1 //===-- LowerGC.cpp - Provide GC support for targets that don't -----------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source 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 as efficient as it would be for targets that natively
13 // support the GC intrinsics, but it is useful for getting new targets
14 // up-and-running quickly.
16 // This pass implements the code transformation described in this paper:
17 // "Accurate Garbage Collection in an Uncooperative Environment"
18 // Fergus Henderson, ISMM, 2002
20 //===----------------------------------------------------------------------===//
22 #define DEBUG_TYPE "lowergc"
23 #include "llvm/Transforms/Scalar.h"
24 #include "llvm/Constants.h"
25 #include "llvm/DerivedTypes.h"
26 #include "llvm/Instructions.h"
27 #include "llvm/Module.h"
28 #include "llvm/Pass.h"
29 #include "llvm/Support/Compiler.h"
33 class VISIBILITY_HIDDEN LowerGC : public FunctionPass {
34 /// GCRootInt, GCReadInt, GCWriteInt - The function prototypes for the
35 /// llvm.gcread/llvm.gcwrite/llvm.gcroot intrinsics.
36 Function *GCRootInt, *GCReadInt, *GCWriteInt;
38 /// GCRead/GCWrite - These are the functions provided by the garbage
39 /// collector for read/write barriers.
40 Constant *GCRead, *GCWrite;
42 /// RootChain - This is the global linked-list that contains the chain of GC
44 GlobalVariable *RootChain;
46 /// MainRootRecordType - This is the type for a function root entry if it
48 const Type *MainRootRecordType;
50 static char ID; // Pass identification, replacement for typeid
51 LowerGC() : FunctionPass((intptr_t)&ID),
52 GCRootInt(0), GCReadInt(0), GCWriteInt(0),
53 GCRead(0), GCWrite(0), RootChain(0), MainRootRecordType(0) {}
54 virtual bool doInitialization(Module &M);
55 virtual bool runOnFunction(Function &F);
58 const StructType *getRootRecordType(unsigned NumRoots);
63 X("lowergc", "Lower GC intrinsics, for GCless code generators");
66 /// createLowerGCPass - This function returns an instance of the "lowergc"
67 /// pass, which lowers garbage collection intrinsics to normal LLVM code.
68 FunctionPass *llvm::createLowerGCPass() {
72 /// getRootRecordType - This function creates and returns the type for a root
73 /// record containing 'NumRoots' roots.
74 const StructType *LowerGC::getRootRecordType(unsigned NumRoots) {
75 // Build a struct that is a type used for meta-data/root pairs.
76 std::vector<const Type *> ST;
77 ST.push_back(GCRootInt->getFunctionType()->getParamType(0));
78 ST.push_back(GCRootInt->getFunctionType()->getParamType(1));
79 StructType *PairTy = StructType::get(ST);
81 // Build the array of pairs.
82 ArrayType *PairArrTy = ArrayType::get(PairTy, NumRoots);
84 // Now build the recursive list type.
85 PATypeHolder RootListH =
86 MainRootRecordType ? (Type*)MainRootRecordType : (Type*)OpaqueType::get();
88 ST.push_back(PointerType::get(RootListH)); // Prev pointer
89 ST.push_back(Type::Int32Ty); // NumElements in array
90 ST.push_back(PairArrTy); // The pairs
91 StructType *RootList = StructType::get(ST);
92 if (MainRootRecordType)
95 assert(NumRoots == 0 && "The main struct type should have zero entries!");
96 cast<OpaqueType>((Type*)RootListH.get())->refineAbstractTypeTo(RootList);
97 MainRootRecordType = RootListH;
98 return cast<StructType>(RootListH.get());
101 /// doInitialization - If this module uses the GC intrinsics, find them now. If
102 /// not, this pass does not do anything.
103 bool LowerGC::doInitialization(Module &M) {
104 GCRootInt = M.getFunction("llvm.gcroot");
105 GCReadInt = M.getFunction("llvm.gcread");
106 GCWriteInt = M.getFunction("llvm.gcwrite");
107 if (!GCRootInt && !GCReadInt && !GCWriteInt) return false;
109 PointerType *VoidPtr = PointerType::get(Type::Int8Ty);
110 PointerType *VoidPtrPtr = PointerType::get(VoidPtr);
112 // If the program is using read/write barriers, find the implementations of
113 // them from the GC runtime library.
114 if (GCReadInt) // Make: sbyte* %llvm_gc_read(sbyte**)
115 GCRead = M.getOrInsertFunction("llvm_gc_read", VoidPtr, VoidPtr, VoidPtrPtr,
117 if (GCWriteInt) // Make: void %llvm_gc_write(sbyte*, sbyte**)
118 GCWrite = M.getOrInsertFunction("llvm_gc_write", Type::VoidTy,
119 VoidPtr, VoidPtr, VoidPtrPtr, (Type *)0);
121 // If the program has GC roots, get or create the global root list.
123 const StructType *RootListTy = getRootRecordType(0);
124 const Type *PRLTy = PointerType::get(RootListTy);
125 M.addTypeName("llvm_gc_root_ty", RootListTy);
127 // Get the root chain if it already exists.
128 RootChain = M.getGlobalVariable("llvm_gc_root_chain", PRLTy);
129 if (RootChain == 0) {
130 // If the root chain does not exist, insert a new one with linkonce
132 RootChain = new GlobalVariable(PRLTy, false,
133 GlobalValue::LinkOnceLinkage,
134 Constant::getNullValue(PRLTy),
135 "llvm_gc_root_chain", &M);
136 } else if (RootChain->hasExternalLinkage() && RootChain->isDeclaration()) {
137 RootChain->setInitializer(Constant::getNullValue(PRLTy));
138 RootChain->setLinkage(GlobalValue::LinkOnceLinkage);
144 /// Coerce - If the specified operand number of the specified instruction does
145 /// not have the specified type, insert a cast. Note that this only uses BitCast
146 /// because the types involved are all pointers.
147 static void Coerce(Instruction *I, unsigned OpNum, Type *Ty) {
148 if (I->getOperand(OpNum)->getType() != Ty) {
149 if (Constant *C = dyn_cast<Constant>(I->getOperand(OpNum)))
150 I->setOperand(OpNum, ConstantExpr::getBitCast(C, Ty));
152 CastInst *CI = new BitCastInst(I->getOperand(OpNum), Ty, "", I);
153 I->setOperand(OpNum, CI);
158 /// runOnFunction - If the program is using GC intrinsics, replace any
159 /// read/write intrinsics with the appropriate read/write barrier calls, then
160 /// inline them. Finally, build the data structures for
161 bool LowerGC::runOnFunction(Function &F) {
162 // Quick exit for programs that are not using GC mechanisms.
163 if (!GCRootInt && !GCReadInt && !GCWriteInt) return false;
165 PointerType *VoidPtr = PointerType::get(Type::Int8Ty);
166 PointerType *VoidPtrPtr = PointerType::get(VoidPtr);
168 // If there are read/write barriers in the program, perform a quick pass over
169 // the function eliminating them. While we are at it, remember where we see
170 // calls to llvm.gcroot.
171 std::vector<CallInst*> GCRoots;
172 std::vector<CallInst*> NormalCalls;
174 bool MadeChange = false;
175 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
176 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;)
177 if (CallInst *CI = dyn_cast<CallInst>(II++)) {
178 if (!CI->getCalledFunction() ||
179 !CI->getCalledFunction()->getIntrinsicID())
180 NormalCalls.push_back(CI); // Remember all normal function calls.
182 if (Function *F = CI->getCalledFunction())
184 GCRoots.push_back(CI);
185 else if (F == GCReadInt || F == GCWriteInt) {
186 if (F == GCWriteInt) {
187 // Change a llvm.gcwrite call to call llvm_gc_write instead.
188 CI->setOperand(0, GCWrite);
189 // Insert casts of the operands as needed.
190 Coerce(CI, 1, VoidPtr);
191 Coerce(CI, 2, VoidPtr);
192 Coerce(CI, 3, VoidPtrPtr);
194 Coerce(CI, 1, VoidPtr);
195 Coerce(CI, 2, VoidPtrPtr);
196 if (CI->getType() == VoidPtr) {
197 CI->setOperand(0, GCRead);
199 // Create a whole new call to replace the old one.
200 CallInst *NC = new CallInst(GCRead, CI->getOperand(1),
203 // These functions only deal with ptr type results so BitCast
204 // is the correct kind of cast (no-op cast).
205 Value *NV = new BitCastInst(NC, CI->getType(), "", CI);
206 CI->replaceAllUsesWith(NV);
207 BB->getInstList().erase(CI);
216 // If there are no GC roots in this function, then there is no need to create
217 // a GC list record for it.
218 if (GCRoots.empty()) return MadeChange;
220 // Okay, there are GC roots in this function. On entry to the function, add a
221 // record to the llvm_gc_root_chain, and remove it on exit.
223 // Create the alloca, and zero it out.
224 const StructType *RootListTy = getRootRecordType(GCRoots.size());
225 AllocaInst *AI = new AllocaInst(RootListTy, 0, "gcroots", F.begin()->begin());
227 // Insert the memset call after all of the allocas in the function.
228 BasicBlock::iterator IP = AI;
229 while (isa<AllocaInst>(IP)) ++IP;
231 Constant *Zero = ConstantInt::get(Type::Int32Ty, 0);
232 Constant *One = ConstantInt::get(Type::Int32Ty, 1);
234 // Get a pointer to the prev pointer.
235 Value *PrevPtrPtr = new GetElementPtrInst(AI, Zero, Zero, "prevptrptr", IP);
237 // Load the previous pointer.
238 Value *PrevPtr = new LoadInst(RootChain, "prevptr", IP);
239 // Store the previous pointer into the prevptrptr
240 new StoreInst(PrevPtr, PrevPtrPtr, IP);
242 // Set the number of elements in this record.
243 Value *NumEltsPtr = new GetElementPtrInst(AI, Zero, One, "numeltsptr", IP);
244 new StoreInst(ConstantInt::get(Type::Int32Ty, GCRoots.size()), NumEltsPtr,IP);
248 Par[1] = ConstantInt::get(Type::Int32Ty, 2);
250 const PointerType *PtrLocTy =
251 cast<PointerType>(GCRootInt->getFunctionType()->getParamType(0));
252 Constant *Null = ConstantPointerNull::get(PtrLocTy);
254 // Initialize all of the gcroot records now, and eliminate them as we go.
255 for (unsigned i = 0, e = GCRoots.size(); i != e; ++i) {
256 // Initialize the meta-data pointer.
257 Par[2] = ConstantInt::get(Type::Int32Ty, i);
259 Value *MetaDataPtr = new GetElementPtrInst(AI, Par, 4, "MetaDataPtr", IP);
260 assert(isa<Constant>(GCRoots[i]->getOperand(2)) && "Must be a constant");
261 new StoreInst(GCRoots[i]->getOperand(2), MetaDataPtr, IP);
263 // Initialize the root pointer to null on entry to the function.
265 Value *RootPtrPtr = new GetElementPtrInst(AI, Par, 4, "RootEntPtr", IP);
266 new StoreInst(Null, RootPtrPtr, IP);
268 // Each occurrance of the llvm.gcroot intrinsic now turns into an
269 // initialization of the slot with the address and a zeroing out of the
270 // address specified.
271 new StoreInst(Constant::getNullValue(PtrLocTy->getElementType()),
272 GCRoots[i]->getOperand(1), GCRoots[i]);
273 new StoreInst(GCRoots[i]->getOperand(1), RootPtrPtr, GCRoots[i]);
274 GCRoots[i]->getParent()->getInstList().erase(GCRoots[i]);
277 // Now that the record is all initialized, store the pointer into the global
279 Value *C = new BitCastInst(AI, PointerType::get(MainRootRecordType), "", IP);
280 new StoreInst(C, RootChain, IP);
282 // On exit from the function we have to remove the entry from the GC root
283 // chain. Doing this is straight-forward for return and unwind instructions:
284 // just insert the appropriate copy.
285 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
286 if (isa<UnwindInst>(BB->getTerminator()) ||
287 isa<ReturnInst>(BB->getTerminator())) {
288 // We could reuse the PrevPtr loaded on entry to the function, but this
289 // would make the value live for the whole function, which is probably a
290 // bad idea. Just reload the value out of our stack entry.
291 PrevPtr = new LoadInst(PrevPtrPtr, "prevptr", BB->getTerminator());
292 new StoreInst(PrevPtr, RootChain, BB->getTerminator());
295 // If an exception is thrown from a callee we have to make sure to
296 // unconditionally take the record off the stack. For this reason, we turn
297 // all call instructions into invoke whose cleanup pops the entry off the
298 // stack. We only insert one cleanup block, which is shared by all invokes.
299 if (!NormalCalls.empty()) {
300 // Create the shared cleanup block.
301 BasicBlock *Cleanup = new BasicBlock("gc_cleanup", &F);
302 UnwindInst *UI = new UnwindInst(Cleanup);
303 PrevPtr = new LoadInst(PrevPtrPtr, "prevptr", UI);
304 new StoreInst(PrevPtr, RootChain, UI);
306 // Loop over all of the function calls, turning them into invokes.
307 while (!NormalCalls.empty()) {
308 CallInst *CI = NormalCalls.back();
309 BasicBlock *CBB = CI->getParent();
310 NormalCalls.pop_back();
312 // Split the basic block containing the function call.
313 BasicBlock *NewBB = CBB->splitBasicBlock(CI, CBB->getName()+".cont");
315 // Remove the unconditional branch inserted at the end of the CBB.
316 CBB->getInstList().pop_back();
317 NewBB->getInstList().remove(CI);
319 // Create a new invoke instruction.
320 std::vector<Value*> Args(CI->op_begin()+1, CI->op_end());
322 Value *II = new InvokeInst(CI->getCalledValue(), NewBB, Cleanup,
323 &Args[0], Args.size(), CI->getName(), CBB);
324 CI->replaceAllUsesWith(II);