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llvm-mirror/lib/CodeGen/ShadowStackGC.cpp
Chandler Carruth 4b51f99c87 Move llvm/Support/IRBuilder.h -> llvm/IRBuilder.h 
This was always part of the VMCore library out of necessity -- it deals
entirely in the IR. The .cpp file in fact was already part of the VMCore
library. This is just a mechanical move.

I've tried to go through and re-apply the coding standard's preferred
header sort, but at 40-ish files, I may have gotten some wrong. Please
let me know if so.

I'll be committing the corresponding updates to Clang and Polly, and
Duncan has DragonEgg.

Thanks to Bill and Eric for giving the green light for this bit of cleanup.

llvm-svn: 159421
2012-06-29 12:38:19 +00:00

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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/IRBuilder.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/Module.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/CodeGen/GCStrategy.h"
#include "llvm/CodeGen/GCs.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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