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      1 //===-- GCRootLowering.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 the lowering for the gc.root mechanism.
     11 //
     12 //===----------------------------------------------------------------------===//
     13 
     14 #include "llvm/CodeGen/GCMetadata.h"
     15 #include "llvm/CodeGen/GCStrategy.h"
     16 #include "llvm/CodeGen/MachineFrameInfo.h"
     17 #include "llvm/CodeGen/MachineFunctionPass.h"
     18 #include "llvm/CodeGen/MachineInstrBuilder.h"
     19 #include "llvm/CodeGen/MachineModuleInfo.h"
     20 #include "llvm/CodeGen/Passes.h"
     21 #include "llvm/IR/Dominators.h"
     22 #include "llvm/IR/IntrinsicInst.h"
     23 #include "llvm/IR/Module.h"
     24 #include "llvm/Support/Debug.h"
     25 #include "llvm/Support/ErrorHandling.h"
     26 #include "llvm/Support/raw_ostream.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/Target/TargetSubtargetInfo.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 LowerIntrinsics : public FunctionPass {
     42   bool PerformDefaultLowering(Function &F, GCStrategy &Coll);
     43 
     44 public:
     45   static char ID;
     46 
     47   LowerIntrinsics();
     48   const char *getPassName() const override;
     49   void getAnalysisUsage(AnalysisUsage &AU) const override;
     50 
     51   bool doInitialization(Module &M) override;
     52   bool runOnFunction(Function &F) override;
     53 };
     54 
     55 /// GCMachineCodeAnalysis - This is a target-independent pass over the machine
     56 /// function representation to identify safe points for the garbage collector
     57 /// in the machine code. It inserts labels at safe points and populates a
     58 /// GCMetadata record for each function.
     59 class GCMachineCodeAnalysis : public MachineFunctionPass {
     60   GCFunctionInfo *FI;
     61   MachineModuleInfo *MMI;
     62   const TargetInstrInfo *TII;
     63 
     64   void FindSafePoints(MachineFunction &MF);
     65   void VisitCallPoint(MachineBasicBlock::iterator MI);
     66   MCSymbol *InsertLabel(MachineBasicBlock &MBB, MachineBasicBlock::iterator MI,
     67                         DebugLoc DL) const;
     68 
     69   void FindStackOffsets(MachineFunction &MF);
     70 
     71 public:
     72   static char ID;
     73 
     74   GCMachineCodeAnalysis();
     75   void getAnalysisUsage(AnalysisUsage &AU) const override;
     76 
     77   bool runOnMachineFunction(MachineFunction &MF) override;
     78 };
     79 }
     80 
     81 // -----------------------------------------------------------------------------
     82 
     83 INITIALIZE_PASS_BEGIN(LowerIntrinsics, "gc-lowering", "GC Lowering", false,
     84                       false)
     85 INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
     86 INITIALIZE_PASS_END(LowerIntrinsics, "gc-lowering", "GC Lowering", false, false)
     87 
     88 FunctionPass *llvm::createGCLoweringPass() { return new LowerIntrinsics(); }
     89 
     90 char LowerIntrinsics::ID = 0;
     91 
     92 LowerIntrinsics::LowerIntrinsics() : FunctionPass(ID) {
     93   initializeLowerIntrinsicsPass(*PassRegistry::getPassRegistry());
     94 }
     95 
     96 const char *LowerIntrinsics::getPassName() const {
     97   return "Lower Garbage Collection Instructions";
     98 }
     99 
    100 void LowerIntrinsics::getAnalysisUsage(AnalysisUsage &AU) const {
    101   FunctionPass::getAnalysisUsage(AU);
    102   AU.addRequired<GCModuleInfo>();
    103   AU.addPreserved<DominatorTreeWrapperPass>();
    104 }
    105 
    106 static bool NeedsDefaultLoweringPass(const GCStrategy &C) {
    107   // Default lowering is necessary only if read or write barriers have a default
    108   // action. The default for roots is no action.
    109   return !C.customWriteBarrier() || !C.customReadBarrier() ||
    110          C.initializeRoots();
    111 }
    112 
    113 /// doInitialization - If this module uses the GC intrinsics, find them now.
    114 bool LowerIntrinsics::doInitialization(Module &M) {
    115   GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
    116   assert(MI && "LowerIntrinsics didn't require GCModuleInfo!?");
    117   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
    118     if (!I->isDeclaration() && I->hasGC())
    119       MI->getFunctionInfo(*I); // Instantiate the GC strategy.
    120 
    121   return false;
    122 }
    123 
    124 /// CouldBecomeSafePoint - Predicate to conservatively determine whether the
    125 /// instruction could introduce a safe point.
    126 static bool CouldBecomeSafePoint(Instruction *I) {
    127   // The natural definition of instructions which could introduce safe points
    128   // are:
    129   //
    130   //   - call, invoke (AfterCall, BeforeCall)
    131   //   - phis (Loops)
    132   //   - invoke, ret, unwind (Exit)
    133   //
    134   // However, instructions as seemingly inoccuous as arithmetic can become
    135   // libcalls upon lowering (e.g., div i64 on a 32-bit platform), so instead
    136   // it is necessary to take a conservative approach.
    137 
    138   if (isa<AllocaInst>(I) || isa<GetElementPtrInst>(I) || isa<StoreInst>(I) ||
    139       isa<LoadInst>(I))
    140     return false;
    141 
    142   // llvm.gcroot is safe because it doesn't do anything at runtime.
    143   if (CallInst *CI = dyn_cast<CallInst>(I))
    144     if (Function *F = CI->getCalledFunction())
    145       if (Intrinsic::ID IID = F->getIntrinsicID())
    146         if (IID == Intrinsic::gcroot)
    147           return false;
    148 
    149   return true;
    150 }
    151 
    152 static bool InsertRootInitializers(Function &F, AllocaInst **Roots,
    153                                    unsigned Count) {
    154   // Scroll past alloca instructions.
    155   BasicBlock::iterator IP = F.getEntryBlock().begin();
    156   while (isa<AllocaInst>(IP))
    157     ++IP;
    158 
    159   // Search for initializers in the initial BB.
    160   SmallPtrSet<AllocaInst *, 16> InitedRoots;
    161   for (; !CouldBecomeSafePoint(&*IP); ++IP)
    162     if (StoreInst *SI = dyn_cast<StoreInst>(IP))
    163       if (AllocaInst *AI =
    164               dyn_cast<AllocaInst>(SI->getOperand(1)->stripPointerCasts()))
    165         InitedRoots.insert(AI);
    166 
    167   // Add root initializers.
    168   bool MadeChange = false;
    169 
    170   for (AllocaInst **I = Roots, **E = Roots + Count; I != E; ++I)
    171     if (!InitedRoots.count(*I)) {
    172       StoreInst *SI = new StoreInst(
    173           ConstantPointerNull::get(cast<PointerType>(
    174               cast<PointerType>((*I)->getType())->getElementType())),
    175           *I);
    176       SI->insertAfter(*I);
    177       MadeChange = true;
    178     }
    179 
    180   return MadeChange;
    181 }
    182 
    183 /// runOnFunction - Replace gcread/gcwrite intrinsics with loads and stores.
    184 /// Leave gcroot intrinsics; the code generator needs to see those.
    185 bool LowerIntrinsics::runOnFunction(Function &F) {
    186   // Quick exit for functions that do not use GC.
    187   if (!F.hasGC())
    188     return false;
    189 
    190   GCFunctionInfo &FI = getAnalysis<GCModuleInfo>().getFunctionInfo(F);
    191   GCStrategy &S = FI.getStrategy();
    192 
    193   bool MadeChange = false;
    194 
    195   if (NeedsDefaultLoweringPass(S))
    196     MadeChange |= PerformDefaultLowering(F, S);
    197 
    198   return MadeChange;
    199 }
    200 
    201 bool LowerIntrinsics::PerformDefaultLowering(Function &F, GCStrategy &S) {
    202   bool LowerWr = !S.customWriteBarrier();
    203   bool LowerRd = !S.customReadBarrier();
    204   bool InitRoots = S.initializeRoots();
    205 
    206   SmallVector<AllocaInst *, 32> Roots;
    207 
    208   bool MadeChange = false;
    209   for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
    210     for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
    211       if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) {
    212         Function *F = CI->getCalledFunction();
    213         switch (F->getIntrinsicID()) {
    214         case Intrinsic::gcwrite:
    215           if (LowerWr) {
    216             // Replace a write barrier with a simple store.
    217             Value *St =
    218                 new StoreInst(CI->getArgOperand(0), CI->getArgOperand(2), CI);
    219             CI->replaceAllUsesWith(St);
    220             CI->eraseFromParent();
    221           }
    222           break;
    223         case Intrinsic::gcread:
    224           if (LowerRd) {
    225             // Replace a read barrier with a simple load.
    226             Value *Ld = new LoadInst(CI->getArgOperand(1), "", CI);
    227             Ld->takeName(CI);
    228             CI->replaceAllUsesWith(Ld);
    229             CI->eraseFromParent();
    230           }
    231           break;
    232         case Intrinsic::gcroot:
    233           if (InitRoots) {
    234             // Initialize the GC root, but do not delete the intrinsic. The
    235             // backend needs the intrinsic to flag the stack slot.
    236             Roots.push_back(
    237                 cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
    238           }
    239           break;
    240         default:
    241           continue;
    242         }
    243 
    244         MadeChange = true;
    245       }
    246     }
    247   }
    248 
    249   if (Roots.size())
    250     MadeChange |= InsertRootInitializers(F, Roots.begin(), Roots.size());
    251 
    252   return MadeChange;
    253 }
    254 
    255 // -----------------------------------------------------------------------------
    256 
    257 char GCMachineCodeAnalysis::ID = 0;
    258 char &llvm::GCMachineCodeAnalysisID = GCMachineCodeAnalysis::ID;
    259 
    260 INITIALIZE_PASS(GCMachineCodeAnalysis, "gc-analysis",
    261                 "Analyze Machine Code For Garbage Collection", false, false)
    262 
    263 GCMachineCodeAnalysis::GCMachineCodeAnalysis() : MachineFunctionPass(ID) {}
    264 
    265 void GCMachineCodeAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
    266   MachineFunctionPass::getAnalysisUsage(AU);
    267   AU.setPreservesAll();
    268   AU.addRequired<MachineModuleInfo>();
    269   AU.addRequired<GCModuleInfo>();
    270 }
    271 
    272 MCSymbol *GCMachineCodeAnalysis::InsertLabel(MachineBasicBlock &MBB,
    273                                              MachineBasicBlock::iterator MI,
    274                                              DebugLoc DL) const {
    275   MCSymbol *Label = MBB.getParent()->getContext().createTempSymbol();
    276   BuildMI(MBB, MI, DL, TII->get(TargetOpcode::GC_LABEL)).addSym(Label);
    277   return Label;
    278 }
    279 
    280 void GCMachineCodeAnalysis::VisitCallPoint(MachineBasicBlock::iterator CI) {
    281   // Find the return address (next instruction), too, so as to bracket the call
    282   // instruction.
    283   MachineBasicBlock::iterator RAI = CI;
    284   ++RAI;
    285 
    286   if (FI->getStrategy().needsSafePoint(GC::PreCall)) {
    287     MCSymbol *Label = InsertLabel(*CI->getParent(), CI, CI->getDebugLoc());
    288     FI->addSafePoint(GC::PreCall, Label, CI->getDebugLoc());
    289   }
    290 
    291   if (FI->getStrategy().needsSafePoint(GC::PostCall)) {
    292     MCSymbol *Label = InsertLabel(*CI->getParent(), RAI, CI->getDebugLoc());
    293     FI->addSafePoint(GC::PostCall, Label, CI->getDebugLoc());
    294   }
    295 }
    296 
    297 void GCMachineCodeAnalysis::FindSafePoints(MachineFunction &MF) {
    298   for (MachineFunction::iterator BBI = MF.begin(), BBE = MF.end(); BBI != BBE;
    299        ++BBI)
    300     for (MachineBasicBlock::iterator MI = BBI->begin(), ME = BBI->end();
    301          MI != ME; ++MI)
    302       if (MI->isCall()) {
    303         // Do not treat tail or sibling call sites as safe points.  This is
    304         // legal since any arguments passed to the callee which live in the
    305         // remnants of the callers frame will be owned and updated by the
    306         // callee if required.
    307         if (MI->isTerminator())
    308           continue;
    309         VisitCallPoint(MI);
    310       }
    311 }
    312 
    313 void GCMachineCodeAnalysis::FindStackOffsets(MachineFunction &MF) {
    314   const TargetFrameLowering *TFI = MF.getSubtarget().getFrameLowering();
    315   assert(TFI && "TargetRegisterInfo not available!");
    316 
    317   for (GCFunctionInfo::roots_iterator RI = FI->roots_begin();
    318        RI != FI->roots_end();) {
    319     // If the root references a dead object, no need to keep it.
    320     if (MF.getFrameInfo()->isDeadObjectIndex(RI->Num)) {
    321       RI = FI->removeStackRoot(RI);
    322     } else {
    323       unsigned FrameReg; // FIXME: surely GCRoot ought to store the
    324                          // register that the offset is from?
    325       RI->StackOffset = TFI->getFrameIndexReference(MF, RI->Num, FrameReg);
    326       ++RI;
    327     }
    328   }
    329 }
    330 
    331 bool GCMachineCodeAnalysis::runOnMachineFunction(MachineFunction &MF) {
    332   // Quick exit for functions that do not use GC.
    333   if (!MF.getFunction()->hasGC())
    334     return false;
    335 
    336   FI = &getAnalysis<GCModuleInfo>().getFunctionInfo(*MF.getFunction());
    337   MMI = &getAnalysis<MachineModuleInfo>();
    338   TII = MF.getSubtarget().getInstrInfo();
    339 
    340   // Find the size of the stack frame.  There may be no correct static frame
    341   // size, we use UINT64_MAX to represent this.
    342   const MachineFrameInfo *MFI = MF.getFrameInfo();
    343   const TargetRegisterInfo *RegInfo = MF.getSubtarget().getRegisterInfo();
    344   const bool DynamicFrameSize = MFI->hasVarSizedObjects() ||
    345     RegInfo->needsStackRealignment(MF);
    346   FI->setFrameSize(DynamicFrameSize ? UINT64_MAX : MFI->getStackSize());
    347 
    348   // Find all safe points.
    349   if (FI->getStrategy().needsSafePoints())
    350     FindSafePoints(MF);
    351 
    352   // Find the concrete stack offsets for all roots (stack slots)
    353   FindStackOffsets(MF);
    354 
    355   return false;
    356 }
    357