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 namespace {
  31
  32 /// LowerIntrinsics - This pass rewrites calls to the llvm.gcread or
  33 /// llvm.gcwrite intrinsics, replacing them with simple loads and stores as
  34 /// directed by the GCStrategy. It also performs automatic root initialization
  35 /// and custom intrinsic lowering.
  36 class LowerIntrinsics : public FunctionPass {
  37   bool DoLowering(Function &F, GCStrategy &S);
  38
  39 public:
  40   static char ID;
  41
  42   LowerIntrinsics();
  43   StringRef getPassName() const override;
  44   void getAnalysisUsage(AnalysisUsage &AU) const override;
  45
  46   bool doInitialization(Module &M) override;
  47   bool runOnFunction(Function &F) override;
  48 };
  49
  50 /// GCMachineCodeAnalysis - This is a target-independent pass over the machine
  51 /// function representation to identify safe points for the garbage collector
  52 /// in the machine code. It inserts labels at safe points and populates a
  53 /// GCMetadata record for each function.
  54 class GCMachineCodeAnalysis : public MachineFunctionPass {
  55   GCFunctionInfo *FI;
  56   const TargetInstrInfo *TII;
  57
  58   void FindSafePoints(MachineFunction &MF);
  59   void VisitCallPoint(MachineBasicBlock::iterator CI);
  60   MCSymbol *InsertLabel(MachineBasicBlock &MBB, MachineBasicBlock::iterator MI,
  61                         const DebugLoc &DL) const;
  62
  63   void FindStackOffsets(MachineFunction &MF);
  64
  65 public:
  66   static char ID;
  67
  68   GCMachineCodeAnalysis();
  69   void getAnalysisUsage(AnalysisUsage &AU) const override;
  70
  71   bool runOnMachineFunction(MachineFunction &MF) override;
  72 };
  73 }
  74
  75 // -----------------------------------------------------------------------------
  76
  77 INITIALIZE_PASS_BEGIN(LowerIntrinsics, "gc-lowering", "GC Lowering", false,
  78                       false)
  79 INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
  80 INITIALIZE_PASS_END(LowerIntrinsics, "gc-lowering", "GC Lowering", false, false)
  81
  82 FunctionPass *llvm::createGCLoweringPass() { return new LowerIntrinsics(); }
  83
  84 char LowerIntrinsics::ID = 0;
  85 char &llvm::GCLoweringID = LowerIntrinsics::ID;
  86
  87 LowerIntrinsics::LowerIntrinsics() : FunctionPass(ID) {
  88   initializeLowerIntrinsicsPass(*PassRegistry::getPassRegistry());
  89 }
  90
  91 StringRef LowerIntrinsics::getPassName() const {
  92   return "Lower Garbage Collection Instructions";
  93 }
  94
  95 void LowerIntrinsics::getAnalysisUsage(AnalysisUsage &AU) const {
  96   FunctionPass::getAnalysisUsage(AU);
  97   AU.addRequired<GCModuleInfo>();
  98   AU.addPreserved<DominatorTreeWrapperPass>();
  99 }
 100
 101 /// doInitialization - If this module uses the GC intrinsics, find them now.
 102 bool LowerIntrinsics::doInitialization(Module &M) {
 103   GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
 104   assert(MI && "LowerIntrinsics didn't require GCModuleInfo!?");
 105   for (Function &F : M)
 106     if (!F.isDeclaration() && F.hasGC())
 107       MI->getFunctionInfo(F); // Instantiate the GC strategy.
 108
 109   return false;
 110 }
 111
 112 /// CouldBecomeSafePoint - Predicate to conservatively determine whether the
 113 /// instruction could introduce a safe point.
 114 static bool CouldBecomeSafePoint(Instruction *I) {
 115   // The natural definition of instructions which could introduce safe points
 116   // are:
 117   //
 118   //   - call, invoke (AfterCall, BeforeCall)
 119   //   - phis (Loops)
 120   //   - invoke, ret, unwind (Exit)
 121   //
 122   // However, instructions as seemingly inoccuous as arithmetic can become
 123   // libcalls upon lowering (e.g., div i64 on a 32-bit platform), so instead
 124   // it is necessary to take a conservative approach.
 125
 126   if (isa<AllocaInst>(I) || isa<GetElementPtrInst>(I) || isa<StoreInst>(I) ||
 127       isa<LoadInst>(I))
 128     return false;
 129
 130   // llvm.gcroot is safe because it doesn't do anything at runtime.
 131   if (CallInst *CI = dyn_cast<CallInst>(I))
 132     if (Function *F = CI->getCalledFunction())
 133       if (Intrinsic::ID IID = F->getIntrinsicID())
 134         if (IID == Intrinsic::gcroot)
 135           return false;
 136
 137   return true;
 138 }
 139
 140 static bool InsertRootInitializers(Function &F, ArrayRef<AllocaInst *> Roots) {
 141   // Scroll past alloca instructions.
 142   BasicBlock::iterator IP = F.getEntryBlock().begin();
 143   while (isa<AllocaInst>(IP))
 144     ++IP;
 145
 146   // Search for initializers in the initial BB.
 147   SmallPtrSet<AllocaInst *, 16> InitedRoots;
 148   for (; !CouldBecomeSafePoint(&*IP); ++IP)
 149     if (StoreInst *SI = dyn_cast<StoreInst>(IP))
 150       if (AllocaInst *AI =
 151               dyn_cast<AllocaInst>(SI->getOperand(1)->stripPointerCasts()))
 152         InitedRoots.insert(AI);
 153
 154   // Add root initializers.
 155   bool MadeChange = false;
 156
 157   for (AllocaInst *Root : Roots)
 158     if (!InitedRoots.count(Root)) {
 159       new StoreInst(
 160           ConstantPointerNull::get(cast<PointerType>(Root->getAllocatedType())),
 161           Root, Root->getNextNode());
 162       MadeChange = true;
 163     }
 164
 165   return MadeChange;
 166 }
 167
 168 /// runOnFunction - Replace gcread/gcwrite intrinsics with loads and stores.
 169 /// Leave gcroot intrinsics; the code generator needs to see those.
 170 bool LowerIntrinsics::runOnFunction(Function &F) {
 171   // Quick exit for functions that do not use GC.
 172   if (!F.hasGC())
 173     return false;
 174
 175   GCFunctionInfo &FI = getAnalysis<GCModuleInfo>().getFunctionInfo(F);
 176   GCStrategy &S = FI.getStrategy();
 177
 178   return DoLowering(F, S);
 179 }
 180
 181 /// Lower barriers out of existance (if the associated GCStrategy hasn't
 182 /// already done so...), and insert initializing stores to roots as a defensive
 183 /// measure.  Given we're going to report all roots live at all safepoints, we
 184 /// need to be able to ensure each root has been initialized by the point the
 185 /// first safepoint is reached.  This really should have been done by the
 186 /// frontend, but the old API made this non-obvious, so we do a potentially
 187 /// redundant store just in case.
 188 bool LowerIntrinsics::DoLowering(Function &F, GCStrategy &S) {
 189   SmallVector<AllocaInst *, 32> Roots;
 190
 191   bool MadeChange = false;
 192   for (BasicBlock &BB : F)
 193     for (Instruction &I : llvm::make_early_inc_range(BB)) {
 194       IntrinsicInst *CI = dyn_cast<IntrinsicInst>(&I);
 195       if (!CI)
 196         continue;
 197
 198       Function *F = CI->getCalledFunction();
 199       switch (F->getIntrinsicID()) {
 200       default: break;
 201       case Intrinsic::gcwrite: {
 202         // Replace a write barrier with a simple store.
 203         Value *St = new StoreInst(CI->getArgOperand(0),
 204                                   CI->getArgOperand(2), CI);
 205         CI->replaceAllUsesWith(St);
 206         CI->eraseFromParent();
 207         MadeChange = true;
 208         break;
 209       }
 210       case Intrinsic::gcread: {
 211         // Replace a read barrier with a simple load.
 212         Value *Ld = new LoadInst(CI->getType(), CI->getArgOperand(1), "", CI);
 213         Ld->takeName(CI);
 214         CI->replaceAllUsesWith(Ld);
 215         CI->eraseFromParent();
 216         MadeChange = true;
 217         break;
 218       }
 219       case Intrinsic::gcroot: {
 220         // Initialize the GC root, but do not delete the intrinsic. The
 221         // backend needs the intrinsic to flag the stack slot.
 222         Roots.push_back(
 223             cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
 224         break;
 225       }
 226       }
 227     }
 228
 229   if (Roots.size())
 230     MadeChange |= InsertRootInitializers(F, Roots);
 231
 232   return MadeChange;
 233 }
 234
 235 // -----------------------------------------------------------------------------
 236
 237 char GCMachineCodeAnalysis::ID = 0;
 238 char &llvm::GCMachineCodeAnalysisID = GCMachineCodeAnalysis::ID;
 239
 240 INITIALIZE_PASS(GCMachineCodeAnalysis, "gc-analysis",
 241                 "Analyze Machine Code For Garbage Collection", false, false)
 242
 243 GCMachineCodeAnalysis::GCMachineCodeAnalysis() : MachineFunctionPass(ID) {}
 244
 245 void GCMachineCodeAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
 246   MachineFunctionPass::getAnalysisUsage(AU);
 247   AU.setPreservesAll();
 248   AU.addRequired<GCModuleInfo>();
 249 }
 250
 251 MCSymbol *GCMachineCodeAnalysis::InsertLabel(MachineBasicBlock &MBB,
 252                                              MachineBasicBlock::iterator MI,
 253                                              const DebugLoc &DL) const {
 254   MCSymbol *Label = MBB.getParent()->getContext().createTempSymbol();
 255   BuildMI(MBB, MI, DL, TII->get(TargetOpcode::GC_LABEL)).addSym(Label);
 256   return Label;
 257 }
 258
 259 void GCMachineCodeAnalysis::VisitCallPoint(MachineBasicBlock::iterator CI) {
 260   // Find the return address (next instruction), since that's what will be on
 261   // the stack when the call is suspended and we need to inspect the stack.
 262   MachineBasicBlock::iterator RAI = CI;
 263   ++RAI;
 264
 265   MCSymbol *Label = InsertLabel(*CI->getParent(), RAI, CI->getDebugLoc());
 266   FI->addSafePoint(Label, CI->getDebugLoc());
 267 }
 268
 269 void GCMachineCodeAnalysis::FindSafePoints(MachineFunction &MF) {
 270   for (MachineBasicBlock &MBB : MF)
 271     for (MachineInstr &MI : MBB)
 272       if (MI.isCall()) {
 273         // Do not treat tail or sibling call sites as safe points.  This is
 274         // legal since any arguments passed to the callee which live in the
 275         // remnants of the callers frame will be owned and updated by the
 276         // callee if required.
 277         if (MI.isTerminator())
 278           continue;
 279         VisitCallPoint(&MI);
 280       }
 281 }
 282
 283 void GCMachineCodeAnalysis::FindStackOffsets(MachineFunction &MF) {
 284   const TargetFrameLowering *TFI = MF.getSubtarget().getFrameLowering();
 285   assert(TFI && "TargetRegisterInfo not available!");
 286
 287   for (GCFunctionInfo::roots_iterator RI = FI->roots_begin();
 288        RI != FI->roots_end();) {
 289     // If the root references a dead object, no need to keep it.
 290     if (MF.getFrameInfo().isDeadObjectIndex(RI->Num)) {
 291       RI = FI->removeStackRoot(RI);
 292     } else {
 293       Register FrameReg; // FIXME: surely GCRoot ought to store the
 294                          // register that the offset is from?
 295       auto FrameOffset = TFI->getFrameIndexReference(MF, RI->Num, FrameReg);
 296       assert(!FrameOffset.getScalable() &&
 297              "Frame offsets with a scalable component are not supported");
 298       RI->StackOffset = FrameOffset.getFixed();
 299       ++RI;
 300     }
 301   }
 302 }
 303
 304 bool GCMachineCodeAnalysis::runOnMachineFunction(MachineFunction &MF) {
 305   // Quick exit for functions that do not use GC.
 306   if (!MF.getFunction().hasGC())
 307     return false;
 308
 309   FI = &getAnalysis<GCModuleInfo>().getFunctionInfo(MF.getFunction());
 310   TII = MF.getSubtarget().getInstrInfo();
 311
 312   // Find the size of the stack frame.  There may be no correct static frame
 313   // size, we use UINT64_MAX to represent this.
 314   const MachineFrameInfo &MFI = MF.getFrameInfo();
 315   const TargetRegisterInfo *RegInfo = MF.getSubtarget().getRegisterInfo();
 316   const bool DynamicFrameSize =
 317       MFI.hasVarSizedObjects() || RegInfo->hasStackRealignment(MF);
 318   FI->setFrameSize(DynamicFrameSize ? UINT64_MAX : MFI.getStackSize());
 319
 320   // Find all safe points.
 321   if (FI->getStrategy().needsSafePoints())
 322     FindSafePoints(MF);
 323
 324   // Find the concrete stack offsets for all roots (stack slots)
 325   FindStackOffsets(MF);
 326
 327   return false;
 328 }