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//===- ShadowStackGCLowering.cpp - Custom lowering for shadow-stack gc ----===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// 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/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;
static char ID;
bool doInitialization(Module &M) override;
bool runOnFunction(Function &F) override;
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;
"Shadow Stack GC Lowering", false, false)
"Shadow Stack GC Lowering", false, false)
FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); }
ShadowStackGCLowering::ShadowStackGCLowering() : FunctionPass(ID) {
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));
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;
for (size_t I = 0; I != Roots.size(); I++)
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;
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. :)
// Specifies length of variable length array.
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");
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()) {
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(
if (IsNullValue(CI->getArgOperand(1)))
// 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.
// 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))
AtEntry.SetInsertPoint(IP->getParent(), IP);
// Initialize the map pointer and load the current head of the shadow stack.
Instruction *CurrentHead = AtEntry.CreateLoad(Head, "gc_currhead");
Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
StackEntry, 0, 1, "");
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;
// 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))
AtEntry.SetInsertPoint(IP->getParent(), IP);
// Push the entry onto the shadow stack.
Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
StackEntry, 0, 0, "");
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,
Value *SavedHead = AtExit->CreateLoad(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) {
return true;