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/CodeGen/Passes.h"
16 #include "llvm/ADT/BitVector.h"
17 #include "llvm/ADT/Statistic.h"
18 #include "llvm/Analysis/CFG.h"
19 #include "llvm/Analysis/LibCallSemantics.h"
20 #include "llvm/Analysis/TargetTransformInfo.h"
21 #include "llvm/IR/Dominators.h"
22 #include "llvm/IR/Function.h"
23 #include "llvm/IR/Instructions.h"
24 #include "llvm/IR/Module.h"
25 #include "llvm/Pass.h"
26 #include "llvm/Target/TargetLowering.h"
27 #include "llvm/Target/TargetSubtargetInfo.h"
28 #include "llvm/Transforms/Utils/Local.h"
29 using namespace llvm;
30 
31 #define DEBUG_TYPE "dwarfehprepare"
32 
33 STATISTIC(NumResumesLowered, "Number of resume calls lowered");
34 
35 namespace {
36   class DwarfEHPrepare : public FunctionPass {
37     const TargetMachine *TM;
38 
39     // RewindFunction - _Unwind_Resume or the target equivalent.
40     Constant *RewindFunction;
41 
42     DominatorTree *DT;
43     const TargetLowering *TLI;
44 
45     bool InsertUnwindResumeCalls(Function &Fn);
46     Value *GetExceptionObject(ResumeInst *RI);
47     size_t
48     pruneUnreachableResumes(Function &Fn,
49                             SmallVectorImpl<ResumeInst *> &Resumes,
50                             SmallVectorImpl<LandingPadInst *> &CleanupLPads);
51 
52   public:
53     static char ID; // Pass identification, replacement for typeid.
54 
55     // INITIALIZE_TM_PASS requires a default constructor, but it isn't used in
56     // practice.
57     DwarfEHPrepare()
58         : FunctionPass(ID), TM(nullptr), RewindFunction(nullptr), DT(nullptr),
59           TLI(nullptr) {}
60 
61     DwarfEHPrepare(const TargetMachine *TM)
62         : FunctionPass(ID), TM(TM), RewindFunction(nullptr), DT(nullptr),
63           TLI(nullptr) {}
64 
65     bool runOnFunction(Function &Fn) override;
66 
67     bool doFinalization(Module &M) override {
68       RewindFunction = nullptr;
69       return false;
70     }
71 
72     void getAnalysisUsage(AnalysisUsage &AU) const override;
73 
74     const char *getPassName() const override {
75       return "Exception handling preparation";
76     }
77   };
78 } // end anonymous namespace
79 
80 char DwarfEHPrepare::ID = 0;
81 INITIALIZE_TM_PASS_BEGIN(DwarfEHPrepare, "dwarfehprepare",
82                          "Prepare DWARF exceptions", false, false)
83 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
84 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
85 INITIALIZE_TM_PASS_END(DwarfEHPrepare, "dwarfehprepare",
86                        "Prepare DWARF exceptions", false, false)
87 
88 FunctionPass *llvm::createDwarfEHPass(const TargetMachine *TM) {
89   return new DwarfEHPrepare(TM);
90 }
91 
92 void DwarfEHPrepare::getAnalysisUsage(AnalysisUsage &AU) const {
93   AU.addRequired<TargetTransformInfoWrapperPass>();
94   AU.addRequired<DominatorTreeWrapperPass>();
95 }
96 
97 /// GetExceptionObject - Return the exception object from the value passed into
98 /// the 'resume' instruction (typically an aggregate). Clean up any dead
99 /// instructions, including the 'resume' instruction.
100 Value *DwarfEHPrepare::GetExceptionObject(ResumeInst *RI) {
101   Value *V = RI->getOperand(0);
102   Value *ExnObj = nullptr;
103   InsertValueInst *SelIVI = dyn_cast<InsertValueInst>(V);
104   LoadInst *SelLoad = nullptr;
105   InsertValueInst *ExcIVI = nullptr;
106   bool EraseIVIs = false;
107 
108   if (SelIVI) {
109     if (SelIVI->getNumIndices() == 1 && *SelIVI->idx_begin() == 1) {
110       ExcIVI = dyn_cast<InsertValueInst>(SelIVI->getOperand(0));
111       if (ExcIVI && isa<UndefValue>(ExcIVI->getOperand(0)) &&
112           ExcIVI->getNumIndices() == 1 && *ExcIVI->idx_begin() == 0) {
113         ExnObj = ExcIVI->getOperand(1);
114         SelLoad = dyn_cast<LoadInst>(SelIVI->getOperand(1));
115         EraseIVIs = true;
116       }
117     }
118   }
119 
120   if (!ExnObj)
121     ExnObj = ExtractValueInst::Create(RI->getOperand(0), 0, "exn.obj", RI);
122 
123   RI->eraseFromParent();
124 
125   if (EraseIVIs) {
126     if (SelIVI->use_empty())
127       SelIVI->eraseFromParent();
128     if (ExcIVI->use_empty())
129       ExcIVI->eraseFromParent();
130     if (SelLoad && SelLoad->use_empty())
131       SelLoad->eraseFromParent();
132   }
133 
134   return ExnObj;
135 }
136 
137 /// Replace resumes that are not reachable from a cleanup landing pad with
138 /// unreachable and then simplify those blocks.
139 size_t DwarfEHPrepare::pruneUnreachableResumes(
140     Function &Fn, SmallVectorImpl<ResumeInst *> &Resumes,
141     SmallVectorImpl<LandingPadInst *> &CleanupLPads) {
142   BitVector ResumeReachable(Resumes.size());
143   size_t ResumeIndex = 0;
144   for (auto *RI : Resumes) {
145     for (auto *LP : CleanupLPads) {
146       if (isPotentiallyReachable(LP, RI, DT)) {
147         ResumeReachable.set(ResumeIndex);
148         break;
149       }
150     }
151     ++ResumeIndex;
152   }
153 
154   // If everything is reachable, there is no change.
155   if (ResumeReachable.all())
156     return Resumes.size();
157 
158   const TargetTransformInfo &TTI =
159       getAnalysis<TargetTransformInfoWrapperPass>().getTTI(Fn);
160   LLVMContext &Ctx = Fn.getContext();
161 
162   // Otherwise, insert unreachable instructions and call simplifycfg.
163   size_t ResumesLeft = 0;
164   for (size_t I = 0, E = Resumes.size(); I < E; ++I) {
165     ResumeInst *RI = Resumes[I];
166     if (ResumeReachable[I]) {
167       Resumes[ResumesLeft++] = RI;
168     } else {
169       BasicBlock *BB = RI->getParent();
170       new UnreachableInst(Ctx, RI);
171       RI->eraseFromParent();
172       SimplifyCFG(BB, TTI, 1);
173     }
174   }
175   Resumes.resize(ResumesLeft);
176   return ResumesLeft;
177 }
178 
179 /// InsertUnwindResumeCalls - Convert the ResumeInsts that are still present
180 /// into calls to the appropriate _Unwind_Resume function.
181 bool DwarfEHPrepare::InsertUnwindResumeCalls(Function &Fn) {
182   SmallVector<ResumeInst*, 16> Resumes;
183   SmallVector<LandingPadInst*, 16> CleanupLPads;
184   bool FoundLP = false;
185   for (BasicBlock &BB : Fn) {
186     if (auto *RI = dyn_cast<ResumeInst>(BB.getTerminator()))
187       Resumes.push_back(RI);
188     if (auto *LP = BB.getLandingPadInst()) {
189       if (LP->isCleanup())
190         CleanupLPads.push_back(LP);
191       // Check the personality on the first landingpad. Don't do anything if
192       // it's for MSVC.
193       if (!FoundLP) {
194         FoundLP = true;
195         EHPersonality Pers = classifyEHPersonality(LP->getPersonalityFn());
196         if (isMSVCEHPersonality(Pers))
197           return false;
198       }
199     }
200   }
201 
202   if (Resumes.empty())
203     return false;
204 
205   LLVMContext &Ctx = Fn.getContext();
206 
207   size_t ResumesLeft = pruneUnreachableResumes(Fn, Resumes, CleanupLPads);
208   if (ResumesLeft == 0)
209     return true; // We pruned them all.
210 
211   // Find the rewind function if we didn't already.
212   if (!RewindFunction) {
213     FunctionType *FTy = FunctionType::get(Type::getVoidTy(Ctx),
214                                           Type::getInt8PtrTy(Ctx), false);
215     const char *RewindName = TLI->getLibcallName(RTLIB::UNWIND_RESUME);
216     RewindFunction = Fn.getParent()->getOrInsertFunction(RewindName, FTy);
217   }
218 
219   // Create the basic block where the _Unwind_Resume call will live.
220   if (ResumesLeft == 1) {
221     // Instead of creating a new BB and PHI node, just append the call to
222     // _Unwind_Resume to the end of the single resume block.
223     ResumeInst *RI = Resumes.front();
224     BasicBlock *UnwindBB = RI->getParent();
225     Value *ExnObj = GetExceptionObject(RI);
226 
227     // Call the _Unwind_Resume function.
228     CallInst *CI = CallInst::Create(RewindFunction, ExnObj, "", UnwindBB);
229     CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
230 
231     // We never expect _Unwind_Resume to return.
232     new UnreachableInst(Ctx, UnwindBB);
233     return true;
234   }
235 
236   BasicBlock *UnwindBB = BasicBlock::Create(Ctx, "unwind_resume", &Fn);
237   PHINode *PN = PHINode::Create(Type::getInt8PtrTy(Ctx), ResumesLeft,
238                                 "exn.obj", UnwindBB);
239 
240   // Extract the exception object from the ResumeInst and add it to the PHI node
241   // that feeds the _Unwind_Resume call.
242   for (ResumeInst *RI : Resumes) {
243     BasicBlock *Parent = RI->getParent();
244     BranchInst::Create(UnwindBB, Parent);
245 
246     Value *ExnObj = GetExceptionObject(RI);
247     PN->addIncoming(ExnObj, Parent);
248 
249     ++NumResumesLowered;
250   }
251 
252   // Call the function.
253   CallInst *CI = CallInst::Create(RewindFunction, PN, "", UnwindBB);
254   CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
255 
256   // We never expect _Unwind_Resume to return.
257   new UnreachableInst(Ctx, UnwindBB);
258   return true;
259 }
260 
261 bool DwarfEHPrepare::runOnFunction(Function &Fn) {
262   assert(TM && "DWARF EH preparation requires a target machine");
263   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
264   TLI = TM->getSubtargetImpl(Fn)->getTargetLowering();
265   bool Changed = InsertUnwindResumeCalls(Fn);
266   DT = nullptr;
267   TLI = nullptr;
268   return Changed;
269 }
270