With this patch, the LowerGC transformation becomes the
ShadowStackCollector, which additionally has reduced overhead with
no sacrifice in portability.
Considering a function @fun with 8 loop-local roots,
ShadowStackCollector introduces the following overhead
(x86):
; shadowstack prologue
movl L_llvm_gc_root_chain$non_lazy_ptr, %eax
movl (%eax), %ecx
movl $___gc_fun, 20(%esp)
movl $0, 24(%esp)
movl $0, 28(%esp)
movl $0, 32(%esp)
movl $0, 36(%esp)
movl $0, 40(%esp)
movl $0, 44(%esp)
movl $0, 48(%esp)
movl $0, 52(%esp)
movl %ecx, 16(%esp)
leal 16(%esp), %ecx
movl %ecx, (%eax)
; shadowstack loop overhead
(none)
; shadowstack epilogue
movl 48(%esp), %edx
movl %edx, (%ecx)
; shadowstack metadata
.align 3
___gc_fun: # __gc_fun
.long 8
.space 4
In comparison to LowerGC:
; lowergc prologue
movl L_llvm_gc_root_chain$non_lazy_ptr, %eax
movl (%eax), %ecx
movl %ecx, 48(%esp)
movl $8, 52(%esp)
movl $0, 60(%esp)
movl $0, 56(%esp)
movl $0, 68(%esp)
movl $0, 64(%esp)
movl $0, 76(%esp)
movl $0, 72(%esp)
movl $0, 84(%esp)
movl $0, 80(%esp)
movl $0, 92(%esp)
movl $0, 88(%esp)
movl $0, 100(%esp)
movl $0, 96(%esp)
movl $0, 108(%esp)
movl $0, 104(%esp)
movl $0, 116(%esp)
movl $0, 112(%esp)
; lowergc loop overhead
leal 44(%esp), %eax
movl %eax, 56(%esp)
leal 40(%esp), %eax
movl %eax, 64(%esp)
leal 36(%esp), %eax
movl %eax, 72(%esp)
leal 32(%esp), %eax
movl %eax, 80(%esp)
leal 28(%esp), %eax
movl %eax, 88(%esp)
leal 24(%esp), %eax
movl %eax, 96(%esp)
leal 20(%esp), %eax
movl %eax, 104(%esp)
leal 16(%esp), %eax
movl %eax, 112(%esp)
; lowergc epilogue
movl 48(%esp), %edx
movl %edx, (%ecx)
; lowergc metadata
(none)
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45670 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/ShadowStackCollector.cpp b/lib/CodeGen/ShadowStackCollector.cpp
new file mode 100644
index 0000000..1b619c9
--- /dev/null
+++ b/lib/CodeGen/ShadowStackCollector.cpp
@@ -0,0 +1,441 @@
+//===-- ShadowStackCollector.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 collectors 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
+// this collector.
+//
+// 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/Collectors.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringExtras.h"
+#include "llvm/CodeGen/Collector.h"
+#include "llvm/Constants.h"
+#include "llvm/DerivedTypes.h"
+#include "llvm/Instructions.h"
+#include "llvm/IntrinsicInst.h"
+#include "llvm/Module.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/LLVMBuilder.h"
+#include "llvm/Analysis/Verifier.h"
+#include <cstdlib>
+
+using namespace llvm;
+
+namespace {
+
+ class VISIBILITY_HIDDEN ShadowStackCollector : public Collector {
+ /// 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.
+ ///
+ const StructType *StackEntryTy;
+
+ /// 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:
+ ShadowStackCollector();
+
+ bool initializeCustomLowering(Module &M);
+ bool performCustomLowering(Function &F);
+
+ private:
+ bool IsNullValue(Value *V);
+ Constant *GetFrameMap(Function &F);
+ const Type* GetConcreteStackEntryType(Function &F);
+ void CollectRoots(Function &F);
+ static GetElementPtrInst *CreateGEP(LLVMBuilder &B, Value *BasePtr,
+ int Idx1, const char *Name);
+ static GetElementPtrInst *CreateGEP(LLVMBuilder &B, Value *BasePtr,
+ int Idx1, int Idx2, const char *Name);
+ };
+
+ CollectorRegistry::Add<ShadowStackCollector>
+ Y("shadow-stack",
+ "Very portable collector for uncooperative code generators");
+
+ /// 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 VISIBILITY_HIDDEN EscapeEnumerator {
+ Function &F;
+ const char *CleanupBBName;
+
+ // State.
+ int State;
+ Function::iterator StateBB, StateE;
+ LLVMBuilder Builder;
+
+ public:
+ EscapeEnumerator(Function &F, const char *N = "cleanup")
+ : F(F), CleanupBBName(N), State(0) {}
+
+ LLVMBuilder *Next() {
+ switch (State) {
+ default:
+ return 0;
+
+ case 0:
+ StateBB = F.begin();
+ StateE = F.end();
+ State = 1;
+
+ case 1:
+ // Find all 'return' and 'unwind' instructions.
+ while (StateBB != StateE) {
+ BasicBlock *CurBB = StateBB++;
+
+ // Branches and invokes do not escape, only unwind and return do.
+ TerminatorInst *TI = CurBB->getTerminator();
+ if (!isa<UnwindInst>(TI) && !isa<ReturnInst>(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.
+ BasicBlock *CleanupBB = new BasicBlock(CleanupBBName, &F);
+ UnwindInst *UI = new UnwindInst(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();
+ Args.append(CI->op_begin() + 1, CI->op_end());
+
+ InvokeInst *II = new InvokeInst(CI->getOperand(0),
+ NewBB, CleanupBB,
+ Args.begin(), Args.end(),
+ CI->getName(), CallBB);
+ II->setCallingConv(CI->getCallingConv());
+ II->setParamAttrs(CI->getParamAttrs());
+ CI->replaceAllUsesWith(II);
+ delete CI;
+ }
+
+ Builder.SetInsertPoint(UI->getParent(), UI);
+ return &Builder;
+ }
+ }
+ };
+
+}
+
+// -----------------------------------------------------------------------------
+
+Collector *llvm::createShadowStackCollector() {
+ return new ShadowStackCollector();
+}
+
+ShadowStackCollector::ShadowStackCollector() : Head(0), StackEntryTy(0) {
+ InitRoots = true;
+ CustomRoots = true;
+}
+
+Constant *ShadowStackCollector::GetFrameMap(Function &F) {
+ // doInitialization creates the abstract type of this value.
+
+ Type *VoidPtr = PointerType::getUnqual(Type::Int8Ty);
+
+ // 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->getOperand(2));
+ if (!C->isNullValue())
+ NumMeta = I + 1;
+ Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
+ }
+
+ Constant *BaseElts[] = {
+ ConstantInt::get(Type::Int32Ty, Roots.size(), false),
+ ConstantInt::get(Type::Int32Ty, NumMeta, false),
+ };
+
+ Constant *DescriptorElts[] = {
+ ConstantStruct::get(BaseElts, 2),
+ ConstantArray::get(ArrayType::get(VoidPtr, NumMeta),
+ Metadata.begin(), NumMeta)
+ };
+
+ Constant *FrameMap = ConstantStruct::get(DescriptorElts, 2);
+
+ std::string TypeName("gc_map.");
+ TypeName += utostr(NumMeta);
+ F.getParent()->addTypeName(TypeName, FrameMap->getType());
+
+ // 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(FrameMap->getType(), true,
+ GlobalVariable::InternalLinkage,
+ FrameMap, "__gc_" + F.getName(),
+ F.getParent());
+
+ Constant *GEPIndices[2] = { ConstantInt::get(Type::Int32Ty, 0),
+ ConstantInt::get(Type::Int32Ty, 0) };
+ return ConstantExpr::getGetElementPtr(GV, GEPIndices, 2);
+}
+
+const Type* ShadowStackCollector::GetConcreteStackEntryType(Function &F) {
+ // doInitialization creates the generic version of this type.
+ std::vector<const Type*> EltTys;
+ EltTys.push_back(StackEntryTy);
+ for (size_t I = 0; I != Roots.size(); I++)
+ EltTys.push_back(Roots[I].second->getAllocatedType());
+ Type *Ty = StructType::get(EltTys);
+
+ std::string TypeName("gc_stackentry.");
+ TypeName += F.getName();
+ F.getParent()->addTypeName(TypeName, Ty);
+
+ return Ty;
+}
+
+/// doInitialization - If this module uses the GC intrinsics, find them now. If
+/// not, exit fast.
+bool ShadowStackCollector::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<const Type*> EltTys;
+ EltTys.push_back(Type::Int32Ty); // 32 bits is ok up to a 32GB stack frame. :)
+ EltTys.push_back(Type::Int32Ty); // Specifies length of variable length array.
+ StructType *FrameMapTy = StructType::get(EltTys);
+ M.addTypeName("gc_map", FrameMapTy);
+ 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).
+ // };
+ OpaqueType *RecursiveTy = OpaqueType::get();
+
+ EltTys.clear();
+ EltTys.push_back(PointerType::getUnqual(RecursiveTy));
+ EltTys.push_back(FrameMapPtrTy);
+ PATypeHolder LinkTyH = StructType::get(EltTys);
+
+ RecursiveTy->refineAbstractTypeTo(LinkTyH.get());
+ StackEntryTy = cast<StructType>(LinkTyH.get());
+ const PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
+ M.addTypeName("gc_stackentry", LinkTyH.get()); // FIXME: Is this safe from
+ // a FunctionPass?
+
+ // 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(StackEntryPtrTy, false,
+ GlobalValue::LinkOnceLinkage,
+ Constant::getNullValue(StackEntryPtrTy),
+ "llvm_gc_root_chain", &M);
+ } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
+ Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
+ Head->setLinkage(GlobalValue::LinkOnceLinkage);
+ }
+
+ return true;
+}
+
+bool ShadowStackCollector::IsNullValue(Value *V) {
+ if (Constant *C = dyn_cast<Constant>(V))
+ return C->isNullValue();
+ return false;
+}
+
+void ShadowStackCollector::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>(
+ IntrinsicInst::StripPointerCasts(CI->getOperand(1))));
+ if (IsNullValue(CI->getOperand(2)))
+ 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 *
+ShadowStackCollector::CreateGEP(LLVMBuilder &B, Value *BasePtr,
+ int Idx, int Idx2, const char *Name) {
+ Value *Indices[] = { ConstantInt::get(Type::Int32Ty, 0),
+ ConstantInt::get(Type::Int32Ty, Idx),
+ ConstantInt::get(Type::Int32Ty, Idx2) };
+ return B.CreateGEP(BasePtr, Indices, Indices + 3, Name);
+}
+
+GetElementPtrInst *
+ShadowStackCollector::CreateGEP(LLVMBuilder &B, Value *BasePtr,
+ int Idx, const char *Name) {
+ Value *Indices[] = { ConstantInt::get(Type::Int32Ty, 0),
+ ConstantInt::get(Type::Int32Ty, Idx) };
+ return B.CreateGEP(BasePtr, Indices, Indices + 2, Name);
+}
+
+/// runOnFunction - Insert code to maintain the shadow stack.
+bool ShadowStackCollector::performCustomLowering(Function &F) {
+ // 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);
+ const Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
+
+ // Build the shadow stack entry at the very start of the function.
+ BasicBlock::iterator IP = F.getEntryBlock().begin();
+ LLVMBuilder 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(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(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 Collector::InitRoots. This isn't
+ // really necessary (the collector would never see the intermediate state),
+ // but it's nicer not to push the half-initialized entry onto the stack.
+ while (isa<StoreInst>(IP)) ++IP;
+ AtEntry.SetInsertPoint(IP->getParent(), IP);
+
+ // Push the entry onto the shadow stack.
+ Instruction *EntryNextPtr = CreateGEP(AtEntry,StackEntry,0,0,"gc_frame.next");
+ Instruction *NewHeadVal = CreateGEP(AtEntry,StackEntry, 0, "gc_newhead");
+ AtEntry.CreateStore(CurrentHead, EntryNextPtr);
+ AtEntry.CreateStore(NewHeadVal, Head);
+
+ // For each instruction that escapes...
+ EscapeEnumerator EE(F, "gc_cleanup");
+ while (LLVMBuilder *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(*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();
+ }
+
+ F.dump();
+
+ Roots.clear();
+ return true;
+}