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 #define DEBUG_TYPE "shadowstackgc"
29 #include "llvm/CodeGen/GCs.h"
30 #include "llvm/ADT/StringExtras.h"
31 #include "llvm/CodeGen/GCStrategy.h"
32 #include "llvm/IntrinsicInst.h"
33 #include "llvm/Module.h"
34 #include "llvm/Support/Compiler.h"
35 #include "llvm/Support/IRBuilder.h"
41 class VISIBILITY_HIDDEN ShadowStackGC : public GCStrategy {
42 /// RootChain - This is the global linked-list that contains the chain of GC
46 /// StackEntryTy - Abstract type of a link in the shadow stack.
48 const StructType *StackEntryTy;
50 /// Roots - GC roots in the current function. Each is a pair of the
51 /// intrinsic call and its corresponding alloca.
52 std::vector<std::pair<CallInst*,AllocaInst*> > Roots;
57 bool initializeCustomLowering(Module &M);
58 bool performCustomLowering(Function &F);
61 bool IsNullValue(Value *V);
62 Constant *GetFrameMap(Function &F);
63 const Type* GetConcreteStackEntryType(Function &F);
64 void CollectRoots(Function &F);
65 static GetElementPtrInst *CreateGEP(LLVMContext &Context,
66 IRBuilder<> &B, Value *BasePtr,
67 int Idx1, const char *Name);
68 static GetElementPtrInst *CreateGEP(LLVMContext &Context,
69 IRBuilder<> &B, Value *BasePtr,
70 int Idx1, int Idx2, const char *Name);
75 static GCRegistry::Add<ShadowStackGC>
76 X("shadow-stack", "Very portable GC for uncooperative code generators");
79 /// EscapeEnumerator - This is a little algorithm to find all escape points
80 /// from a function so that "finally"-style code can be inserted. In addition
81 /// to finding the existing return and unwind instructions, it also (if
82 /// necessary) transforms any call instructions into invokes and sends them to
85 /// It's wrapped up in a state machine using the same transform C# uses for
86 /// 'yield return' enumerators, This transform allows it to be non-allocating.
87 class VISIBILITY_HIDDEN EscapeEnumerator {
89 const char *CleanupBBName;
93 Function::iterator StateBB, StateE;
97 EscapeEnumerator(Function &F, const char *N = "cleanup")
98 : F(F), CleanupBBName(N), State(0), Builder(F.getContext()) {}
100 IRBuilder<> *Next() {
111 // Find all 'return' and 'unwind' instructions.
112 while (StateBB != StateE) {
113 BasicBlock *CurBB = StateBB++;
115 // Branches and invokes do not escape, only unwind and return do.
116 TerminatorInst *TI = CurBB->getTerminator();
117 if (!isa<UnwindInst>(TI) && !isa<ReturnInst>(TI))
120 Builder.SetInsertPoint(TI->getParent(), TI);
126 // Find all 'call' instructions.
127 SmallVector<Instruction*,16> Calls;
128 for (Function::iterator BB = F.begin(),
129 E = F.end(); BB != E; ++BB)
130 for (BasicBlock::iterator II = BB->begin(),
131 EE = BB->end(); II != EE; ++II)
132 if (CallInst *CI = dyn_cast<CallInst>(II))
133 if (!CI->getCalledFunction() ||
134 !CI->getCalledFunction()->getIntrinsicID())
140 // Create a cleanup block.
141 BasicBlock *CleanupBB = BasicBlock::Create(CleanupBBName, &F);
142 UnwindInst *UI = new UnwindInst(CleanupBB);
144 // Transform the 'call' instructions into 'invoke's branching to the
145 // cleanup block. Go in reverse order to make prettier BB names.
146 SmallVector<Value*,16> Args;
147 for (unsigned I = Calls.size(); I != 0; ) {
148 CallInst *CI = cast<CallInst>(Calls[--I]);
150 // Split the basic block containing the function call.
151 BasicBlock *CallBB = CI->getParent();
153 CallBB->splitBasicBlock(CI, CallBB->getName() + ".cont");
155 // Remove the unconditional branch inserted at the end of CallBB.
156 CallBB->getInstList().pop_back();
157 NewBB->getInstList().remove(CI);
159 // Create a new invoke instruction.
161 Args.append(CI->op_begin() + 1, CI->op_end());
163 InvokeInst *II = InvokeInst::Create(CI->getOperand(0),
165 Args.begin(), Args.end(),
166 CI->getName(), CallBB);
167 II->setCallingConv(CI->getCallingConv());
168 II->setAttributes(CI->getAttributes());
169 CI->replaceAllUsesWith(II);
173 Builder.SetInsertPoint(UI->getParent(), UI);
180 // -----------------------------------------------------------------------------
182 void llvm::linkShadowStackGC() { }
184 ShadowStackGC::ShadowStackGC() : Head(0), StackEntryTy(0) {
189 Constant *ShadowStackGC::GetFrameMap(Function &F) {
190 // doInitialization creates the abstract type of this value.
191 LLVMContext &Context = F.getContext();
193 Type *VoidPtr = PointerType::getUnqual(Type::Int8Ty);
195 // Truncate the ShadowStackDescriptor if some metadata is null.
196 unsigned NumMeta = 0;
197 SmallVector<Constant*,16> Metadata;
198 for (unsigned I = 0; I != Roots.size(); ++I) {
199 Constant *C = cast<Constant>(Roots[I].first->getOperand(2));
200 if (!C->isNullValue())
202 Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
205 Constant *BaseElts[] = {
206 Context.getConstantInt(Type::Int32Ty, Roots.size(), false),
207 Context.getConstantInt(Type::Int32Ty, NumMeta, false),
210 Constant *DescriptorElts[] = {
211 Context.getConstantStruct(BaseElts, 2),
212 Context.getConstantArray(Context.getArrayType(VoidPtr, NumMeta),
213 Metadata.begin(), NumMeta)
216 Constant *FrameMap = Context.getConstantStruct(DescriptorElts, 2);
218 std::string TypeName("gc_map.");
219 TypeName += utostr(NumMeta);
220 F.getParent()->addTypeName(TypeName, FrameMap->getType());
222 // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
223 // that, short of multithreaded LLVM, it should be safe; all that is
224 // necessary is that a simple Module::iterator loop not be invalidated.
225 // Appending to the GlobalVariable list is safe in that sense.
227 // All of the output passes emit globals last. The ExecutionEngine
228 // explicitly supports adding globals to the module after
231 // Still, if it isn't deemed acceptable, then this transformation needs
232 // to be a ModulePass (which means it cannot be in the 'llc' pipeline
233 // (which uses a FunctionPassManager (which segfaults (not asserts) if
234 // provided a ModulePass))).
235 Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
236 GlobalVariable::InternalLinkage,
237 FrameMap, "__gc_" + F.getName());
239 Constant *GEPIndices[2] = { Context.getConstantInt(Type::Int32Ty, 0),
240 Context.getConstantInt(Type::Int32Ty, 0) };
241 return Context.getConstantExprGetElementPtr(GV, GEPIndices, 2);
244 const Type* ShadowStackGC::GetConcreteStackEntryType(Function &F) {
245 // doInitialization creates the generic version of this type.
246 std::vector<const Type*> EltTys;
247 EltTys.push_back(StackEntryTy);
248 for (size_t I = 0; I != Roots.size(); I++)
249 EltTys.push_back(Roots[I].second->getAllocatedType());
250 Type *Ty = StructType::get(EltTys);
252 std::string TypeName("gc_stackentry.");
253 TypeName += F.getName();
254 F.getParent()->addTypeName(TypeName, Ty);
259 /// doInitialization - If this module uses the GC intrinsics, find them now. If
261 bool ShadowStackGC::initializeCustomLowering(Module &M) {
263 // int32_t NumRoots; // Number of roots in stack frame.
264 // int32_t NumMeta; // Number of metadata descriptors. May be < NumRoots.
265 // void *Meta[]; // May be absent for roots without metadata.
267 std::vector<const Type*> EltTys;
268 EltTys.push_back(Type::Int32Ty); // 32 bits is ok up to a 32GB stack frame. :)
269 EltTys.push_back(Type::Int32Ty); // Specifies length of variable length array.
270 StructType *FrameMapTy = StructType::get(EltTys);
271 M.addTypeName("gc_map", FrameMapTy);
272 PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);
274 // struct StackEntry {
275 // ShadowStackEntry *Next; // Caller's stack entry.
276 // FrameMap *Map; // Pointer to constant FrameMap.
277 // void *Roots[]; // Stack roots (in-place array, so we pretend).
279 OpaqueType *RecursiveTy = OpaqueType::get();
282 EltTys.push_back(PointerType::getUnqual(RecursiveTy));
283 EltTys.push_back(FrameMapPtrTy);
284 PATypeHolder LinkTyH = StructType::get(EltTys);
286 RecursiveTy->refineAbstractTypeTo(LinkTyH.get());
287 StackEntryTy = cast<StructType>(LinkTyH.get());
288 const PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
289 M.addTypeName("gc_stackentry", LinkTyH.get()); // FIXME: Is this safe from
292 // Get the root chain if it already exists.
293 Head = M.getGlobalVariable("llvm_gc_root_chain");
295 // If the root chain does not exist, insert a new one with linkonce
297 Head = new GlobalVariable(M, StackEntryPtrTy, false,
298 GlobalValue::LinkOnceAnyLinkage,
299 M.getContext().getNullValue(StackEntryPtrTy),
300 "llvm_gc_root_chain");
301 } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
302 Head->setInitializer(M.getContext().getNullValue(StackEntryPtrTy));
303 Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
309 bool ShadowStackGC::IsNullValue(Value *V) {
310 if (Constant *C = dyn_cast<Constant>(V))
311 return C->isNullValue();
315 void ShadowStackGC::CollectRoots(Function &F) {
316 // FIXME: Account for original alignment. Could fragment the root array.
317 // Approach 1: Null initialize empty slots at runtime. Yuck.
318 // Approach 2: Emit a map of the array instead of just a count.
320 assert(Roots.empty() && "Not cleaned up?");
322 SmallVector<std::pair<CallInst*,AllocaInst*>,16> MetaRoots;
324 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
325 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;)
326 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++))
327 if (Function *F = CI->getCalledFunction())
328 if (F->getIntrinsicID() == Intrinsic::gcroot) {
329 std::pair<CallInst*,AllocaInst*> Pair = std::make_pair(
330 CI, cast<AllocaInst>(CI->getOperand(1)->stripPointerCasts()));
331 if (IsNullValue(CI->getOperand(2)))
332 Roots.push_back(Pair);
334 MetaRoots.push_back(Pair);
337 // Number roots with metadata (usually empty) at the beginning, so that the
338 // FrameMap::Meta array can be elided.
339 Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
343 ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr,
344 int Idx, int Idx2, const char *Name) {
345 Value *Indices[] = { Context.getConstantInt(Type::Int32Ty, 0),
346 Context.getConstantInt(Type::Int32Ty, Idx),
347 Context.getConstantInt(Type::Int32Ty, Idx2) };
348 Value* Val = B.CreateGEP(BasePtr, Indices, Indices + 3, Name);
350 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
352 return dyn_cast<GetElementPtrInst>(Val);
356 ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr,
357 int Idx, const char *Name) {
358 Value *Indices[] = { Context.getConstantInt(Type::Int32Ty, 0),
359 Context.getConstantInt(Type::Int32Ty, Idx) };
360 Value *Val = B.CreateGEP(BasePtr, Indices, Indices + 2, Name);
362 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
364 return dyn_cast<GetElementPtrInst>(Val);
367 /// runOnFunction - Insert code to maintain the shadow stack.
368 bool ShadowStackGC::performCustomLowering(Function &F) {
369 LLVMContext &Context = F.getContext();
371 // Find calls to llvm.gcroot.
374 // If there are no roots in this function, then there is no need to add a
375 // stack map entry for it.
379 // Build the constant map and figure the type of the shadow stack entry.
380 Value *FrameMap = GetFrameMap(F);
381 const Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
383 // Build the shadow stack entry at the very start of the function.
384 BasicBlock::iterator IP = F.getEntryBlock().begin();
385 IRBuilder<> AtEntry(IP->getParent(), IP);
387 Instruction *StackEntry = AtEntry.CreateAlloca(ConcreteStackEntryTy, 0,
390 while (isa<AllocaInst>(IP)) ++IP;
391 AtEntry.SetInsertPoint(IP->getParent(), IP);
393 // Initialize the map pointer and load the current head of the shadow stack.
394 Instruction *CurrentHead = AtEntry.CreateLoad(Head, "gc_currhead");
395 Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, StackEntry,
397 AtEntry.CreateStore(FrameMap, EntryMapPtr);
399 // After all the allocas...
400 for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
401 // For each root, find the corresponding slot in the aggregate...
402 Value *SlotPtr = CreateGEP(Context, AtEntry, StackEntry, 1 + I, "gc_root");
404 // And use it in lieu of the alloca.
405 AllocaInst *OriginalAlloca = Roots[I].second;
406 SlotPtr->takeName(OriginalAlloca);
407 OriginalAlloca->replaceAllUsesWith(SlotPtr);
410 // Move past the original stores inserted by GCStrategy::InitRoots. This isn't
411 // really necessary (the collector would never see the intermediate state at
412 // runtime), but it's nicer not to push the half-initialized entry onto the
414 while (isa<StoreInst>(IP)) ++IP;
415 AtEntry.SetInsertPoint(IP->getParent(), IP);
417 // Push the entry onto the shadow stack.
418 Instruction *EntryNextPtr = CreateGEP(Context, AtEntry,
419 StackEntry,0,0,"gc_frame.next");
420 Instruction *NewHeadVal = CreateGEP(Context, AtEntry,
421 StackEntry, 0, "gc_newhead");
422 AtEntry.CreateStore(CurrentHead, EntryNextPtr);
423 AtEntry.CreateStore(NewHeadVal, Head);
425 // For each instruction that escapes...
426 EscapeEnumerator EE(F, "gc_cleanup");
427 while (IRBuilder<> *AtExit = EE.Next()) {
428 // Pop the entry from the shadow stack. Don't reuse CurrentHead from
429 // AtEntry, since that would make the value live for the entire function.
430 Instruction *EntryNextPtr2 = CreateGEP(Context, *AtExit, StackEntry, 0, 0,
432 Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead");
433 AtExit->CreateStore(SavedHead, Head);
436 // Delete the original allocas (which are no longer used) and the intrinsic
437 // calls (which are no longer valid). Doing this last avoids invalidating
439 for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
440 Roots[I].first->eraseFromParent();
441 Roots[I].second->eraseFromParent();