1 //===-- ShadowStackGC.cpp - GC support for uncooperative targets ----------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements lowering for the llvm.gc* intrinsics for targets that do
11 // not natively support them (which includes the C backend). Note that the code
12 // generated is not quite as efficient as algorithms which generate stack maps
15 // This pass implements the code transformation described in this paper:
16 // "Accurate Garbage Collection in an Uncooperative Environment"
17 // Fergus Henderson, ISMM, 2002
19 // In runtime/GC/SemiSpace.cpp is a prototype runtime which is compatible with
22 // In order to support this particular transformation, all stack roots are
23 // coallocated in the stack. This allows a fully target-independent stack map
24 // while introducing only minor runtime overhead.
26 //===----------------------------------------------------------------------===//
28 #include "llvm/CodeGen/GCs.h"
29 #include "llvm/ADT/StringExtras.h"
30 #include "llvm/CodeGen/GCStrategy.h"
31 #include "llvm/IR/CallSite.h"
32 #include "llvm/IR/IRBuilder.h"
33 #include "llvm/IR/IntrinsicInst.h"
34 #include "llvm/IR/Module.h"
38 #define DEBUG_TYPE "shadowstackgc"
42 class ShadowStackGC : public GCStrategy {
43 /// RootChain - This is the global linked-list that contains the chain of GC
47 /// StackEntryTy - Abstract type of a link in the shadow stack.
49 StructType *StackEntryTy;
50 StructType *FrameMapTy;
52 /// Roots - GC roots in the current function. Each is a pair of the
53 /// intrinsic call and its corresponding alloca.
54 std::vector<std::pair<CallInst*,AllocaInst*> > Roots;
59 bool initializeCustomLowering(Module &M) override;
60 bool performCustomLowering(Function &F) override;
63 bool IsNullValue(Value *V);
64 Constant *GetFrameMap(Function &F);
65 Type* GetConcreteStackEntryType(Function &F);
66 void CollectRoots(Function &F);
67 static GetElementPtrInst *CreateGEP(LLVMContext &Context,
68 IRBuilder<> &B, Value *BasePtr,
69 int Idx1, const char *Name);
70 static GetElementPtrInst *CreateGEP(LLVMContext &Context,
71 IRBuilder<> &B, Value *BasePtr,
72 int Idx1, int Idx2, const char *Name);
77 static GCRegistry::Add<ShadowStackGC>
78 X("shadow-stack", "Very portable GC for uncooperative code generators");
81 /// EscapeEnumerator - This is a little algorithm to find all escape points
82 /// from a function so that "finally"-style code can be inserted. In addition
83 /// to finding the existing return and unwind instructions, it also (if
84 /// necessary) transforms any call instructions into invokes and sends them to
87 /// It's wrapped up in a state machine using the same transform C# uses for
88 /// 'yield return' enumerators, This transform allows it to be non-allocating.
89 class EscapeEnumerator {
91 const char *CleanupBBName;
95 Function::iterator StateBB, StateE;
99 EscapeEnumerator(Function &F, const char *N = "cleanup")
100 : F(F), CleanupBBName(N), State(0), Builder(F.getContext()) {}
102 IRBuilder<> *Next() {
113 // Find all 'return', 'resume', and 'unwind' instructions.
114 while (StateBB != StateE) {
115 BasicBlock *CurBB = StateBB++;
117 // Branches and invokes do not escape, only unwind, resume, and return
119 TerminatorInst *TI = CurBB->getTerminator();
120 if (!isa<ReturnInst>(TI) && !isa<ResumeInst>(TI))
123 Builder.SetInsertPoint(TI->getParent(), TI);
129 // Find all 'call' instructions.
130 SmallVector<Instruction*,16> Calls;
131 for (Function::iterator BB = F.begin(),
132 E = F.end(); BB != E; ++BB)
133 for (BasicBlock::iterator II = BB->begin(),
134 EE = BB->end(); II != EE; ++II)
135 if (CallInst *CI = dyn_cast<CallInst>(II))
136 if (!CI->getCalledFunction() ||
137 !CI->getCalledFunction()->getIntrinsicID())
143 // Create a cleanup block.
144 LLVMContext &C = F.getContext();
145 BasicBlock *CleanupBB = BasicBlock::Create(C, CleanupBBName, &F);
146 Type *ExnTy = StructType::get(Type::getInt8PtrTy(C),
147 Type::getInt32Ty(C), nullptr);
150 getOrInsertFunction("__gcc_personality_v0",
151 FunctionType::get(Type::getInt32Ty(C), true));
152 LandingPadInst *LPad = LandingPadInst::Create(ExnTy, PersFn, 1,
155 LPad->setCleanup(true);
156 ResumeInst *RI = ResumeInst::Create(LPad, CleanupBB);
158 // Transform the 'call' instructions into 'invoke's branching to the
159 // cleanup block. Go in reverse order to make prettier BB names.
160 SmallVector<Value*,16> Args;
161 for (unsigned I = Calls.size(); I != 0; ) {
162 CallInst *CI = cast<CallInst>(Calls[--I]);
164 // Split the basic block containing the function call.
165 BasicBlock *CallBB = CI->getParent();
167 CallBB->splitBasicBlock(CI, CallBB->getName() + ".cont");
169 // Remove the unconditional branch inserted at the end of CallBB.
170 CallBB->getInstList().pop_back();
171 NewBB->getInstList().remove(CI);
173 // Create a new invoke instruction.
176 Args.append(CS.arg_begin(), CS.arg_end());
178 InvokeInst *II = InvokeInst::Create(CI->getCalledValue(),
180 Args, CI->getName(), CallBB);
181 II->setCallingConv(CI->getCallingConv());
182 II->setAttributes(CI->getAttributes());
183 CI->replaceAllUsesWith(II);
187 Builder.SetInsertPoint(RI->getParent(), RI);
194 // -----------------------------------------------------------------------------
196 void llvm::linkShadowStackGC() { }
198 ShadowStackGC::ShadowStackGC() : Head(nullptr), StackEntryTy(nullptr) {
203 Constant *ShadowStackGC::GetFrameMap(Function &F) {
204 // doInitialization creates the abstract type of this value.
205 Type *VoidPtr = Type::getInt8PtrTy(F.getContext());
207 // Truncate the ShadowStackDescriptor if some metadata is null.
208 unsigned NumMeta = 0;
209 SmallVector<Constant*, 16> Metadata;
210 for (unsigned I = 0; I != Roots.size(); ++I) {
211 Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1));
212 if (!C->isNullValue())
214 Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
216 Metadata.resize(NumMeta);
218 Type *Int32Ty = Type::getInt32Ty(F.getContext());
220 Constant *BaseElts[] = {
221 ConstantInt::get(Int32Ty, Roots.size(), false),
222 ConstantInt::get(Int32Ty, NumMeta, false),
225 Constant *DescriptorElts[] = {
226 ConstantStruct::get(FrameMapTy, BaseElts),
227 ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)
230 Type *EltTys[] = { DescriptorElts[0]->getType(),DescriptorElts[1]->getType()};
231 StructType *STy = StructType::create(EltTys, "gc_map."+utostr(NumMeta));
233 Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts);
235 // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
236 // that, short of multithreaded LLVM, it should be safe; all that is
237 // necessary is that a simple Module::iterator loop not be invalidated.
238 // Appending to the GlobalVariable list is safe in that sense.
240 // All of the output passes emit globals last. The ExecutionEngine
241 // explicitly supports adding globals to the module after
244 // Still, if it isn't deemed acceptable, then this transformation needs
245 // to be a ModulePass (which means it cannot be in the 'llc' pipeline
246 // (which uses a FunctionPassManager (which segfaults (not asserts) if
247 // provided a ModulePass))).
248 Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
249 GlobalVariable::InternalLinkage,
250 FrameMap, "__gc_" + F.getName());
252 Constant *GEPIndices[2] = {
253 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0),
254 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)
256 return ConstantExpr::getGetElementPtr(GV, GEPIndices);
259 Type* ShadowStackGC::GetConcreteStackEntryType(Function &F) {
260 // doInitialization creates the generic version of this type.
261 std::vector<Type*> EltTys;
262 EltTys.push_back(StackEntryTy);
263 for (size_t I = 0; I != Roots.size(); I++)
264 EltTys.push_back(Roots[I].second->getAllocatedType());
266 return StructType::create(EltTys, "gc_stackentry."+F.getName().str());
269 /// doInitialization - If this module uses the GC intrinsics, find them now. If
271 bool ShadowStackGC::initializeCustomLowering(Module &M) {
273 // int32_t NumRoots; // Number of roots in stack frame.
274 // int32_t NumMeta; // Number of metadata descriptors. May be < NumRoots.
275 // void *Meta[]; // May be absent for roots without metadata.
277 std::vector<Type*> EltTys;
278 // 32 bits is ok up to a 32GB stack frame. :)
279 EltTys.push_back(Type::getInt32Ty(M.getContext()));
280 // Specifies length of variable length array.
281 EltTys.push_back(Type::getInt32Ty(M.getContext()));
282 FrameMapTy = StructType::create(EltTys, "gc_map");
283 PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);
285 // struct StackEntry {
286 // ShadowStackEntry *Next; // Caller's stack entry.
287 // FrameMap *Map; // Pointer to constant FrameMap.
288 // void *Roots[]; // Stack roots (in-place array, so we pretend).
291 StackEntryTy = StructType::create(M.getContext(), "gc_stackentry");
294 EltTys.push_back(PointerType::getUnqual(StackEntryTy));
295 EltTys.push_back(FrameMapPtrTy);
296 StackEntryTy->setBody(EltTys);
297 PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
299 // Get the root chain if it already exists.
300 Head = M.getGlobalVariable("llvm_gc_root_chain");
302 // If the root chain does not exist, insert a new one with linkonce
304 Head = new GlobalVariable(M, StackEntryPtrTy, false,
305 GlobalValue::LinkOnceAnyLinkage,
306 Constant::getNullValue(StackEntryPtrTy),
307 "llvm_gc_root_chain");
308 } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
309 Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
310 Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
316 bool ShadowStackGC::IsNullValue(Value *V) {
317 if (Constant *C = dyn_cast<Constant>(V))
318 return C->isNullValue();
322 void ShadowStackGC::CollectRoots(Function &F) {
323 // FIXME: Account for original alignment. Could fragment the root array.
324 // Approach 1: Null initialize empty slots at runtime. Yuck.
325 // Approach 2: Emit a map of the array instead of just a count.
327 assert(Roots.empty() && "Not cleaned up?");
329 SmallVector<std::pair<CallInst*, AllocaInst*>, 16> MetaRoots;
331 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
332 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;)
333 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++))
334 if (Function *F = CI->getCalledFunction())
335 if (F->getIntrinsicID() == Intrinsic::gcroot) {
336 std::pair<CallInst*, AllocaInst*> Pair = std::make_pair(
337 CI, cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
338 if (IsNullValue(CI->getArgOperand(1)))
339 Roots.push_back(Pair);
341 MetaRoots.push_back(Pair);
344 // Number roots with metadata (usually empty) at the beginning, so that the
345 // FrameMap::Meta array can be elided.
346 Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
350 ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr,
351 int Idx, int Idx2, const char *Name) {
352 Value *Indices[] = { ConstantInt::get(Type::getInt32Ty(Context), 0),
353 ConstantInt::get(Type::getInt32Ty(Context), Idx),
354 ConstantInt::get(Type::getInt32Ty(Context), Idx2) };
355 Value* Val = B.CreateGEP(BasePtr, Indices, Name);
357 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
359 return dyn_cast<GetElementPtrInst>(Val);
363 ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr,
364 int Idx, const char *Name) {
365 Value *Indices[] = { ConstantInt::get(Type::getInt32Ty(Context), 0),
366 ConstantInt::get(Type::getInt32Ty(Context), Idx) };
367 Value *Val = B.CreateGEP(BasePtr, Indices, Name);
369 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
371 return dyn_cast<GetElementPtrInst>(Val);
374 /// runOnFunction - Insert code to maintain the shadow stack.
375 bool ShadowStackGC::performCustomLowering(Function &F) {
376 LLVMContext &Context = F.getContext();
378 // Find calls to llvm.gcroot.
381 // If there are no roots in this function, then there is no need to add a
382 // stack map entry for it.
386 // Build the constant map and figure the type of the shadow stack entry.
387 Value *FrameMap = GetFrameMap(F);
388 Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
390 // Build the shadow stack entry at the very start of the function.
391 BasicBlock::iterator IP = F.getEntryBlock().begin();
392 IRBuilder<> AtEntry(IP->getParent(), IP);
394 Instruction *StackEntry = AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr,
397 while (isa<AllocaInst>(IP)) ++IP;
398 AtEntry.SetInsertPoint(IP->getParent(), IP);
400 // Initialize the map pointer and load the current head of the shadow stack.
401 Instruction *CurrentHead = AtEntry.CreateLoad(Head, "gc_currhead");
402 Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, StackEntry,
404 AtEntry.CreateStore(FrameMap, EntryMapPtr);
406 // After all the allocas...
407 for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
408 // For each root, find the corresponding slot in the aggregate...
409 Value *SlotPtr = CreateGEP(Context, AtEntry, StackEntry, 1 + I, "gc_root");
411 // And use it in lieu of the alloca.
412 AllocaInst *OriginalAlloca = Roots[I].second;
413 SlotPtr->takeName(OriginalAlloca);
414 OriginalAlloca->replaceAllUsesWith(SlotPtr);
417 // Move past the original stores inserted by GCStrategy::InitRoots. This isn't
418 // really necessary (the collector would never see the intermediate state at
419 // runtime), but it's nicer not to push the half-initialized entry onto the
421 while (isa<StoreInst>(IP)) ++IP;
422 AtEntry.SetInsertPoint(IP->getParent(), IP);
424 // Push the entry onto the shadow stack.
425 Instruction *EntryNextPtr = CreateGEP(Context, AtEntry,
426 StackEntry,0,0,"gc_frame.next");
427 Instruction *NewHeadVal = CreateGEP(Context, AtEntry,
428 StackEntry, 0, "gc_newhead");
429 AtEntry.CreateStore(CurrentHead, EntryNextPtr);
430 AtEntry.CreateStore(NewHeadVal, Head);
432 // For each instruction that escapes...
433 EscapeEnumerator EE(F, "gc_cleanup");
434 while (IRBuilder<> *AtExit = EE.Next()) {
435 // Pop the entry from the shadow stack. Don't reuse CurrentHead from
436 // AtEntry, since that would make the value live for the entire function.
437 Instruction *EntryNextPtr2 = CreateGEP(Context, *AtExit, StackEntry, 0, 0,
439 Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead");
440 AtExit->CreateStore(SavedHead, Head);
443 // Delete the original allocas (which are no longer used) and the intrinsic
444 // calls (which are no longer valid). Doing this last avoids invalidating
446 for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
447 Roots[I].first->eraseFromParent();
448 Roots[I].second->eraseFromParent();