoint - Predicate to conservatively determine whether the
 208 /// instruction could introduce a safe point.
 209 bool LowerIntrinsics::CouldBecomeSafePoint(Instruction *I) {
 210   // The natural definition of instructions which could introduce safe points
 211   // are:
 212   //
 213   //   - call, invoke (AfterCall, BeforeCall)
 214   //   - phis (Loops)
 215   //   - invoke, ret, unwind (Exit)
 216   //
 217   // However, instructions as seemingly inoccuous as arithmetic can become
 218   // libcalls upon lowering (e.g., div i64 on a 32-bit platform), so instead
 219   // it is necessary to take a conservative approach.
 220
 221   if (isa<AllocaInst>(I) || isa<GetElementPtrInst>(I) ||
 222       isa<StoreInst>(I) || isa<LoadInst>(I))
 223     return false;
 224
 225   // llvm.gcroot is safe because it doesn't do anything at runtime.
 226   if (CallInst *CI = dyn_cast<CallInst>(I))
 227     if (Function *F = CI->getCalledFunction())
 228       if (unsigned IID = F->getIntrinsicID())
 229         if (IID == Intrinsic::gcroot)
 230           return false;
 231
 232   return true;
 233 }
 234
 235 /// runOnFunction - Replace gcread/gcwrite intrinsics with loads and stores.
 236 /// Leave gcroot intrinsics; the code generator needs to see those.
 237 bool LowerIntrinsics::runOnFunction(Function &F) {
 238   // Quick exit for functions that do not use GC.
 239   if (!F.hasGC())
 240     return false;
 241
 242   GCFunctionInfo &FI = getAnalysis<GCModuleInfo>().getFunctionInfo(F);
 243   GCStrategy &S = FI.getStrategy();
 244
 245   bool MadeChange = false;
 246
 247   if (NeedsDefaultLoweringPass(S))
 248     MadeChange |= PerformDefaultLowering(F, S);
 249
 250   if (NeedsCustomLoweringPass(S))
 251     MadeChange |= S.performCustomLowering(F);
 252
 253   return MadeChange;
 254 }
 255
 256 bool LowerIntrinsics::PerformDefaultLowering(Function &F, GCStrategy &S) {
 257   bool LowerWr = !S.customWriteBarrier();
 258   bool LowerRd = !S.customReadBarrier();
 259   bool InitRoots = S.initializeRoots();
 260
 261   SmallVector<AllocaInst*,32> Roots;
 262
 263   bool MadeChange = false;
 264   for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
 265     for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
 266       if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) {
 267         Function *F = CI->getCalledFunction();
 268         switch (F->getIntrinsicID()) {
 269         case Intrinsic::gcwrite:
 270           if (LowerWr) {
 271             // Replace a write barrier with a simple store.
 272             Value *St = new StoreInst(CI->getOperand(1), CI->getOperand(3), CI);
 273             CI->replaceAllUsesWith(St);
 274             CI->eraseFromParent();
 275           }
 276           break;
 277         case Intrinsic::gcread:
 278           if (LowerRd) {
 279             // Replace a read barrier with a simple load.
 280             Value *Ld = new LoadInst(CI->getOperand(2), "", CI);
 281             Ld->takeName(CI);
 282             CI->replaceAllUsesWith(Ld);
 283             CI->eraseFromParent();
 284           }
 285           break;
 286         case Intrinsic::gcroot:
 287           if (InitRoots) {
 288             // Initialize the GC root, but do not delete the intrinsic. The
 289             // backend needs the intrinsic to flag the stack slot.
 290             Roots.push_back(cast<AllocaInst>(
 291                               CI->getOperand(1)->stripPointerCasts()));
 292           }
 293           break;
 294         default:
 295           continue;
 296         }
 297
 298         MadeChange = true;
 299       }
 300     }
 301   }
 302
 303   if (Roots.size())
 304     MadeChange |= InsertRootInitializers(F, Roots.begin(), Roots.size());
 305
 306   return MadeChange;
 307 }
 308
 309 // -----------------------------------------------------------------------------
 310
 311 FunctionPass *llvm::createGCMachineCodeAnalysisPass() {
 312   return new MachineCodeAnalysis();
 313 }
 314
 315 char MachineCodeAnalysis::ID = 0;
 316
 317 MachineCodeAnalysis::MachineCodeAnalysis()
 318   : MachineFunctionPass(&ID) {}
 319
 320 const char *MachineCodeAnalysis::getPassName() const {
 321   return "Analyze Machine Code For Garbage Collection";
 322 }
 323
 324 void MachineCodeAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
 325   MachineFunctionPass::getAnalysisUsage(AU);
 326   AU.setPreservesAll();
 327   AU.addRequired<MachineModuleInfo>();
 328   AU.addRequired<GCModuleInfo>();
 329 }
 330
 331 unsigned MachineCodeAnalysis::InsertLabel(MachineBasicBlock &MBB,
 332                                      MachineBasicBlock::iterator MI) const {
 333   unsigned Label = MMI->NextLabelID();
 334   // N.B. we assume that MI is *not* equal to the "end()" iterator.
 335   BuildMI(MBB, MI, MI->getDebugLoc(),
 336           TII->get(TargetInstrInfo::GC_LABEL)).addImm(Label);
 337   return Label;
 338 }
 339
 340 void 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
 349   if (FI->getStrategy().needsSafePoint(GC::PostCall))
 350     FI->addSafePoint(GC::PostCall, InsertLabel(*CI->getParent(), RAI));
 351 }
 352
 353 void MachineCodeAnalysis::FindSafePoints(MachineFunction &MF) {
 354   for (MachineFunction::iterator BBI = MF.begin(),
 355                                  BBE = MF.end(); BBI != BBE; ++BBI)
 356     for (MachineBasicBlock::iterator MI = BBI->begin(),
 357                                      ME = BBI->end(); MI != ME; ++MI)
 358       if (MI->getDesc().isCall())
 359         VisitCallPoint(MI);
 360 }
 361
 362 void MachineCodeAnalysis::FindStackOffsets(MachineFunction &MF) {
 363   const TargetRegisterInfo *TRI = TM->getRegisterInfo();
 364   assert(TRI && "TargetRegisterInfo not available!");
 365
 366   for (GCFunctionInfo::roots_iterator RI = FI->roots_begin(),
 367                                       RE = FI->roots_end(); RI != RE; ++RI)
 368     RI->StackOffset = TRI->getFrameIndexOffset(MF, RI->Num);
 369 }
 370
 371 bool MachineCodeAnalysis::runOnMachineFunction(MachineFunction &MF) {
 372   // Quick exit for functions that do not use GC.
 373   if (!MF.getFunction()->hasGC())
 374     return false;
 375
 376   FI = &getAnalysis<GCModuleInfo>().getFunctionInfo(*MF.getFunction());
 377   if (!FI->getStrategy().needsSafePoints())
 378     return false;
 379
 380   TM = &MF.getTarget();
 381   MMI = &getAnalysis<MachineModuleInfo>();
 382   TII = TM->getInstrInfo();
 383
 384   // Find the size of the stack frame.
 385   FI->setFrameSize(MF.getFrameInfo()->getStackSize());
 386
 387   // Find all safe points.
 388   FindSafePoints(MF);
 389
 390   // Find the stack offsets for all roots.
 391   FindStackOffsets(MF);
 392
 393   return false;
 394 }