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