1 //===-- DwarfEHPrepare - Prepare exception handling for code generation ---===//
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 pass mulches exception handling code into a form adapted to code
11 // generation. Required if using dwarf exception handling.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #include "llvm/ADT/BitVector.h"
16 #include "llvm/ADT/Statistic.h"
17 #include "llvm/Analysis/CFG.h"
18 #include "llvm/Analysis/EHPersonalities.h"
19 #include "llvm/Analysis/TargetTransformInfo.h"
20 #include "llvm/CodeGen/Passes.h"
21 #include "llvm/CodeGen/TargetPassConfig.h"
22 #include "llvm/IR/Dominators.h"
23 #include "llvm/IR/Function.h"
24 #include "llvm/IR/Instructions.h"
25 #include "llvm/IR/Module.h"
26 #include "llvm/Pass.h"
27 #include "llvm/Target/TargetLowering.h"
28 #include "llvm/Target/TargetSubtargetInfo.h"
29 #include "llvm/Transforms/Utils/Local.h"
30 using namespace llvm;
31 
32 #define DEBUG_TYPE "dwarfehprepare"
33 
34 STATISTIC(NumResumesLowered, "Number of resume calls lowered");
35 
36 namespace {
37   class DwarfEHPrepare : public FunctionPass {
38     // RewindFunction - _Unwind_Resume or the target equivalent.
39     Constant *RewindFunction;
40 
41     DominatorTree *DT;
42     const TargetLowering *TLI;
43 
44     bool InsertUnwindResumeCalls(Function &Fn);
45     Value *GetExceptionObject(ResumeInst *RI);
46     size_t
47     pruneUnreachableResumes(Function &Fn,
48                             SmallVectorImpl<ResumeInst *> &Resumes,
49                             SmallVectorImpl<LandingPadInst *> &CleanupLPads);
50 
51   public:
52     static char ID; // Pass identification, replacement for typeid.
53 
54     DwarfEHPrepare()
55         : FunctionPass(ID), RewindFunction(nullptr), DT(nullptr), TLI(nullptr) {
56     }
57 
58     bool runOnFunction(Function &Fn) override;
59 
60     bool doFinalization(Module &M) override {
61       RewindFunction = nullptr;
62       return false;
63     }
64 
65     void getAnalysisUsage(AnalysisUsage &AU) const override;
66 
67     StringRef getPassName() const override {
68       return "Exception handling preparation";
69     }
70   };
71 } // end anonymous namespace
72 
73 char DwarfEHPrepare::ID = 0;
74 INITIALIZE_PASS_BEGIN(DwarfEHPrepare, DEBUG_TYPE,
75                       "Prepare DWARF exceptions", false, false)
76 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
77 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
78 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
79 INITIALIZE_PASS_END(DwarfEHPrepare, DEBUG_TYPE,
80                     "Prepare DWARF exceptions", false, false)
81 
82 FunctionPass *llvm::createDwarfEHPass() { return new DwarfEHPrepare(); }
83 
84 void DwarfEHPrepare::getAnalysisUsage(AnalysisUsage &AU) const {
85   AU.addRequired<TargetPassConfig>();
86   AU.addRequired<TargetTransformInfoWrapperPass>();
87   AU.addRequired<DominatorTreeWrapperPass>();
88 }
89 
90 /// GetExceptionObject - Return the exception object from the value passed into
91 /// the 'resume' instruction (typically an aggregate). Clean up any dead
92 /// instructions, including the 'resume' instruction.
93 Value *DwarfEHPrepare::GetExceptionObject(ResumeInst *RI) {
94   Value *V = RI->getOperand(0);
95   Value *ExnObj = nullptr;
96   InsertValueInst *SelIVI = dyn_cast<InsertValueInst>(V);
97   LoadInst *SelLoad = nullptr;
98   InsertValueInst *ExcIVI = nullptr;
99   bool EraseIVIs = false;
100 
101   if (SelIVI) {
102     if (SelIVI->getNumIndices() == 1 && *SelIVI->idx_begin() == 1) {
103       ExcIVI = dyn_cast<InsertValueInst>(SelIVI->getOperand(0));
104       if (ExcIVI && isa<UndefValue>(ExcIVI->getOperand(0)) &&
105           ExcIVI->getNumIndices() == 1 && *ExcIVI->idx_begin() == 0) {
106         ExnObj = ExcIVI->getOperand(1);
107         SelLoad = dyn_cast<LoadInst>(SelIVI->getOperand(1));
108         EraseIVIs = true;
109       }
110     }
111   }
112 
113   if (!ExnObj)
114     ExnObj = ExtractValueInst::Create(RI->getOperand(0), 0, "exn.obj", RI);
115 
116   RI->eraseFromParent();
117 
118   if (EraseIVIs) {
119     if (SelIVI->use_empty())
120       SelIVI->eraseFromParent();
121     if (ExcIVI->use_empty())
122       ExcIVI->eraseFromParent();
123     if (SelLoad && SelLoad->use_empty())
124       SelLoad->eraseFromParent();
125   }
126 
127   return ExnObj;
128 }
129 
130 /// Replace resumes that are not reachable from a cleanup landing pad with
131 /// unreachable and then simplify those blocks.
132 size_t DwarfEHPrepare::pruneUnreachableResumes(
133     Function &Fn, SmallVectorImpl<ResumeInst *> &Resumes,
134     SmallVectorImpl<LandingPadInst *> &CleanupLPads) {
135   BitVector ResumeReachable(Resumes.size());
136   size_t ResumeIndex = 0;
137   for (auto *RI : Resumes) {
138     for (auto *LP : CleanupLPads) {
139       if (isPotentiallyReachable(LP, RI, DT)) {
140         ResumeReachable.set(ResumeIndex);
141         break;
142       }
143     }
144     ++ResumeIndex;
145   }
146 
147   // If everything is reachable, there is no change.
148   if (ResumeReachable.all())
149     return Resumes.size();
150 
151   const TargetTransformInfo &TTI =
152       getAnalysis<TargetTransformInfoWrapperPass>().getTTI(Fn);
153   LLVMContext &Ctx = Fn.getContext();
154 
155   // Otherwise, insert unreachable instructions and call simplifycfg.
156   size_t ResumesLeft = 0;
157   for (size_t I = 0, E = Resumes.size(); I < E; ++I) {
158     ResumeInst *RI = Resumes[I];
159     if (ResumeReachable[I]) {
160       Resumes[ResumesLeft++] = RI;
161     } else {
162       BasicBlock *BB = RI->getParent();
163       new UnreachableInst(Ctx, RI);
164       RI->eraseFromParent();
165       SimplifyCFG(BB, TTI, 1);
166     }
167   }
168   Resumes.resize(ResumesLeft);
169   return ResumesLeft;
170 }
171 
172 /// InsertUnwindResumeCalls - Convert the ResumeInsts that are still present
173 /// into calls to the appropriate _Unwind_Resume function.
174 bool DwarfEHPrepare::InsertUnwindResumeCalls(Function &Fn) {
175   SmallVector<ResumeInst*, 16> Resumes;
176   SmallVector<LandingPadInst*, 16> CleanupLPads;
177   for (BasicBlock &BB : Fn) {
178     if (auto *RI = dyn_cast<ResumeInst>(BB.getTerminator()))
179       Resumes.push_back(RI);
180     if (auto *LP = BB.getLandingPadInst())
181       if (LP->isCleanup())
182         CleanupLPads.push_back(LP);
183   }
184 
185   if (Resumes.empty())
186     return false;
187 
188   // Check the personality, don't do anything if it's funclet-based.
189   EHPersonality Pers = classifyEHPersonality(Fn.getPersonalityFn());
190   if (isFuncletEHPersonality(Pers))
191     return false;
192 
193   LLVMContext &Ctx = Fn.getContext();
194 
195   size_t ResumesLeft = pruneUnreachableResumes(Fn, Resumes, CleanupLPads);
196   if (ResumesLeft == 0)
197     return true; // We pruned them all.
198 
199   // Find the rewind function if we didn't already.
200   if (!RewindFunction) {
201     FunctionType *FTy = FunctionType::get(Type::getVoidTy(Ctx),
202                                           Type::getInt8PtrTy(Ctx), false);
203     const char *RewindName = TLI->getLibcallName(RTLIB::UNWIND_RESUME);
204     RewindFunction = Fn.getParent()->getOrInsertFunction(RewindName, FTy);
205   }
206 
207   // Create the basic block where the _Unwind_Resume call will live.
208   if (ResumesLeft == 1) {
209     // Instead of creating a new BB and PHI node, just append the call to
210     // _Unwind_Resume to the end of the single resume block.
211     ResumeInst *RI = Resumes.front();
212     BasicBlock *UnwindBB = RI->getParent();
213     Value *ExnObj = GetExceptionObject(RI);
214 
215     // Call the _Unwind_Resume function.
216     CallInst *CI = CallInst::Create(RewindFunction, ExnObj, "", UnwindBB);
217     CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
218 
219     // We never expect _Unwind_Resume to return.
220     new UnreachableInst(Ctx, UnwindBB);
221     return true;
222   }
223 
224   BasicBlock *UnwindBB = BasicBlock::Create(Ctx, "unwind_resume", &Fn);
225   PHINode *PN = PHINode::Create(Type::getInt8PtrTy(Ctx), ResumesLeft,
226                                 "exn.obj", UnwindBB);
227 
228   // Extract the exception object from the ResumeInst and add it to the PHI node
229   // that feeds the _Unwind_Resume call.
230   for (ResumeInst *RI : Resumes) {
231     BasicBlock *Parent = RI->getParent();
232     BranchInst::Create(UnwindBB, Parent);
233 
234     Value *ExnObj = GetExceptionObject(RI);
235     PN->addIncoming(ExnObj, Parent);
236 
237     ++NumResumesLowered;
238   }
239 
240   // Call the function.
241   CallInst *CI = CallInst::Create(RewindFunction, PN, "", UnwindBB);
242   CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
243 
244   // We never expect _Unwind_Resume to return.
245   new UnreachableInst(Ctx, UnwindBB);
246   return true;
247 }
248 
249 bool DwarfEHPrepare::runOnFunction(Function &Fn) {
250   const TargetMachine &TM =
251       getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
252   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
253   TLI = TM.getSubtargetImpl(Fn)->getTargetLowering();
254   bool Changed = InsertUnwindResumeCalls(Fn);
255   DT = nullptr;
256   TLI = nullptr;
257   return Changed;
258 }
259