1//===-- ShadowStackGC.cpp - GC support for uncooperative targets ----------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
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
13// to identify roots.
14//
15// This pass implements the code transformation described in this paper:
16//   "Accurate Garbage Collection in an Uncooperative Environment"
17//   Fergus Henderson, ISMM, 2002
18//
19// In runtime/GC/SemiSpace.cpp is a prototype runtime which is compatible with
20// ShadowStackGC.
21//
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.
25//
26//===----------------------------------------------------------------------===//
27
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/IR/IRBuilder.h"
33#include "llvm/IR/IntrinsicInst.h"
34#include "llvm/IR/Module.h"
35#include "llvm/Support/CallSite.h"
36
37using namespace llvm;
38
39namespace {
40
41  class ShadowStackGC : public GCStrategy {
42    /// RootChain - This is the global linked-list that contains the chain of GC
43    /// roots.
44    GlobalVariable *Head;
45
46    /// StackEntryTy - Abstract type of a link in the shadow stack.
47    ///
48    StructType *StackEntryTy;
49    StructType *FrameMapTy;
50
51    /// Roots - GC roots in the current function. Each is a pair of the
52    /// intrinsic call and its corresponding alloca.
53    std::vector<std::pair<CallInst*,AllocaInst*> > Roots;
54
55  public:
56    ShadowStackGC();
57
58    bool initializeCustomLowering(Module &M);
59    bool performCustomLowering(Function &F);
60
61  private:
62    bool IsNullValue(Value *V);
63    Constant *GetFrameMap(Function &F);
64    Type* GetConcreteStackEntryType(Function &F);
65    void CollectRoots(Function &F);
66    static GetElementPtrInst *CreateGEP(LLVMContext &Context,
67                                        IRBuilder<> &B, Value *BasePtr,
68                                        int Idx1, const char *Name);
69    static GetElementPtrInst *CreateGEP(LLVMContext &Context,
70                                        IRBuilder<> &B, Value *BasePtr,
71                                        int Idx1, int Idx2, const char *Name);
72  };
73
74}
75
76static GCRegistry::Add<ShadowStackGC>
77X("shadow-stack", "Very portable GC for uncooperative code generators");
78
79namespace {
80  /// EscapeEnumerator - This is a little algorithm to find all escape points
81  /// from a function so that "finally"-style code can be inserted. In addition
82  /// to finding the existing return and unwind instructions, it also (if
83  /// necessary) transforms any call instructions into invokes and sends them to
84  /// a landing pad.
85  ///
86  /// It's wrapped up in a state machine using the same transform C# uses for
87  /// 'yield return' enumerators, This transform allows it to be non-allocating.
88  class EscapeEnumerator {
89    Function &F;
90    const char *CleanupBBName;
91
92    // State.
93    int State;
94    Function::iterator StateBB, StateE;
95    IRBuilder<> Builder;
96
97  public:
98    EscapeEnumerator(Function &F, const char *N = "cleanup")
99      : F(F), CleanupBBName(N), State(0), Builder(F.getContext()) {}
100
101    IRBuilder<> *Next() {
102      switch (State) {
103      default:
104        return 0;
105
106      case 0:
107        StateBB = F.begin();
108        StateE = F.end();
109        State = 1;
110
111      case 1:
112        // Find all 'return', 'resume', and 'unwind' instructions.
113        while (StateBB != StateE) {
114          BasicBlock *CurBB = StateBB++;
115
116          // Branches and invokes do not escape, only unwind, resume, and return
117          // do.
118          TerminatorInst *TI = CurBB->getTerminator();
119          if (!isa<ReturnInst>(TI) && !isa<ResumeInst>(TI))
120            continue;
121
122          Builder.SetInsertPoint(TI->getParent(), TI);
123          return &Builder;
124        }
125
126        State = 2;
127
128        // Find all 'call' instructions.
129        SmallVector<Instruction*,16> Calls;
130        for (Function::iterator BB = F.begin(),
131                                E = F.end(); BB != E; ++BB)
132          for (BasicBlock::iterator II = BB->begin(),
133                                    EE = BB->end(); II != EE; ++II)
134            if (CallInst *CI = dyn_cast<CallInst>(II))
135              if (!CI->getCalledFunction() ||
136                  !CI->getCalledFunction()->getIntrinsicID())
137                Calls.push_back(CI);
138
139        if (Calls.empty())
140          return 0;
141
142        // Create a cleanup block.
143        LLVMContext &C = F.getContext();
144        BasicBlock *CleanupBB = BasicBlock::Create(C, CleanupBBName, &F);
145        Type *ExnTy = StructType::get(Type::getInt8PtrTy(C),
146                                      Type::getInt32Ty(C), NULL);
147        Constant *PersFn =
148          F.getParent()->
149          getOrInsertFunction("__gcc_personality_v0",
150                              FunctionType::get(Type::getInt32Ty(C), true));
151        LandingPadInst *LPad = LandingPadInst::Create(ExnTy, PersFn, 1,
152                                                      "cleanup.lpad",
153                                                      CleanupBB);
154        LPad->setCleanup(true);
155        ResumeInst *RI = ResumeInst::Create(LPad, CleanupBB);
156
157        // Transform the 'call' instructions into 'invoke's branching to the
158        // cleanup block. Go in reverse order to make prettier BB names.
159        SmallVector<Value*,16> Args;
160        for (unsigned I = Calls.size(); I != 0; ) {
161          CallInst *CI = cast<CallInst>(Calls[--I]);
162
163          // Split the basic block containing the function call.
164          BasicBlock *CallBB = CI->getParent();
165          BasicBlock *NewBB =
166            CallBB->splitBasicBlock(CI, CallBB->getName() + ".cont");
167
168          // Remove the unconditional branch inserted at the end of CallBB.
169          CallBB->getInstList().pop_back();
170          NewBB->getInstList().remove(CI);
171
172          // Create a new invoke instruction.
173          Args.clear();
174          CallSite CS(CI);
175          Args.append(CS.arg_begin(), CS.arg_end());
176
177          InvokeInst *II = InvokeInst::Create(CI->getCalledValue(),
178                                              NewBB, CleanupBB,
179                                              Args, CI->getName(), CallBB);
180          II->setCallingConv(CI->getCallingConv());
181          II->setAttributes(CI->getAttributes());
182          CI->replaceAllUsesWith(II);
183          delete CI;
184        }
185
186        Builder.SetInsertPoint(RI->getParent(), RI);
187        return &Builder;
188      }
189    }
190  };
191}
192
193// -----------------------------------------------------------------------------
194
195void llvm::linkShadowStackGC() { }
196
197ShadowStackGC::ShadowStackGC() : Head(0), StackEntryTy(0) {
198  InitRoots = true;
199  CustomRoots = true;
200}
201
202Constant *ShadowStackGC::GetFrameMap(Function &F) {
203  // doInitialization creates the abstract type of this value.
204  Type *VoidPtr = Type::getInt8PtrTy(F.getContext());
205
206  // Truncate the ShadowStackDescriptor if some metadata is null.
207  unsigned NumMeta = 0;
208  SmallVector<Constant*, 16> Metadata;
209  for (unsigned I = 0; I != Roots.size(); ++I) {
210    Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1));
211    if (!C->isNullValue())
212      NumMeta = I + 1;
213    Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
214  }
215  Metadata.resize(NumMeta);
216
217  Type *Int32Ty = Type::getInt32Ty(F.getContext());
218
219  Constant *BaseElts[] = {
220    ConstantInt::get(Int32Ty, Roots.size(), false),
221    ConstantInt::get(Int32Ty, NumMeta, false),
222  };
223
224  Constant *DescriptorElts[] = {
225    ConstantStruct::get(FrameMapTy, BaseElts),
226    ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)
227  };
228
229  Type *EltTys[] = { DescriptorElts[0]->getType(),DescriptorElts[1]->getType()};
230  StructType *STy = StructType::create(EltTys, "gc_map."+utostr(NumMeta));
231
232  Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts);
233
234  // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
235  //        that, short of multithreaded LLVM, it should be safe; all that is
236  //        necessary is that a simple Module::iterator loop not be invalidated.
237  //        Appending to the GlobalVariable list is safe in that sense.
238  //
239  //        All of the output passes emit globals last. The ExecutionEngine
240  //        explicitly supports adding globals to the module after
241  //        initialization.
242  //
243  //        Still, if it isn't deemed acceptable, then this transformation needs
244  //        to be a ModulePass (which means it cannot be in the 'llc' pipeline
245  //        (which uses a FunctionPassManager (which segfaults (not asserts) if
246  //        provided a ModulePass))).
247  Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
248                                    GlobalVariable::InternalLinkage,
249                                    FrameMap, "__gc_" + F.getName());
250
251  Constant *GEPIndices[2] = {
252                          ConstantInt::get(Type::getInt32Ty(F.getContext()), 0),
253                          ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)
254                          };
255  return ConstantExpr::getGetElementPtr(GV, GEPIndices);
256}
257
258Type* ShadowStackGC::GetConcreteStackEntryType(Function &F) {
259  // doInitialization creates the generic version of this type.
260  std::vector<Type*> EltTys;
261  EltTys.push_back(StackEntryTy);
262  for (size_t I = 0; I != Roots.size(); I++)
263    EltTys.push_back(Roots[I].second->getAllocatedType());
264
265  return StructType::create(EltTys, "gc_stackentry."+F.getName().str());
266}
267
268/// doInitialization - If this module uses the GC intrinsics, find them now. If
269/// not, exit fast.
270bool ShadowStackGC::initializeCustomLowering(Module &M) {
271  // struct FrameMap {
272  //   int32_t NumRoots; // Number of roots in stack frame.
273  //   int32_t NumMeta;  // Number of metadata descriptors. May be < NumRoots.
274  //   void *Meta[];     // May be absent for roots without metadata.
275  // };
276  std::vector<Type*> EltTys;
277  // 32 bits is ok up to a 32GB stack frame. :)
278  EltTys.push_back(Type::getInt32Ty(M.getContext()));
279  // Specifies length of variable length array.
280  EltTys.push_back(Type::getInt32Ty(M.getContext()));
281  FrameMapTy = StructType::create(EltTys, "gc_map");
282  PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);
283
284  // struct StackEntry {
285  //   ShadowStackEntry *Next; // Caller's stack entry.
286  //   FrameMap *Map;          // Pointer to constant FrameMap.
287  //   void *Roots[];          // Stack roots (in-place array, so we pretend).
288  // };
289
290  StackEntryTy = StructType::create(M.getContext(), "gc_stackentry");
291
292  EltTys.clear();
293  EltTys.push_back(PointerType::getUnqual(StackEntryTy));
294  EltTys.push_back(FrameMapPtrTy);
295  StackEntryTy->setBody(EltTys);
296  PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
297
298  // Get the root chain if it already exists.
299  Head = M.getGlobalVariable("llvm_gc_root_chain");
300  if (!Head) {
301    // If the root chain does not exist, insert a new one with linkonce
302    // linkage!
303    Head = new GlobalVariable(M, StackEntryPtrTy, false,
304                              GlobalValue::LinkOnceAnyLinkage,
305                              Constant::getNullValue(StackEntryPtrTy),
306                              "llvm_gc_root_chain");
307  } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
308    Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
309    Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
310  }
311
312  return true;
313}
314
315bool ShadowStackGC::IsNullValue(Value *V) {
316  if (Constant *C = dyn_cast<Constant>(V))
317    return C->isNullValue();
318  return false;
319}
320
321void ShadowStackGC::CollectRoots(Function &F) {
322  // FIXME: Account for original alignment. Could fragment the root array.
323  //   Approach 1: Null initialize empty slots at runtime. Yuck.
324  //   Approach 2: Emit a map of the array instead of just a count.
325
326  assert(Roots.empty() && "Not cleaned up?");
327
328  SmallVector<std::pair<CallInst*, AllocaInst*>, 16> MetaRoots;
329
330  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
331    for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;)
332      if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++))
333        if (Function *F = CI->getCalledFunction())
334          if (F->getIntrinsicID() == Intrinsic::gcroot) {
335            std::pair<CallInst*, AllocaInst*> Pair = std::make_pair(
336              CI, cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
337            if (IsNullValue(CI->getArgOperand(1)))
338              Roots.push_back(Pair);
339            else
340              MetaRoots.push_back(Pair);
341          }
342
343  // Number roots with metadata (usually empty) at the beginning, so that the
344  // FrameMap::Meta array can be elided.
345  Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
346}
347
348GetElementPtrInst *
349ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr,
350                         int Idx, int Idx2, const char *Name) {
351  Value *Indices[] = { ConstantInt::get(Type::getInt32Ty(Context), 0),
352                       ConstantInt::get(Type::getInt32Ty(Context), Idx),
353                       ConstantInt::get(Type::getInt32Ty(Context), Idx2) };
354  Value* Val = B.CreateGEP(BasePtr, Indices, Name);
355
356  assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
357
358  return dyn_cast<GetElementPtrInst>(Val);
359}
360
361GetElementPtrInst *
362ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr,
363                         int Idx, const char *Name) {
364  Value *Indices[] = { ConstantInt::get(Type::getInt32Ty(Context), 0),
365                       ConstantInt::get(Type::getInt32Ty(Context), Idx) };
366  Value *Val = B.CreateGEP(BasePtr, Indices, Name);
367
368  assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
369
370  return dyn_cast<GetElementPtrInst>(Val);
371}
372
373/// runOnFunction - Insert code to maintain the shadow stack.
374bool ShadowStackGC::performCustomLowering(Function &F) {
375  LLVMContext &Context = F.getContext();
376
377  // Find calls to llvm.gcroot.
378  CollectRoots(F);
379
380  // If there are no roots in this function, then there is no need to add a
381  // stack map entry for it.
382  if (Roots.empty())
383    return false;
384
385  // Build the constant map and figure the type of the shadow stack entry.
386  Value *FrameMap = GetFrameMap(F);
387  Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
388
389  // Build the shadow stack entry at the very start of the function.
390  BasicBlock::iterator IP = F.getEntryBlock().begin();
391  IRBuilder<> AtEntry(IP->getParent(), IP);
392
393  Instruction *StackEntry   = AtEntry.CreateAlloca(ConcreteStackEntryTy, 0,
394                                                   "gc_frame");
395
396  while (isa<AllocaInst>(IP)) ++IP;
397  AtEntry.SetInsertPoint(IP->getParent(), IP);
398
399  // Initialize the map pointer and load the current head of the shadow stack.
400  Instruction *CurrentHead  = AtEntry.CreateLoad(Head, "gc_currhead");
401  Instruction *EntryMapPtr  = CreateGEP(Context, AtEntry, StackEntry,
402                                        0,1,"gc_frame.map");
403  AtEntry.CreateStore(FrameMap, EntryMapPtr);
404
405  // After all the allocas...
406  for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
407    // For each root, find the corresponding slot in the aggregate...
408    Value *SlotPtr = CreateGEP(Context, AtEntry, StackEntry, 1 + I, "gc_root");
409
410    // And use it in lieu of the alloca.
411    AllocaInst *OriginalAlloca = Roots[I].second;
412    SlotPtr->takeName(OriginalAlloca);
413    OriginalAlloca->replaceAllUsesWith(SlotPtr);
414  }
415
416  // Move past the original stores inserted by GCStrategy::InitRoots. This isn't
417  // really necessary (the collector would never see the intermediate state at
418  // runtime), but it's nicer not to push the half-initialized entry onto the
419  // shadow stack.
420  while (isa<StoreInst>(IP)) ++IP;
421  AtEntry.SetInsertPoint(IP->getParent(), IP);
422
423  // Push the entry onto the shadow stack.
424  Instruction *EntryNextPtr = CreateGEP(Context, AtEntry,
425                                        StackEntry,0,0,"gc_frame.next");
426  Instruction *NewHeadVal   = CreateGEP(Context, AtEntry,
427                                        StackEntry, 0, "gc_newhead");
428  AtEntry.CreateStore(CurrentHead, EntryNextPtr);
429  AtEntry.CreateStore(NewHeadVal, Head);
430
431  // For each instruction that escapes...
432  EscapeEnumerator EE(F, "gc_cleanup");
433  while (IRBuilder<> *AtExit = EE.Next()) {
434    // Pop the entry from the shadow stack. Don't reuse CurrentHead from
435    // AtEntry, since that would make the value live for the entire function.
436    Instruction *EntryNextPtr2 = CreateGEP(Context, *AtExit, StackEntry, 0, 0,
437                                           "gc_frame.next");
438    Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead");
439                       AtExit->CreateStore(SavedHead, Head);
440  }
441
442  // Delete the original allocas (which are no longer used) and the intrinsic
443  // calls (which are no longer valid). Doing this last avoids invalidating
444  // iterators.
445  for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
446    Roots[I].first->eraseFromParent();
447    Roots[I].second->eraseFromParent();
448  }
449
450  Roots.clear();
451  return true;
452}
453