1 //===-- GCRootLowering.cpp - Garbage collection infrastructure ------------===//
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 the lowering for the gc.root mechanism.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/CodeGen/GCMetadata.h"
15 #include "llvm/CodeGen/MachineFrameInfo.h"
16 #include "llvm/CodeGen/MachineFunctionPass.h"
17 #include "llvm/CodeGen/MachineInstrBuilder.h"
18 #include "llvm/CodeGen/MachineModuleInfo.h"
19 #include "llvm/CodeGen/GCStrategy.h"
20 #include "llvm/CodeGen/Passes.h"
21 #include "llvm/IR/Dominators.h"
22 #include "llvm/IR/IntrinsicInst.h"
23 #include "llvm/IR/Module.h"
24 #include "llvm/Support/Debug.h"
25 #include "llvm/Support/ErrorHandling.h"
26 #include "llvm/Support/raw_ostream.h"
27 #include "llvm/Target/TargetFrameLowering.h"
28 #include "llvm/Target/TargetInstrInfo.h"
29 #include "llvm/Target/TargetMachine.h"
30 #include "llvm/Target/TargetRegisterInfo.h"
31 #include "llvm/Target/TargetSubtargetInfo.h"
32 
33 using namespace llvm;
34 
35 namespace {
36 
37 /// LowerIntrinsics - This pass rewrites calls to the llvm.gcread or
38 /// llvm.gcwrite intrinsics, replacing them with simple loads and stores as
39 /// directed by the GCStrategy. It also performs automatic root initialization
40 /// and custom intrinsic lowering.
41 class LowerIntrinsics : public FunctionPass {
42   bool PerformDefaultLowering(Function &F, GCStrategy &Coll);
43 
44 public:
45   static char ID;
46 
47   LowerIntrinsics();
48   const char *getPassName() const override;
49   void getAnalysisUsage(AnalysisUsage &AU) const override;
50 
51   bool doInitialization(Module &M) override;
52   bool runOnFunction(Function &F) override;
53 };
54 
55 /// GCMachineCodeAnalysis - This is a target-independent pass over the machine
56 /// function representation to identify safe points for the garbage collector
57 /// in the machine code. It inserts labels at safe points and populates a
58 /// GCMetadata record for each function.
59 class GCMachineCodeAnalysis : public MachineFunctionPass {
60   const TargetMachine *TM;
61   GCFunctionInfo *FI;
62   MachineModuleInfo *MMI;
63   const TargetInstrInfo *TII;
64 
65   void FindSafePoints(MachineFunction &MF);
66   void VisitCallPoint(MachineBasicBlock::iterator MI);
67   MCSymbol *InsertLabel(MachineBasicBlock &MBB, MachineBasicBlock::iterator MI,
68                         DebugLoc DL) const;
69 
70   void FindStackOffsets(MachineFunction &MF);
71 
72 public:
73   static char ID;
74 
75   GCMachineCodeAnalysis();
76   void getAnalysisUsage(AnalysisUsage &AU) const override;
77 
78   bool runOnMachineFunction(MachineFunction &MF) override;
79 };
80 }
81 
82 // -----------------------------------------------------------------------------
83 
84 INITIALIZE_PASS_BEGIN(LowerIntrinsics, "gc-lowering", "GC Lowering", false,
85                       false)
86 INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
87 INITIALIZE_PASS_END(LowerIntrinsics, "gc-lowering", "GC Lowering", false, false)
88 
89 FunctionPass *llvm::createGCLoweringPass() { return new LowerIntrinsics(); }
90 
91 char LowerIntrinsics::ID = 0;
92 
93 LowerIntrinsics::LowerIntrinsics() : FunctionPass(ID) {
94   initializeLowerIntrinsicsPass(*PassRegistry::getPassRegistry());
95 }
96 
97 const char *LowerIntrinsics::getPassName() const {
98   return "Lower Garbage Collection Instructions";
99 }
100 
101 void LowerIntrinsics::getAnalysisUsage(AnalysisUsage &AU) const {
102   FunctionPass::getAnalysisUsage(AU);
103   AU.addRequired<GCModuleInfo>();
104   AU.addPreserved<DominatorTreeWrapperPass>();
105 }
106 
107 static bool NeedsDefaultLoweringPass(const GCStrategy &C) {
108   // Default lowering is necessary only if read or write barriers have a default
109   // action. The default for roots is no action.
110   return !C.customWriteBarrier() || !C.customReadBarrier() ||
111          C.initializeRoots();
112 }
113 
114 /// doInitialization - If this module uses the GC intrinsics, find them now.
115 bool LowerIntrinsics::doInitialization(Module &M) {
116   GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
117   assert(MI && "LowerIntrinsics didn't require GCModuleInfo!?");
118   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
119     if (!I->isDeclaration() && I->hasGC())
120       MI->getFunctionInfo(*I); // Instantiate the GC strategy.
121 
122   return false;
123 }
124 
125 /// CouldBecomeSafePoint - Predicate to conservatively determine whether the
126 /// instruction could introduce a safe point.
127 static bool CouldBecomeSafePoint(Instruction *I) {
128   // The natural definition of instructions which could introduce safe points
129   // are:
130   //
131   //   - call, invoke (AfterCall, BeforeCall)
132   //   - phis (Loops)
133   //   - invoke, ret, unwind (Exit)
134   //
135   // However, instructions as seemingly inoccuous as arithmetic can become
136   // libcalls upon lowering (e.g., div i64 on a 32-bit platform), so instead
137   // it is necessary to take a conservative approach.
138 
139   if (isa<AllocaInst>(I) || isa<GetElementPtrInst>(I) || isa<StoreInst>(I) ||
140       isa<LoadInst>(I))
141     return false;
142 
143   // llvm.gcroot is safe because it doesn't do anything at runtime.
144   if (CallInst *CI = dyn_cast<CallInst>(I))
145     if (Function *F = CI->getCalledFunction())
146       if (unsigned IID = F->getIntrinsicID())
147         if (IID == Intrinsic::gcroot)
148           return false;
149 
150   return true;
151 }
152 
153 static bool InsertRootInitializers(Function &F, AllocaInst **Roots,
154                                    unsigned Count) {
155   // Scroll past alloca instructions.
156   BasicBlock::iterator IP = F.getEntryBlock().begin();
157   while (isa<AllocaInst>(IP))
158     ++IP;
159 
160   // Search for initializers in the initial BB.
161   SmallPtrSet<AllocaInst *, 16> InitedRoots;
162   for (; !CouldBecomeSafePoint(IP); ++IP)
163     if (StoreInst *SI = dyn_cast<StoreInst>(IP))
164       if (AllocaInst *AI =
165               dyn_cast<AllocaInst>(SI->getOperand(1)->stripPointerCasts()))
166         InitedRoots.insert(AI);
167 
168   // Add root initializers.
169   bool MadeChange = false;
170 
171   for (AllocaInst **I = Roots, **E = Roots + Count; I != E; ++I)
172     if (!InitedRoots.count(*I)) {
173       StoreInst *SI = new StoreInst(
174           ConstantPointerNull::get(cast<PointerType>(
175               cast<PointerType>((*I)->getType())->getElementType())),
176           *I);
177       SI->insertAfter(*I);
178       MadeChange = true;
179     }
180 
181   return MadeChange;
182 }
183 
184 /// runOnFunction - Replace gcread/gcwrite intrinsics with loads and stores.
185 /// Leave gcroot intrinsics; the code generator needs to see those.
186 bool LowerIntrinsics::runOnFunction(Function &F) {
187   // Quick exit for functions that do not use GC.
188   if (!F.hasGC())
189     return false;
190 
191   GCFunctionInfo &FI = getAnalysis<GCModuleInfo>().getFunctionInfo(F);
192   GCStrategy &S = FI.getStrategy();
193 
194   bool MadeChange = false;
195 
196   if (NeedsDefaultLoweringPass(S))
197     MadeChange |= PerformDefaultLowering(F, S);
198 
199   return MadeChange;
200 }
201 
202 bool LowerIntrinsics::PerformDefaultLowering(Function &F, GCStrategy &S) {
203   bool LowerWr = !S.customWriteBarrier();
204   bool LowerRd = !S.customReadBarrier();
205   bool InitRoots = S.initializeRoots();
206 
207   SmallVector<AllocaInst *, 32> Roots;
208 
209   bool MadeChange = false;
210   for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
211     for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
212       if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) {
213         Function *F = CI->getCalledFunction();
214         switch (F->getIntrinsicID()) {
215         case Intrinsic::gcwrite:
216           if (LowerWr) {
217             // Replace a write barrier with a simple store.
218             Value *St =
219                 new StoreInst(CI->getArgOperand(0), CI->getArgOperand(2), CI);
220             CI->replaceAllUsesWith(St);
221             CI->eraseFromParent();
222           }
223           break;
224         case Intrinsic::gcread:
225           if (LowerRd) {
226             // Replace a read barrier with a simple load.
227             Value *Ld = new LoadInst(CI->getArgOperand(1), "", CI);
228             Ld->takeName(CI);
229             CI->replaceAllUsesWith(Ld);
230             CI->eraseFromParent();
231           }
232           break;
233         case Intrinsic::gcroot:
234           if (InitRoots) {
235             // Initialize the GC root, but do not delete the intrinsic. The
236             // backend needs the intrinsic to flag the stack slot.
237             Roots.push_back(
238                 cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
239           }
240           break;
241         default:
242           continue;
243         }
244 
245         MadeChange = true;
246       }
247     }
248   }
249 
250   if (Roots.size())
251     MadeChange |= InsertRootInitializers(F, Roots.begin(), Roots.size());
252 
253   return MadeChange;
254 }
255 
256 // -----------------------------------------------------------------------------
257 
258 char GCMachineCodeAnalysis::ID = 0;
259 char &llvm::GCMachineCodeAnalysisID = GCMachineCodeAnalysis::ID;
260 
261 INITIALIZE_PASS(GCMachineCodeAnalysis, "gc-analysis",
262                 "Analyze Machine Code For Garbage Collection", false, false)
263 
264 GCMachineCodeAnalysis::GCMachineCodeAnalysis() : MachineFunctionPass(ID) {}
265 
266 void GCMachineCodeAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
267   MachineFunctionPass::getAnalysisUsage(AU);
268   AU.setPreservesAll();
269   AU.addRequired<MachineModuleInfo>();
270   AU.addRequired<GCModuleInfo>();
271 }
272 
273 MCSymbol *GCMachineCodeAnalysis::InsertLabel(MachineBasicBlock &MBB,
274                                              MachineBasicBlock::iterator MI,
275                                              DebugLoc DL) const {
276   MCSymbol *Label = MBB.getParent()->getContext().CreateTempSymbol();
277   BuildMI(MBB, MI, DL, TII->get(TargetOpcode::GC_LABEL)).addSym(Label);
278   return Label;
279 }
280 
281 void GCMachineCodeAnalysis::VisitCallPoint(MachineBasicBlock::iterator CI) {
282   // Find the return address (next instruction), too, so as to bracket the call
283   // instruction.
284   MachineBasicBlock::iterator RAI = CI;
285   ++RAI;
286 
287   if (FI->getStrategy().needsSafePoint(GC::PreCall)) {
288     MCSymbol *Label = InsertLabel(*CI->getParent(), CI, CI->getDebugLoc());
289     FI->addSafePoint(GC::PreCall, Label, CI->getDebugLoc());
290   }
291 
292   if (FI->getStrategy().needsSafePoint(GC::PostCall)) {
293     MCSymbol *Label = InsertLabel(*CI->getParent(), RAI, CI->getDebugLoc());
294     FI->addSafePoint(GC::PostCall, Label, CI->getDebugLoc());
295   }
296 }
297 
298 void GCMachineCodeAnalysis::FindSafePoints(MachineFunction &MF) {
299   for (MachineFunction::iterator BBI = MF.begin(), BBE = MF.end(); BBI != BBE;
300        ++BBI)
301     for (MachineBasicBlock::iterator MI = BBI->begin(), ME = BBI->end();
302          MI != ME; ++MI)
303       if (MI->isCall()) {
304         // Do not treat tail or sibling call sites as safe points.  This is
305         // legal since any arguments passed to the callee which live in the
306         // remnants of the callers frame will be owned and updated by the
307         // callee if required.
308         if (MI->isTerminator())
309           continue;
310         VisitCallPoint(MI);
311       }
312 }
313 
314 void GCMachineCodeAnalysis::FindStackOffsets(MachineFunction &MF) {
315   const TargetFrameLowering *TFI = TM->getSubtargetImpl()->getFrameLowering();
316   assert(TFI && "TargetRegisterInfo not available!");
317 
318   for (GCFunctionInfo::roots_iterator RI = FI->roots_begin();
319        RI != FI->roots_end();) {
320     // If the root references a dead object, no need to keep it.
321     if (MF.getFrameInfo()->isDeadObjectIndex(RI->Num)) {
322       RI = FI->removeStackRoot(RI);
323     } else {
324       RI->StackOffset = TFI->getFrameIndexOffset(MF, RI->Num);
325       ++RI;
326     }
327   }
328 }
329 
330 bool GCMachineCodeAnalysis::runOnMachineFunction(MachineFunction &MF) {
331   // Quick exit for functions that do not use GC.
332   if (!MF.getFunction()->hasGC())
333     return false;
334 
335   FI = &getAnalysis<GCModuleInfo>().getFunctionInfo(*MF.getFunction());
336   if (!FI->getStrategy().needsSafePoints())
337     return false;
338 
339   TM = &MF.getTarget();
340   MMI = &getAnalysis<MachineModuleInfo>();
341   TII = TM->getSubtargetImpl()->getInstrInfo();
342 
343   // Find the size of the stack frame.
344   FI->setFrameSize(MF.getFrameInfo()->getStackSize());
345 
346   // Find all safe points.
347   FindSafePoints(MF);
348 
349   // Find the stack offsets for all roots.
350   FindStackOffsets(MF);
351 
352   return false;
353 }
354