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📄 AggressiveAntiDepBreaker.cpp(37.23 KB)
📄 AggressiveAntiDepBreaker.h(6.8 KB)
📄 AllocationOrder.cpp(1.96 KB)
📄 AllocationOrder.h(2.96 KB)
📄 Analysis.cpp(32.62 KB)
📁 AsmPrinter
📄 AtomicExpandPass.cpp(71.86 KB)
📄 BBSectionsPrepare.cpp(18.8 KB)
📄 BasicTargetTransformInfo.cpp(1.53 KB)
📄 BranchFolding.cpp(77.92 KB)
📄 BranchFolding.h(7.36 KB)
📄 BranchRelaxation.cpp(19.45 KB)
📄 BreakFalseDeps.cpp(9.79 KB)
📄 BuiltinGCs.cpp(4.88 KB)
📄 CFGuardLongjmp.cpp(3.73 KB)
📄 CFIInstrInserter.cpp(17.53 KB)
📄 CalcSpillWeights.cpp(10.22 KB)
📄 CallingConvLower.cpp(10.4 KB)
📄 CodeGen.cpp(5.28 KB)
📄 CodeGenPrepare.cpp(295.01 KB)
📄 CommandFlags.cpp(24.89 KB)
📄 CriticalAntiDepBreaker.cpp(27.91 KB)
📄 CriticalAntiDepBreaker.h(4.22 KB)
📄 DFAPacketizer.cpp(10.91 KB)
📄 DeadMachineInstructionElim.cpp(6.52 KB)
📄 DetectDeadLanes.cpp(20.74 KB)
📄 DwarfEHPrepare.cpp(9.01 KB)
📄 EarlyIfConversion.cpp(37.51 KB)
📄 EdgeBundles.cpp(3.21 KB)
📄 ExecutionDomainFix.cpp(14.67 KB)
📄 ExpandMemCmp.cpp(33.66 KB)
📄 ExpandPostRAPseudos.cpp(7.28 KB)
📄 ExpandReductions.cpp(7.23 KB)
📄 FEntryInserter.cpp(1.81 KB)
📄 FaultMaps.cpp(4.99 KB)
📄 FinalizeISel.cpp(2.65 KB)
📄 FixupStatepointCallerSaved.cpp(11.06 KB)
📄 FuncletLayout.cpp(2.21 KB)
📄 GCMetadata.cpp(5.1 KB)
📄 GCMetadataPrinter.cpp(748 B)
📄 GCRootLowering.cpp(11.46 KB)
📄 GCStrategy.cpp(708 B)
📁 GlobalISel
📄 GlobalMerge.cpp(24.52 KB)
📄 HardwareLoops.cpp(18.44 KB)
📄 IfConversion.cpp(89.43 KB)
📄 ImplicitNullChecks.cpp(25.14 KB)
📄 IndirectBrExpandPass.cpp(7.79 KB)
📄 InlineSpiller.cpp(58.24 KB)
📄 InterferenceCache.cpp(8.83 KB)
📄 InterferenceCache.h(7.22 KB)
📄 InterleavedAccessPass.cpp(16.59 KB)
📄 InterleavedLoadCombinePass.cpp(42.35 KB)
📄 IntrinsicLowering.cpp(17.08 KB)
📄 LLVMTargetMachine.cpp(10.25 KB)
📄 LatencyPriorityQueue.cpp(5.64 KB)
📄 LazyMachineBlockFrequencyInfo.cpp(3.36 KB)
📄 LexicalScopes.cpp(12.16 KB)
📄 LiveDebugValues.cpp(78.98 KB)
📄 LiveDebugVariables.cpp(51.79 KB)
📄 LiveDebugVariables.h(2.15 KB)
📄 LiveInterval.cpp(46.67 KB)
📄 LiveIntervalCalc.cpp(7.62 KB)
📄 LiveIntervalUnion.cpp(6.36 KB)
📄 LiveIntervals.cpp(64.59 KB)
📄 LivePhysRegs.cpp(11.08 KB)
📄 LiveRangeCalc.cpp(15.72 KB)
📄 LiveRangeEdit.cpp(17.03 KB)
📄 LiveRangeShrink.cpp(8.69 KB)
📄 LiveRangeUtils.h(2.12 KB)
📄 LiveRegMatrix.cpp(7.47 KB)
📄 LiveRegUnits.cpp(4.72 KB)
📄 LiveStacks.cpp(2.95 KB)
📄 LiveVariables.cpp(30.26 KB)
📄 LocalStackSlotAllocation.cpp(17.26 KB)
📄 LoopTraversal.cpp(2.89 KB)
📄 LowLevelType.cpp(1.93 KB)
📄 LowerEmuTLS.cpp(5.66 KB)
📄 MBFIWrapper.cpp(1.57 KB)
📄 MIRCanonicalizerPass.cpp(12.46 KB)
📄 MIRNamerPass.cpp(2.16 KB)
📁 MIRParser
📄 MIRPrinter.cpp(32.67 KB)
📄 MIRPrintingPass.cpp(1.99 KB)
📄 MIRVRegNamerUtils.cpp(6.04 KB)
📄 MIRVRegNamerUtils.h(3.25 KB)
📄 MachineBasicBlock.cpp(50.47 KB)
📄 MachineBlockFrequencyInfo.cpp(10.13 KB)
📄 MachineBlockPlacement.cpp(137.61 KB)
📄 MachineBranchProbabilityInfo.cpp(3.5 KB)
📄 MachineCSE.cpp(31.82 KB)
📄 MachineCombiner.cpp(28.13 KB)
📄 MachineCopyPropagation.cpp(29.21 KB)
📄 MachineDebugify.cpp(6.47 KB)
📄 MachineDominanceFrontier.cpp(1.83 KB)
📄 MachineDominators.cpp(4.86 KB)
📄 MachineFrameInfo.cpp(9.77 KB)
📄 MachineFunction.cpp(42.97 KB)
📄 MachineFunctionPass.cpp(4.78 KB)
📄 MachineFunctionPrinterPass.cpp(2.3 KB)
📄 MachineInstr.cpp(76.39 KB)
📄 MachineInstrBundle.cpp(11.49 KB)
📄 MachineLICM.cpp(57.05 KB)
📄 MachineLoopInfo.cpp(4.98 KB)
📄 MachineLoopUtils.cpp(5.16 KB)
📄 MachineModuleInfo.cpp(9.9 KB)
📄 MachineModuleInfoImpls.cpp(1.5 KB)
📄 MachineOperand.cpp(39.6 KB)
📄 MachineOptimizationRemarkEmitter.cpp(3.29 KB)
📄 MachineOutliner.cpp(42.13 KB)
📄 MachinePipeliner.cpp(111.33 KB)
📄 MachinePostDominators.cpp(2.42 KB)
📄 MachineRegionInfo.cpp(4.75 KB)
📄 MachineRegisterInfo.cpp(22.97 KB)
📄 MachineSSAUpdater.cpp(12.99 KB)
📄 MachineScheduler.cpp(136.89 KB)
📄 MachineSink.cpp(51.94 KB)
📄 MachineSizeOpts.cpp(8.76 KB)
📄 MachineStripDebug.cpp(3.76 KB)
📄 MachineTraceMetrics.cpp(49.58 KB)
📄 MachineVerifier.cpp(107.98 KB)
📄 MacroFusion.cpp(7.55 KB)
📄 ModuloSchedule.cpp(85.09 KB)
📄 NonRelocatableStringpool.cpp(1.65 KB)
📄 OptimizePHIs.cpp(6.7 KB)
📄 PHIElimination.cpp(27.73 KB)
📄 PHIEliminationUtils.cpp(2.56 KB)
📄 PHIEliminationUtils.h(972 B)
📄 ParallelCG.cpp(3.71 KB)
📄 PatchableFunction.cpp(3.44 KB)
📄 PeepholeOptimizer.cpp(78.41 KB)
📄 PostRAHazardRecognizer.cpp(3.5 KB)
📄 PostRASchedulerList.cpp(24.31 KB)
📄 PreISelIntrinsicLowering.cpp(7.91 KB)
📄 ProcessImplicitDefs.cpp(5.4 KB)
📄 PrologEpilogInserter.cpp(50.45 KB)
📄 PseudoSourceValue.cpp(4.71 KB)
📄 RDFGraph.cpp(58.39 KB)
📄 RDFLiveness.cpp(40.7 KB)
📄 RDFRegisters.cpp(11.29 KB)
📄 ReachingDefAnalysis.cpp(21.74 KB)
📄 RegAllocBase.cpp(6.31 KB)
📄 RegAllocBase.h(4.63 KB)
📄 RegAllocBasic.cpp(11.33 KB)
📄 RegAllocFast.cpp(45.78 KB)
📄 RegAllocGreedy.cpp(123.32 KB)
📄 RegAllocPBQP.cpp(33.14 KB)
📄 RegUsageInfoCollector.cpp(7.39 KB)
📄 RegUsageInfoPropagate.cpp(5.07 KB)
📄 RegisterClassInfo.cpp(6.62 KB)
📄 RegisterCoalescer.cpp(151.71 KB)
📄 RegisterCoalescer.h(4.04 KB)
📄 RegisterPressure.cpp(48.86 KB)
📄 RegisterScavenging.cpp(27.48 KB)
📄 RegisterUsageInfo.cpp(3.18 KB)
📄 RenameIndependentSubregs.cpp(14.79 KB)
📄 ResetMachineFunctionPass.cpp(3.48 KB)
📄 SafeStack.cpp(34.12 KB)
📄 SafeStackLayout.cpp(5.3 KB)
📄 SafeStackLayout.h(2.41 KB)
📄 ScalarizeMaskedMemIntrin.cpp(31.46 KB)
📄 ScheduleDAG.cpp(21.34 KB)
📄 ScheduleDAGInstrs.cpp(54.59 KB)
📄 ScheduleDAGPrinter.cpp(3.21 KB)
📄 ScoreboardHazardRecognizer.cpp(7.96 KB)
📁 SelectionDAG
📄 ShadowStackGCLowering.cpp(14.16 KB)
📄 ShrinkWrap.cpp(23.03 KB)
📄 SjLjEHPrepare.cpp(18.93 KB)
📄 SlotIndexes.cpp(9.35 KB)
📄 SpillPlacement.cpp(12.58 KB)
📄 SpillPlacement.h(6.67 KB)
📄 SplitKit.cpp(66.39 KB)
📄 SplitKit.h(23.7 KB)
📄 StackColoring.cpp(49.03 KB)
📄 StackMapLivenessAnalysis.cpp(6.16 KB)
📄 StackMaps.cpp(19.74 KB)
📄 StackProtector.cpp(22.94 KB)
📄 StackSlotColoring.cpp(17.12 KB)
📄 SwiftErrorValueTracking.cpp(11.37 KB)
📄 SwitchLoweringUtils.cpp(18.33 KB)
📄 TailDuplication.cpp(3.32 KB)
📄 TailDuplicator.cpp(38.29 KB)
📄 TargetFrameLoweringImpl.cpp(6.24 KB)
📄 TargetInstrInfo.cpp(51.1 KB)
📄 TargetLoweringBase.cpp(82.53 KB)
📄 TargetLoweringObjectFileImpl.cpp(80.52 KB)
📄 TargetOptionsImpl.cpp(2 KB)
📄 TargetPassConfig.cpp(48.89 KB)
📄 TargetRegisterInfo.cpp(19.15 KB)
📄 TargetSchedule.cpp(13.16 KB)
📄 TargetSubtargetInfo.cpp(1.89 KB)
📄 TwoAddressInstructionPass.cpp(62.08 KB)
📄 TypePromotion.cpp(32.46 KB)
📄 UnreachableBlockElim.cpp(7.48 KB)
📄 ValueTypes.cpp(19.87 KB)
📄 VirtRegMap.cpp(21.4 KB)
📄 WasmEHPrepare.cpp(17.48 KB)
📄 WinEHPrepare.cpp(51.16 KB)
📄 XRayInstrumentation.cpp(9.66 KB)

Editing: ShadowStackGCLowering.cpp

//===- ShadowStackGCLowering.cpp - Custom lowering for shadow-stack gc ----===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file contains the custom lowering code required by the shadow-stack GC
// strategy.
//
// This pass implements the code transformation described in this paper:
//   "Accurate Garbage Collection in an Uncooperative Environment"
//   Fergus Henderson, ISMM, 2002
//
//===----------------------------------------------------------------------===//

#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Transforms/Utils/EscapeEnumerator.h"
#include <cassert>
#include <cstddef>
#include <string>
#include <utility>
#include <vector>

using namespace llvm;

#define DEBUG_TYPE "shadow-stack-gc-lowering"

namespace {

class ShadowStackGCLowering : public FunctionPass {
  /// RootChain - This is the global linked-list that contains the chain of GC
  /// roots.
  GlobalVariable *Head = nullptr;

/// StackEntryTy - Abstract type of a link in the shadow stack.
  StructType *StackEntryTy = nullptr;
  StructType *FrameMapTy = nullptr;

/// Roots - GC roots in the current function. Each is a pair of the
  /// intrinsic call and its corresponding alloca.
  std::vector<std::pair<CallInst *, AllocaInst *>> Roots;

public:
  static char ID;

ShadowStackGCLowering();

bool doInitialization(Module &M) override;
  bool runOnFunction(Function &F) override;

private:
  bool IsNullValue(Value *V);
  Constant *GetFrameMap(Function &F);
  Type *GetConcreteStackEntryType(Function &F);
  void CollectRoots(Function &F);

static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
                                      Type *Ty, Value *BasePtr, int Idx1,
                                      const char *Name);
  static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
                                      Type *Ty, Value *BasePtr, int Idx1, int Idx2,
                                      const char *Name);
};

} // end anonymous namespace

char ShadowStackGCLowering::ID = 0;

INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, DEBUG_TYPE,
                      "Shadow Stack GC Lowering", false, false)
INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
INITIALIZE_PASS_END(ShadowStackGCLowering, DEBUG_TYPE,
                    "Shadow Stack GC Lowering", false, false)

FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); }

ShadowStackGCLowering::ShadowStackGCLowering() : FunctionPass(ID) {
  initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry());
}

Constant *ShadowStackGCLowering::GetFrameMap(Function &F) {
  // doInitialization creates the abstract type of this value.
  Type *VoidPtr = Type::getInt8PtrTy(F.getContext());

// Truncate the ShadowStackDescriptor if some metadata is null.
  unsigned NumMeta = 0;
  SmallVector<Constant *, 16> Metadata;
  for (unsigned I = 0; I != Roots.size(); ++I) {
    Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1));
    if (!C->isNullValue())
      NumMeta = I + 1;
    Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
  }
  Metadata.resize(NumMeta);

Type *Int32Ty = Type::getInt32Ty(F.getContext());

Constant *BaseElts[] = {
      ConstantInt::get(Int32Ty, Roots.size(), false),
      ConstantInt::get(Int32Ty, NumMeta, false),
  };

Constant *DescriptorElts[] = {
      ConstantStruct::get(FrameMapTy, BaseElts),
      ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)};

Type *EltTys[] = {DescriptorElts[0]->getType(), DescriptorElts[1]->getType()};
  StructType *STy = StructType::create(EltTys, "gc_map." + utostr(NumMeta));

Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts);

// FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
  //        that, short of multithreaded LLVM, it should be safe; all that is
  //        necessary is that a simple Module::iterator loop not be invalidated.
  //        Appending to the GlobalVariable list is safe in that sense.
  //
  //        All of the output passes emit globals last. The ExecutionEngine
  //        explicitly supports adding globals to the module after
  //        initialization.
  //
  //        Still, if it isn't deemed acceptable, then this transformation needs
  //        to be a ModulePass (which means it cannot be in the 'llc' pipeline
  //        (which uses a FunctionPassManager (which segfaults (not asserts) if
  //        provided a ModulePass))).
  Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
                                    GlobalVariable::InternalLinkage, FrameMap,
                                    "__gc_" + F.getName());

Constant *GEPIndices[2] = {
      ConstantInt::get(Type::getInt32Ty(F.getContext()), 0),
      ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)};
  return ConstantExpr::getGetElementPtr(FrameMap->getType(), GV, GEPIndices);
}

Type *ShadowStackGCLowering::GetConcreteStackEntryType(Function &F) {
  // doInitialization creates the generic version of this type.
  std::vector<Type *> EltTys;
  EltTys.push_back(StackEntryTy);
  for (size_t I = 0; I != Roots.size(); I++)
    EltTys.push_back(Roots[I].second->getAllocatedType());

return StructType::create(EltTys, ("gc_stackentry." + F.getName()).str());
}

/// doInitialization - If this module uses the GC intrinsics, find them now. If
/// not, exit fast.
bool ShadowStackGCLowering::doInitialization(Module &M) {
  bool Active = false;
  for (Function &F : M) {
    if (F.hasGC() && F.getGC() == std::string("shadow-stack")) {
      Active = true;
      break;
    }
  }
  if (!Active)
    return false;

// struct FrameMap {
  //   int32_t NumRoots; // Number of roots in stack frame.
  //   int32_t NumMeta;  // Number of metadata descriptors. May be < NumRoots.
  //   void *Meta[];     // May be absent for roots without metadata.
  // };
  std::vector<Type *> EltTys;
  // 32 bits is ok up to a 32GB stack frame. :)
  EltTys.push_back(Type::getInt32Ty(M.getContext()));
  // Specifies length of variable length array.
  EltTys.push_back(Type::getInt32Ty(M.getContext()));
  FrameMapTy = StructType::create(EltTys, "gc_map");
  PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);

// struct StackEntry {
  //   ShadowStackEntry *Next; // Caller's stack entry.
  //   FrameMap *Map;          // Pointer to constant FrameMap.
  //   void *Roots[];          // Stack roots (in-place array, so we pretend).
  // };

StackEntryTy = StructType::create(M.getContext(), "gc_stackentry");

EltTys.clear();
  EltTys.push_back(PointerType::getUnqual(StackEntryTy));
  EltTys.push_back(FrameMapPtrTy);
  StackEntryTy->setBody(EltTys);
  PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);

// Get the root chain if it already exists.
  Head = M.getGlobalVariable("llvm_gc_root_chain");
  if (!Head) {
    // If the root chain does not exist, insert a new one with linkonce
    // linkage!
    Head = new GlobalVariable(
        M, StackEntryPtrTy, false, GlobalValue::LinkOnceAnyLinkage,
        Constant::getNullValue(StackEntryPtrTy), "llvm_gc_root_chain");
  } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
    Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
    Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
  }

return true;
}

bool ShadowStackGCLowering::IsNullValue(Value *V) {
  if (Constant *C = dyn_cast<Constant>(V))
    return C->isNullValue();
  return false;
}

void ShadowStackGCLowering::CollectRoots(Function &F) {
  // FIXME: Account for original alignment. Could fragment the root array.
  //   Approach 1: Null initialize empty slots at runtime. Yuck.
  //   Approach 2: Emit a map of the array instead of just a count.

assert(Roots.empty() && "Not cleaned up?");

SmallVector<std::pair<CallInst *, AllocaInst *>, 16> MetaRoots;

for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
    for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;)
      if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++))
        if (Function *F = CI->getCalledFunction())
          if (F->getIntrinsicID() == Intrinsic::gcroot) {
            std::pair<CallInst *, AllocaInst *> Pair = std::make_pair(
                CI,
                cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
            if (IsNullValue(CI->getArgOperand(1)))
              Roots.push_back(Pair);
            else
              MetaRoots.push_back(Pair);
          }

// Number roots with metadata (usually empty) at the beginning, so that the
  // FrameMap::Meta array can be elided.
  Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
}

GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
                                                    IRBuilder<> &B, Type *Ty,
                                                    Value *BasePtr, int Idx,
                                                    int Idx2,
                                                    const char *Name) {
  Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
                      ConstantInt::get(Type::getInt32Ty(Context), Idx),
                      ConstantInt::get(Type::getInt32Ty(Context), Idx2)};
  Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);

assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");

return dyn_cast<GetElementPtrInst>(Val);
}

GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
                                            IRBuilder<> &B, Type *Ty, Value *BasePtr,
                                            int Idx, const char *Name) {
  Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
                      ConstantInt::get(Type::getInt32Ty(Context), Idx)};
  Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);

assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");

return dyn_cast<GetElementPtrInst>(Val);
}

/// runOnFunction - Insert code to maintain the shadow stack.
bool ShadowStackGCLowering::runOnFunction(Function &F) {
  // Quick exit for functions that do not use the shadow stack GC.
  if (!F.hasGC() ||
      F.getGC() != std::string("shadow-stack"))
    return false;

LLVMContext &Context = F.getContext();

// Find calls to llvm.gcroot.
  CollectRoots(F);

// If there are no roots in this function, then there is no need to add a
  // stack map entry for it.
  if (Roots.empty())
    return false;

// Build the constant map and figure the type of the shadow stack entry.
  Value *FrameMap = GetFrameMap(F);
  Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);

// Build the shadow stack entry at the very start of the function.
  BasicBlock::iterator IP = F.getEntryBlock().begin();
  IRBuilder<> AtEntry(IP->getParent(), IP);

Instruction *StackEntry =
      AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr, "gc_frame");

while (isa<AllocaInst>(IP))
    ++IP;
  AtEntry.SetInsertPoint(IP->getParent(), IP);

// Initialize the map pointer and load the current head of the shadow stack.
  Instruction *CurrentHead =
      AtEntry.CreateLoad(StackEntryTy->getPointerTo(), Head, "gc_currhead");
  Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
                                       StackEntry, 0, 1, "gc_frame.map");
  AtEntry.CreateStore(FrameMap, EntryMapPtr);

// After all the allocas...
  for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
    // For each root, find the corresponding slot in the aggregate...
    Value *SlotPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
                               StackEntry, 1 + I, "gc_root");

// And use it in lieu of the alloca.
    AllocaInst *OriginalAlloca = Roots[I].second;
    SlotPtr->takeName(OriginalAlloca);
    OriginalAlloca->replaceAllUsesWith(SlotPtr);
  }

// Move past the original stores inserted by GCStrategy::InitRoots. This isn't
  // really necessary (the collector would never see the intermediate state at
  // runtime), but it's nicer not to push the half-initialized entry onto the
  // shadow stack.
  while (isa<StoreInst>(IP))
    ++IP;
  AtEntry.SetInsertPoint(IP->getParent(), IP);

// Push the entry onto the shadow stack.
  Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
                                        StackEntry, 0, 0, "gc_frame.next");
  Instruction *NewHeadVal = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
                                      StackEntry, 0, "gc_newhead");
  AtEntry.CreateStore(CurrentHead, EntryNextPtr);
  AtEntry.CreateStore(NewHeadVal, Head);

// For each instruction that escapes...
  EscapeEnumerator EE(F, "gc_cleanup");
  while (IRBuilder<> *AtExit = EE.Next()) {
    // Pop the entry from the shadow stack. Don't reuse CurrentHead from
    // AtEntry, since that would make the value live for the entire function.
    Instruction *EntryNextPtr2 =
        CreateGEP(Context, *AtExit, ConcreteStackEntryTy, StackEntry, 0, 0,
                  "gc_frame.next");
    Value *SavedHead = AtExit->CreateLoad(StackEntryTy->getPointerTo(),
                                          EntryNextPtr2, "gc_savedhead");
    AtExit->CreateStore(SavedHead, Head);
  }

// Delete the original allocas (which are no longer used) and the intrinsic
  // calls (which are no longer valid). Doing this last avoids invalidating
  // iterators.
  for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
    Roots[I].first->eraseFromParent();
    Roots[I].second->eraseFromParent();
  }

Roots.clear();
  return true;
}

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Editing: ShadowStackGCLowering.cpp

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