llvm/lib/CodeGen/GCRootLowering.cpp

   1 //===-- GCRootLowering.cpp - Garbage collection infrastructure ------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements the lowering for the gc.root mechanism.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "llvm/CodeGen/GCMetadata.h"
  14 #include "llvm/CodeGen/MachineFrameInfo.h"
  15 #include "llvm/CodeGen/MachineFunctionPass.h"
  16 #include "llvm/CodeGen/MachineInstrBuilder.h"
  17 #include "llvm/CodeGen/Passes.h"
  18 #include "llvm/CodeGen/TargetFrameLowering.h"
  19 #include "llvm/CodeGen/TargetInstrInfo.h"
  20 #include "llvm/CodeGen/TargetRegisterInfo.h"
  21 #include "llvm/CodeGen/TargetSubtargetInfo.h"
  22 #include "llvm/IR/Dominators.h"
  23 #include "llvm/IR/IntrinsicInst.h"
  24 #include "llvm/IR/Module.h"
  25 #include "llvm/InitializePasses.h"
  26 #include "llvm/MC/MCContext.h"
  27
  28 using namespace llvm;
  29
  30 /// Lower barriers out of existence (if the associated GCStrategy hasn't
  31 /// already done so...), and insert initializing stores to roots as a defensive
  32 /// measure.  Given we're going to report all roots live at all safepoints, we
  33 /// need to be able to ensure each root has been initialized by the point the
  34 /// first safepoint is reached.  This really should have been done by the
  35 /// frontend, but the old API made this non-obvious, so we do a potentially
  36 /// redundant store just in case.
  37 static bool DoLowering(Function &F, GCStrategy &S);
  38
  39 namespace {
  40
  41 /// LowerIntrinsics - This pass rewrites calls to the llvm.gcread or
  42 /// llvm.gcwrite intrinsics, replacing them with simple loads and stores as
  43 /// directed by the GCStrategy. It also performs automatic root initialization
  44 /// and custom intrinsic lowering.
  45 class LowerIntrinsics : public FunctionPass {
  46 public:
  47   static char ID;
  48
  49   LowerIntrinsics();
  50   StringRef getPassName() const override;
  51   void getAnalysisUsage(AnalysisUsage &AU) const override;
  52
  53   bool doInitialization(Module &M) override;
  54   bool runOnFunction(Function &F) override;
  55 };
  56
  57 /// GCMachineCodeAnalysis - This is a target-independent pass over the machine
  58 /// function representation to identify safe points for the garbage collector
  59 /// in the machine code. It inserts labels at safe points and populates a
  60 /// GCMetadata record for each function.
  61 class GCMachineCodeAnalysis : public MachineFunctionPass {
  62   GCFunctionInfo *FI = nullptr;
  63   const TargetInstrInfo *TII = nullptr;
  64
  65   void FindSafePoints(MachineFunction &MF);
  66   void VisitCallPoint(MachineBasicBlock::iterator CI);
  67   MCSymbol *InsertLabel(MachineBasicBlock &MBB, MachineBasicBlock::iterator MI,
  68                         const DebugLoc &DL) const;
  69
  70   void FindStackOffsets(MachineFunction &MF);
  71
  72 public:
  73   static char ID;
  74
  75   GCMachineCodeAnalysis();
  76   void getAnalysisUsage(AnalysisUsage &AU) const override;
  77
  78   bool runOnMachineFunction(MachineFunction &MF) override;
  79 };
  80 }
  81
  82 PreservedAnalyses GCLoweringPass::run(Function &F,
  83                                       FunctionAnalysisManager &FAM) {
  84   if (!F.hasGC())
  85     return PreservedAnalyses::all();
  86
  87   auto &Info = FAM.getResult<GCFunctionAnalysis>(F);
  88
  89   bool Changed = DoLowering(F, Info.getStrategy());
  90
  91   if (!Changed)
  92     return PreservedAnalyses::all();
  93   PreservedAnalyses PA;
  94   PA.preserve<DominatorTreeAnalysis>();
  95   return PA;
  96 }
  97
  98 // -----------------------------------------------------------------------------
  99
 100 INITIALIZE_PASS_BEGIN(LowerIntrinsics, "gc-lowering", "GC Lowering", false,
 101                       false)
 102 INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
 103 INITIALIZE_PASS_END(LowerIntrinsics, "gc-lowering", "GC Lowering", false, false)
 104
 105 FunctionPass *llvm::createGCLoweringPass() { return new LowerIntrinsics(); }
 106
 107 char LowerIntrinsics::ID = 0;
 108 char &llvm::GCLoweringID = LowerIntrinsics::ID;
 109
 110 LowerIntrinsics::LowerIntrinsics() : FunctionPass(ID) {
 111   initializeLowerIntrinsicsPass(*PassRegistry::getPassRegistry());
 112 }
 113
 114 StringRef LowerIntrinsics::getPassName() const {
 115   return "Lower Garbage Collection Instructions";
 116 }
 117
 118 void LowerIntrinsics::getAnalysisUsage(AnalysisUsage &AU) const {
 119   FunctionPass::getAnalysisUsage(AU);
 120   AU.addRequired<GCModuleInfo>();
 121   AU.addPreserved<DominatorTreeWrapperPass>();
 122 }
 123
 124 /// doInitialization - If this module uses the GC intrinsics, find them now.
 125 bool LowerIntrinsics::doInitialization(Module &M) {
 126   GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
 127   assert(MI && "LowerIntrinsics didn't require GCModuleInfo!?");
 128   for (Function &F : M)
 129     if (!F.isDeclaration() && F.hasGC())
 130       MI->getFunctionInfo(F); // Instantiate the GC strategy.
 131
 132   return false;
 133 }
 134
 135 /// CouldBecomeSafePoint - Predicate to conservatively determine whether the
 136 /// instruction could introduce a safe point.
 137 static bool CouldBecomeSafePoint(Instruction *I) {
 138   // The natural definition of instructions which could introduce safe points
 139   // are:
 140   //
 141   //   - call, invoke (AfterCall, BeforeCall)
 142   //   - phis (Loops)
 143   //   - invoke, ret, unwind (Exit)
 144   //
 145   // However, instructions as seemingly inoccuous as arithmetic can become
 146   // libcalls upon lowering (e.g., div i64 on a 32-bit platform), so instead
 147   // it is necessary to take a conservative approach.
 148
 149   if (isa<AllocaInst>(I) || isa<GetElementPtrInst>(I) || isa<StoreInst>(I) ||
 150       isa<LoadInst>(I))
 151     return false;
 152
 153   // llvm.gcroot is safe because it doesn't do anything at runtime.
 154   if (CallInst *CI = dyn_cast<CallInst>(I))
 155     if (Function *F = CI->getCalledFunction())
 156       if (Intrinsic::ID IID = F->getIntrinsicID())
 157         if (IID == Intrinsic::gcroot)
 158           return false;
 159
 160   return true;
 161 }
 162
 163 static bool InsertRootInitializers(Function &F, ArrayRef<AllocaInst *> Roots) {
 164   // Scroll past alloca instructions.
 165   BasicBlock::iterator IP = F.getEntryBlock().begin();
 166   while (isa<AllocaInst>(IP))
 167     ++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 *Root : Roots)
 181     if (!InitedRoots.count(Root)) {
 182       new StoreInst(
 183           ConstantPointerNull::get(cast<PointerType>(Root->getAllocatedType())),
 184           Root, std::next(Root->getIterator()));
 185       MadeChange = true;
 186     }
 187
 188   return MadeChange;
 189 }
 190
 191 /// runOnFunction - Replace gcread/gcwrite intrinsics with loads and stores.
 192 /// Leave gcroot intrinsics; the code generator needs to see those.
 193 bool LowerIntrinsics::runOnFunction(Function &F) {
 194   // Quick exit for functions that do not use GC.
 195   if (!F.hasGC())
 196     return false;
 197
 198   GCFunctionInfo &FI = getAnalysis<GCModuleInfo>().getFunctionInfo(F);
 199   GCStrategy &S = FI.getStrategy();
 200
 201   return DoLowering(F, S);
 202 }
 203
 204 bool DoLowering(Function &F, GCStrategy &S) {
 205   SmallVector<AllocaInst *, 32> Roots;
 206
 207   bool MadeChange = false;
 208   for (BasicBlock &BB : F)
 209     for (Instruction &I : llvm::make_early_inc_range(BB)) {
 210       IntrinsicInst *CI = dyn_cast<IntrinsicInst>(&I);
 211       if (!CI)
 212         continue;
 213
 214       Function *F = CI->getCalledFunction();
 215       switch (F->getIntrinsicID()) {
 216       default: break;
 217       case Intrinsic::gcwrite: {
 218         // Replace a write barrier with a simple store.
 219         Value *St = new StoreInst(CI->getArgOperand(0), CI->getArgOperand(2),
 220                                   CI->getIterator());
 221         CI->replaceAllUsesWith(St);
 222         CI->eraseFromParent();
 223         MadeChange = true;
 224         break;
 225       }
 226       case Intrinsic::gcread: {
 227         // Replace a read barrier with a simple load.
 228         Value *Ld = new LoadInst(CI->getType(), CI->getArgOperand(1), "",
 229                                  CI->getIterator());
 230         Ld->takeName(CI);
 231         CI->replaceAllUsesWith(Ld);
 232         CI->eraseFromParent();
 233         MadeChange = true;
 234         break;
 235       }
 236       case Intrinsic::gcroot: {
 237         // Initialize the GC root, but do not delete the intrinsic. The
 238         // backend needs the intrinsic to flag the stack slot.
 239         Roots.push_back(
 240             cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
 241         break;
 242       }
 243       }
 244     }
 245
 246   if (Roots.size())
 247     MadeChange |= InsertRootInitializers(F, Roots);
 248
 249   return MadeChange;
 250 }
 251
 252 // -----------------------------------------------------------------------------
 253
 254 char GCMachineCodeAnalysis::ID = 0;
 255 char &llvm::GCMachineCodeAnalysisID = GCMachineCodeAnalysis::ID;
 256
 257 INITIALIZE_PASS(GCMachineCodeAnalysis, "gc-analysis",
 258                 "Analyze Machine Code For Garbage Collection", false, false)
 259
 260 GCMachineCodeAnalysis::GCMachineCodeAnalysis() : MachineFunctionPass(ID) {}
 261
 262 void GCMachineCodeAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
 263   MachineFunctionPass::getAnalysisUsage(AU);
 264   AU.setPreservesAll();
 265   AU.addRequired<GCModuleInfo>();
 266 }
 267
 268 MCSymbol *GCMachineCodeAnalysis::InsertLabel(MachineBasicBlock &MBB,
 269                                              MachineBasicBlock::iterator MI,
 270                                              const DebugLoc &DL) const {
 271   MCSymbol *Label = MBB.getParent()->getContext().createTempSymbol();
 272   BuildMI(MBB, MI, DL, TII->get(TargetOpcode::GC_LABEL)).addSym(Label);
 273   return Label;
 274 }
 275
 276 void GCMachineCodeAnalysis::VisitCallPoint(MachineBasicBlock::iterator CI) {
 277   // Find the return address (next instruction), since that's what will be on
 278   // the stack when the call is suspended and we need to inspect the stack.
 279   MachineBasicBlock::iterator RAI = CI;
 280   ++RAI;
 281
 282   MCSymbol *Label = InsertLabel(*CI->getParent(), RAI, CI->getDebugLoc());
 283   FI->addSafePoint(Label, CI->getDebugLoc());
 284 }
 285
 286 void GCMachineCodeAnalysis::FindSafePoints(MachineFunction &MF) {
 287   for (MachineBasicBlock &MBB : MF)
 288     for (MachineInstr &MI : MBB)
 289       if (MI.isCall()) {
 290         // Do not treat tail or sibling call sites as safe points.  This is
 291         // legal since any arguments passed to the callee which live in the
 292         // remnants of the callers frame will be owned and updated by the
 293         // callee if required.
 294         if (MI.isTerminator())
 295           continue;
 296         VisitCallPoint(&MI);
 297       }
 298 }
 299
 300 void GCMachineCodeAnalysis::FindStackOffsets(MachineFunction &MF) {
 301   const TargetFrameLowering *TFI = MF.getSubtarget().getFrameLowering();
 302   assert(TFI && "TargetRegisterInfo not available!");
 303
 304   for (GCFunctionInfo::roots_iterator RI = FI->roots_begin();
 305        RI != FI->roots_end();) {
 306     // If the root references a dead object, no need to keep it.
 307     if (MF.getFrameInfo().isDeadObjectIndex(RI->Num)) {
 308       RI = FI->removeStackRoot(RI);
 309     } else {
 310       Register FrameReg; // FIXME: surely GCRoot ought to store the
 311                          // register that the offset is from?
 312       auto FrameOffset = TFI->getFrameIndexReference(MF, RI->Num, FrameReg);
 313       assert(!FrameOffset.getScalable() &&
 314              "Frame offsets with a scalable component are not supported");
 315       RI->StackOffset = FrameOffset.getFixed();
 316       ++RI;
 317     }
 318   }
 319 }
 320
 321 bool GCMachineCodeAnalysis::runOnMachineFunction(MachineFunction &MF) {
 322   // Quick exit for functions that do not use GC.
 323   if (!MF.getFunction().hasGC())
 324     return false;
 325
 326   FI = &getAnalysis<GCModuleInfo>().getFunctionInfo(MF.getFunction());
 327   TII = MF.getSubtarget().getInstrInfo();
 328
 329   // Find the size of the stack frame.  There may be no correct static frame
 330   // size, we use UINT64_MAX to represent this.
 331   const MachineFrameInfo &MFI = MF.getFrameInfo();
 332   const TargetRegisterInfo *RegInfo = MF.getSubtarget().getRegisterInfo();
 333   const bool DynamicFrameSize =
 334       MFI.hasVarSizedObjects() || RegInfo->hasStackRealignment(MF);
 335   FI->setFrameSize(DynamicFrameSize ? UINT64_MAX : MFI.getStackSize());
 336
 337   // Find all safe points.
 338   if (FI->getStrategy().needsSafePoints())
 339     FindSafePoints(MF);
 340
 341   // Find the concrete stack offsets for all roots (stack slots)
 342   FindStackOffsets(MF);
 343
 344   return false;
 345 }