--- /dev/null
+//===-- LowerGC.cpp - Provide GC support for targets that don't -----------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file was developed by the LLVM research group and 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 as efficient as it would be for targets that natively
+// support the GC intrinsics, but it is useful for getting new targets
+// up-and-running quickly.
+//
+// This pass implements the code transformation described in this paper:
+// "Accurate Garbage Collection in an Uncooperative Environment"
+// Fergus Hendersen, ISMM, 2002
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "lowergc"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Constants.h"
+#include "llvm/DerivedTypes.h"
+#include "llvm/Instructions.h"
+#include "llvm/Module.h"
+#include "llvm/Pass.h"
+#include "llvm/Transforms/Utils/Cloning.h"
+using namespace llvm;
+
+namespace {
+ class LowerGC : public FunctionPass {
+ /// GCRootInt, GCReadInt, GCWriteInt - The function prototypes for the
+ /// llvm.gcread/llvm.gcwrite/llvm.gcroot intrinsics.
+ Function *GCRootInt, *GCReadInt, *GCWriteInt;
+
+ /// GCRead/GCWrite - These are the functions provided by the garbage
+ /// collector for read/write barriers.
+ Function *GCRead, *GCWrite;
+
+ /// RootChain - This is the global linked-list that contains the chain of GC
+ /// roots.
+ GlobalVariable *RootChain;
+
+ /// MainRootRecordType - This is the type for a function root entry if it
+ /// had zero roots.
+ const Type *MainRootRecordType;
+ public:
+ LowerGC() : GCRootInt(0), GCReadInt(0), GCWriteInt(0),
+ GCRead(0), GCWrite(0), RootChain(0), MainRootRecordType(0) {}
+ virtual bool doInitialization(Module &M);
+ virtual bool runOnFunction(Function &F);
+
+ private:
+ const StructType *getRootRecordType(unsigned NumRoots);
+ };
+
+ RegisterOpt<LowerGC>
+ X("lowergc", "Lower GC intrinsics, for GCless code generators");
+}
+
+/// createLowerGCPass - This function returns an instance of the "lowergc"
+/// pass, which lowers garbage collection intrinsics to normal LLVM code.
+FunctionPass *llvm::createLowerGCPass() {
+ return new LowerGC();
+}
+
+/// getRootRecordType - This function creates and returns the type for a root
+/// record containing 'NumRoots' roots.
+const StructType *LowerGC::getRootRecordType(unsigned NumRoots) {
+ // Build a struct that is a type used for meta-data/root pairs.
+ std::vector<const Type *> ST;
+ ST.push_back(GCRootInt->getFunctionType()->getParamType(0));
+ ST.push_back(GCRootInt->getFunctionType()->getParamType(1));
+ StructType *PairTy = StructType::get(ST);
+
+ // Build the array of pairs.
+ ArrayType *PairArrTy = ArrayType::get(PairTy, NumRoots);
+
+ // Now build the recursive list type.
+ PATypeHolder RootListH =
+ MainRootRecordType ? (Type*)MainRootRecordType : (Type*)OpaqueType::get();
+ ST.clear();
+ ST.push_back(PointerType::get(RootListH)); // Prev pointer
+ ST.push_back(Type::UIntTy); // NumElements in array
+ ST.push_back(PairArrTy); // The pairs
+ StructType *RootList = StructType::get(ST);
+ if (MainRootRecordType)
+ return RootList;
+
+ assert(NumRoots == 0 && "The main struct type should have zero entries!");
+ cast<OpaqueType>((Type*)RootListH.get())->refineAbstractTypeTo(RootList);
+ MainRootRecordType = RootListH;
+ return cast<StructType>(RootListH.get());
+}
+
+/// doInitialization - If this module uses the GC intrinsics, find them now. If
+/// not, this pass does not do anything.
+bool LowerGC::doInitialization(Module &M) {
+ GCRootInt = M.getNamedFunction("llvm.gcroot");
+ GCReadInt = M.getNamedFunction("llvm.gcread");
+ GCWriteInt = M.getNamedFunction("llvm.gcwrite");
+ if (!GCRootInt && !GCReadInt && !GCWriteInt) return false;
+
+ PointerType *VoidPtr = PointerType::get(Type::SByteTy);
+ PointerType *VoidPtrPtr = PointerType::get(VoidPtr);
+
+ // If the program is using read/write barriers, find the implementations of
+ // them from the GC runtime library.
+ if (GCReadInt) // Make: sbyte* %llvm_gc_read(sbyte**)
+ GCRead = M.getOrInsertFunction("llvm_gc_read", VoidPtr, VoidPtrPtr, 0);
+ if (GCWriteInt) // Make: void %llvm_gc_write(sbyte*, sbyte**)
+ GCWrite = M.getOrInsertFunction("llvm_gc_write", Type::VoidTy,
+ VoidPtr, VoidPtrPtr, 0);
+
+ // If the program has GC roots, get or create the global root list.
+ if (GCRootInt) {
+ const StructType *RootListTy = getRootRecordType(0);
+ const Type *PRLTy = PointerType::get(RootListTy);
+ M.addTypeName("llvm_gc_root_ty", RootListTy);
+
+ // Get the root chain if it already exists.
+ RootChain = M.getGlobalVariable("llvm_gc_root_chain", PRLTy);
+ if (RootChain == 0) {
+ // If the root chain does not exist, insert a new one with linkonce
+ // linkage!
+ RootChain = new GlobalVariable(PRLTy, false, GlobalValue::LinkOnceLinkage,
+ Constant::getNullValue(RootListTy),
+ "llvm_gc_root_chain", &M);
+ } else if (RootChain->hasExternalLinkage() && RootChain->isExternal()) {
+ RootChain->setInitializer(Constant::getNullValue(PRLTy));
+ RootChain->setLinkage(GlobalValue::LinkOnceLinkage);
+ }
+ }
+ return true;
+}
+
+/// Coerce - If the specified operand number of the specified instruction does
+/// not have the specified type, insert a cast.
+static void Coerce(Instruction *I, unsigned OpNum, Type *Ty) {
+ if (I->getOperand(OpNum)->getType() != Ty) {
+ if (Constant *C = dyn_cast<Constant>(I->getOperand(OpNum)))
+ I->setOperand(OpNum, ConstantExpr::getCast(C, Ty));
+ else {
+ CastInst *C = new CastInst(I->getOperand(OpNum), Ty, "", I);
+ I->setOperand(OpNum, C);
+ }
+ }
+}
+
+/// runOnFunction - If the program is using GC intrinsics, replace any
+/// read/write intrinsics with the appropriate read/write barrier calls, then
+/// inline them. Finally, build the data structures for
+bool LowerGC::runOnFunction(Function &F) {
+ // Quick exit for programs that are not using GC mechanisms.
+ if (!GCRootInt && !GCReadInt && !GCWriteInt) return false;
+
+ PointerType *VoidPtr = PointerType::get(Type::SByteTy);
+ PointerType *VoidPtrPtr = PointerType::get(VoidPtr);
+
+ // If there are read/write barriers in the program, perform a quick pass over
+ // the function eliminating them. While we are at it, remember where we see
+ // calls to llvm.gcroot.
+ std::vector<CallInst*> GCRoots;
+ std::vector<CallInst*> NormalCalls;
+
+ bool MadeChange = false;
+ for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
+ for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;)
+ if (CallInst *CI = dyn_cast<CallInst>(II++)) {
+ if (!CI->getCalledFunction() ||
+ !CI->getCalledFunction()->getIntrinsicID())
+ NormalCalls.push_back(CI); // Remember all normal function calls.
+
+ if (Function *F = CI->getCalledFunction())
+ if (F == GCRootInt)
+ GCRoots.push_back(CI);
+ else if (F == GCReadInt || F == GCWriteInt) {
+ if (F == GCWriteInt) {
+ // Change a llvm.gcwrite call to call llvm_gc_write instead.
+ CI->setOperand(0, GCWrite);
+ // Insert casts of the operands as needed.
+ Coerce(CI, 1, VoidPtr);
+ Coerce(CI, 2, VoidPtrPtr);
+ } else {
+ Coerce(CI, 1, VoidPtrPtr);
+ if (CI->getType() == VoidPtr) {
+ CI->setOperand(0, GCRead);
+ } else {
+ // Create a whole new call to replace the old one.
+ CallInst *NC = new CallInst(GCRead, CI->getOperand(1),
+ CI->getName(), CI);
+ Value *NV = new CastInst(NC, CI->getType(), "", CI);
+ CI->replaceAllUsesWith(NV);
+ BB->getInstList().erase(CI);
+ CI = NC;
+ }
+ }
+
+ // Now that we made the replacement, inline expand the call if
+ // possible, otherwise things will be too horribly expensive.
+ InlineFunction(CI);
+ MadeChange = true;
+ }
+ }
+
+ // If there are no GC roots in this function, then there is no need to create
+ // a GC list record for it.
+ if (GCRoots.empty()) return MadeChange;
+
+ // Okay, there are GC roots in this function. On entry to the function, add a
+ // record to the llvm_gc_root_chain, and remove it on exit.
+
+ // Create the alloca, and zero it out.
+ const StructType *RootListTy = getRootRecordType(GCRoots.size());
+ AllocaInst *AI = new AllocaInst(RootListTy, 0, "gcroots", F.begin()->begin());
+
+ // Insert the memset call after all of the allocas in the function.
+ BasicBlock::iterator IP = AI;
+ while (isa<AllocaInst>(IP)) ++IP;
+
+ Constant *Zero = ConstantUInt::get(Type::UIntTy, 0);
+ Constant *One = ConstantUInt::get(Type::UIntTy, 1);
+
+ // Get a pointer to the prev pointer.
+ std::vector<Value*> Par;
+ Par.push_back(Zero);
+ Par.push_back(Zero);
+ Value *PrevPtrPtr = new GetElementPtrInst(AI, Par, "prevptrptr", IP);
+
+ // Load the previous pointer.
+ Value *PrevPtr = new LoadInst(RootChain, "prevptr", IP);
+ // Store the previous pointer into the prevptrptr
+ new StoreInst(PrevPtr, PrevPtrPtr, IP);
+
+ // Set the number of elements in this record.
+ Par[1] = ConstantUInt::get(Type::UIntTy, 1);
+ Value *NumEltsPtr = new GetElementPtrInst(AI, Par, "numeltsptr", IP);
+ new StoreInst(ConstantUInt::get(Type::UIntTy, GCRoots.size()), NumEltsPtr,IP);
+
+ Par[1] = ConstantUInt::get(Type::UIntTy, 2);
+ Par.resize(4);
+
+ const PointerType *PtrLocTy =
+ cast<PointerType>(GCRootInt->getFunctionType()->getParamType(0));
+ Constant *Null = ConstantPointerNull::get(PtrLocTy);
+
+ // Initialize all of the gcroot records now, and eliminate them as we go.
+ for (unsigned i = 0, e = GCRoots.size(); i != e; ++i) {
+ // Initialize the meta-data pointer.
+ Par[2] = ConstantUInt::get(Type::UIntTy, i);
+ Par[3] = One;
+ Value *MetaDataPtr = new GetElementPtrInst(AI, Par, "MetaDataPtr", IP);
+ assert(isa<Constant>(GCRoots[i]->getOperand(2)) ||
+ isa<GlobalValue>(GCRoots[i]->getOperand(2)));
+ new StoreInst(GCRoots[i]->getOperand(2), MetaDataPtr, IP);
+
+ // Initialize the root pointer to null on entry to the function.
+ Par[3] = Zero;
+ Value *RootPtrPtr = new GetElementPtrInst(AI, Par, "RootEntPtr", IP);
+ new StoreInst(Null, RootPtrPtr, IP);
+
+ // Each occurrance of the llvm.gcroot intrinsic now turns into an
+ // initialization of the slot with the address and a zeroing out of the
+ // address specified.
+ new StoreInst(Constant::getNullValue(PtrLocTy->getElementType()),
+ GCRoots[i]->getOperand(1), GCRoots[i]);
+ new StoreInst(GCRoots[i]->getOperand(1), RootPtrPtr, GCRoots[i]);
+ GCRoots[i]->getParent()->getInstList().erase(GCRoots[i]);
+ }
+
+ // Now that the record is all initialized, store the pointer into the global
+ // pointer.
+ Value *C = new CastInst(AI, PointerType::get(MainRootRecordType), "", IP);
+ new StoreInst(C, RootChain, IP);
+
+ // On exit from the function we have to remove the entry from the GC root
+ // chain. Doing this is straight-forward for return and unwind instructions:
+ // just insert the appropriate copy.
+ for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
+ if (isa<UnwindInst>(BB->getTerminator()) ||
+ isa<ReturnInst>(BB->getTerminator())) {
+ // We could reuse the PrevPtr loaded on entry to the function, but this
+ // would make the value live for the whole function, which is probably a
+ // bad idea. Just reload the value out of our stack entry.
+ PrevPtr = new LoadInst(PrevPtrPtr, "prevptr", BB->getTerminator());
+ new StoreInst(PrevPtr, RootChain, BB->getTerminator());
+ }
+
+ // If an exception is thrown from a callee we have to make sure to
+ // unconditionally take the record off the stack. For this reason, we turn
+ // all call instructions into invoke whose cleanup pops the entry off the
+ // stack. We only insert one cleanup block, which is shared by all invokes.
+ if (!NormalCalls.empty()) {
+ // Create the shared cleanup block.
+ BasicBlock *Cleanup = new BasicBlock("gc_cleanup", &F);
+ UnwindInst *UI = new UnwindInst(Cleanup);
+ PrevPtr = new LoadInst(PrevPtrPtr, "prevptr", UI);
+ new StoreInst(PrevPtr, RootChain, UI);
+
+ // Loop over all of the function calls, turning them into invokes.
+ while (!NormalCalls.empty()) {
+ CallInst *CI = NormalCalls.back();
+ BasicBlock *CBB = CI->getParent();
+ NormalCalls.pop_back();
+
+ // Split the basic block containing the function call.
+ BasicBlock *NewBB = CBB->splitBasicBlock(CI, CBB->getName()+".cont");
+
+ // Remove the unconditional branch inserted at the end of the CBB.
+ CBB->getInstList().pop_back();
+ NewBB->getInstList().remove(CI);
+
+ // Create a new invoke instruction.
+ Value *II = new InvokeInst(CI->getCalledValue(), NewBB, Cleanup,
+ std::vector<Value*>(CI->op_begin()+1,
+ CI->op_end()),
+ CI->getName(), CBB);
+ CI->replaceAllUsesWith(II);
+ delete CI;
+ }
+ }
+
+ return true;
+}
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