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+//===-- ShadowStackGC.cpp - GC support for uncooperative targets ----------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements lowering for the llvm.gc* intrinsics for targets that do
+// not natively support them (which includes the C backend). Note that the code
+// generated is not quite as efficient as algorithms which generate stack maps
+// to identify roots.
+//
+// This pass implements the code transformation described in this paper:
+// "Accurate Garbage Collection in an Uncooperative Environment"
+// Fergus Henderson, ISMM, 2002
+//
+// In runtime/GC/SemiSpace.cpp is a prototype runtime which is compatible with
+// ShadowStackGC.
+//
+// In order to support this particular transformation, all stack roots are
+// coallocated in the stack. This allows a fully target-independent stack map
+// while introducing only minor runtime overhead.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "shadowstackgc"
+#include "llvm/CodeGen/GCs.h"
+#include "llvm/ADT/StringExtras.h"
+#include "llvm/CodeGen/GCStrategy.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Support/CallSite.h"
+
+using namespace llvm;
+
+namespace {
+
+ class ShadowStackGC : public GCStrategy {
+ /// RootChain - This is the global linked-list that contains the chain of GC
+ /// roots.
+ GlobalVariable *Head;
+
+ /// StackEntryTy - Abstract type of a link in the shadow stack.
+ ///
+ StructType *StackEntryTy;
+ StructType *FrameMapTy;
+
+ /// 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:
+ ShadowStackGC();
+
+ bool initializeCustomLowering(Module &M);
+ bool performCustomLowering(Function &F);
+
+ private:
+ bool IsNullValue(Value *V);
+ Constant *GetFrameMap(Function &F);
+ Type* GetConcreteStackEntryType(Function &F);
+ void CollectRoots(Function &F);
+ static GetElementPtrInst *CreateGEP(LLVMContext &Context,
+ IRBuilder<> &B, Value *BasePtr,
+ int Idx1, const char *Name);
+ static GetElementPtrInst *CreateGEP(LLVMContext &Context,
+ IRBuilder<> &B, Value *BasePtr,
+ int Idx1, int Idx2, const char *Name);
+ };
+
+}
+
+static GCRegistry::Add<ShadowStackGC>
+X("shadow-stack", "Very portable GC for uncooperative code generators");
+
+namespace {
+ /// EscapeEnumerator - This is a little algorithm to find all escape points
+ /// from a function so that "finally"-style code can be inserted. In addition
+ /// to finding the existing return and unwind instructions, it also (if
+ /// necessary) transforms any call instructions into invokes and sends them to
+ /// a landing pad.
+ ///
+ /// It's wrapped up in a state machine using the same transform C# uses for
+ /// 'yield return' enumerators, This transform allows it to be non-allocating.
+ class EscapeEnumerator {
+ Function &F;
+ const char *CleanupBBName;
+
+ // State.
+ int State;
+ Function::iterator StateBB, StateE;
+ IRBuilder<> Builder;
+
+ public:
+ EscapeEnumerator(Function &F, const char *N = "cleanup")
+ : F(F), CleanupBBName(N), State(0), Builder(F.getContext()) {}
+
+ IRBuilder<> *Next() {
+ switch (State) {
+ default:
+ return 0;
+
+ case 0:
+ StateBB = F.begin();
+ StateE = F.end();
+ State = 1;
+
+ case 1:
+ // Find all 'return', 'resume', and 'unwind' instructions.
+ while (StateBB != StateE) {
+ BasicBlock *CurBB = StateBB++;
+
+ // Branches and invokes do not escape, only unwind, resume, and return
+ // do.
+ TerminatorInst *TI = CurBB->getTerminator();
+ if (!isa<ReturnInst>(TI) && !isa<ResumeInst>(TI))
+ continue;
+
+ Builder.SetInsertPoint(TI->getParent(), TI);
+ return &Builder;
+ }
+
+ State = 2;
+
+ // Find all 'call' instructions.
+ SmallVector<Instruction*,16> Calls;
+ for (Function::iterator BB = F.begin(),
+ E = F.end(); BB != E; ++BB)
+ for (BasicBlock::iterator II = BB->begin(),
+ EE = BB->end(); II != EE; ++II)
+ if (CallInst *CI = dyn_cast<CallInst>(II))
+ if (!CI->getCalledFunction() ||
+ !CI->getCalledFunction()->getIntrinsicID())
+ Calls.push_back(CI);
+
+ if (Calls.empty())
+ return 0;
+
+ // Create a cleanup block.
+ LLVMContext &C = F.getContext();
+ BasicBlock *CleanupBB = BasicBlock::Create(C, CleanupBBName, &F);
+ Type *ExnTy = StructType::get(Type::getInt8PtrTy(C),
+ Type::getInt32Ty(C), NULL);
+ Constant *PersFn =
+ F.getParent()->
+ getOrInsertFunction("__gcc_personality_v0",
+ FunctionType::get(Type::getInt32Ty(C), true));
+ LandingPadInst *LPad = LandingPadInst::Create(ExnTy, PersFn, 1,
+ "cleanup.lpad",
+ CleanupBB);
+ LPad->setCleanup(true);
+ ResumeInst *RI = ResumeInst::Create(LPad, CleanupBB);
+
+ // Transform the 'call' instructions into 'invoke's branching to the
+ // cleanup block. Go in reverse order to make prettier BB names.
+ SmallVector<Value*,16> Args;
+ for (unsigned I = Calls.size(); I != 0; ) {
+ CallInst *CI = cast<CallInst>(Calls[--I]);
+
+ // Split the basic block containing the function call.
+ BasicBlock *CallBB = CI->getParent();
+ BasicBlock *NewBB =
+ CallBB->splitBasicBlock(CI, CallBB->getName() + ".cont");
+
+ // Remove the unconditional branch inserted at the end of CallBB.
+ CallBB->getInstList().pop_back();
+ NewBB->getInstList().remove(CI);
+
+ // Create a new invoke instruction.
+ Args.clear();
+ CallSite CS(CI);
+ Args.append(CS.arg_begin(), CS.arg_end());
+
+ InvokeInst *II = InvokeInst::Create(CI->getCalledValue(),
+ NewBB, CleanupBB,
+ Args, CI->getName(), CallBB);
+ II->setCallingConv(CI->getCallingConv());
+ II->setAttributes(CI->getAttributes());
+ CI->replaceAllUsesWith(II);
+ delete CI;
+ }
+
+ Builder.SetInsertPoint(RI->getParent(), RI);
+ return &Builder;
+ }
+ }
+ };
+}
+
+// -----------------------------------------------------------------------------
+
+void llvm::linkShadowStackGC() { }
+
+ShadowStackGC::ShadowStackGC() : Head(0), StackEntryTy(0) {
+ InitRoots = true;
+ CustomRoots = true;
+}
+
+Constant *ShadowStackGC::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(GV, GEPIndices);
+}
+
+Type* ShadowStackGC::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 ShadowStackGC::initializeCustomLowering(Module &M) {
+ // 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 ShadowStackGC::IsNullValue(Value *V) {
+ if (Constant *C = dyn_cast<Constant>(V))
+ return C->isNullValue();
+ return false;
+}
+
+void ShadowStackGC::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 *
+ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, 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(BasePtr, Indices, Name);
+
+ assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
+
+ return dyn_cast<GetElementPtrInst>(Val);
+}
+
+GetElementPtrInst *
+ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, 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(BasePtr, Indices, Name);
+
+ assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
+
+ return dyn_cast<GetElementPtrInst>(Val);
+}
+
+/// runOnFunction - Insert code to maintain the shadow stack.
+bool ShadowStackGC::performCustomLowering(Function &F) {
+ 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, 0,
+ "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(Head, "gc_currhead");
+ Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, 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, 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,
+ StackEntry,0,0,"gc_frame.next");
+ Instruction *NewHeadVal = CreateGEP(Context, AtEntry,
+ 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, StackEntry, 0, 0,
+ "gc_frame.next");
+ 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) {
+ Roots[I].first->eraseFromParent();
+ Roots[I].second->eraseFromParent();
+ }
+
+ Roots.clear();
+ return true;
+}
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