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/Statistic.h"
17 #include "llvm/IR/Function.h"
18 #include "llvm/IR/Instructions.h"
19 #include "llvm/IR/Module.h"
20 #include "llvm/Pass.h"
21 #include "llvm/Target/TargetLowering.h"
22 #include "llvm/Target/TargetSubtargetInfo.h"
23 using namespace llvm;
24 
25 #define DEBUG_TYPE "dwarfehprepare"
26 
27 STATISTIC(NumResumesLowered, "Number of resume calls lowered");
28 
29 namespace {
30   class DwarfEHPrepare : public FunctionPass {
31     const TargetMachine *TM;
32 
33     // RewindFunction - _Unwind_Resume or the target equivalent.
34     Constant *RewindFunction;
35 
36     bool InsertUnwindResumeCalls(Function &Fn);
37     Value *GetExceptionObject(ResumeInst *RI);
38 
39   public:
40     static char ID; // Pass identification, replacement for typeid.
41 
42     // INITIALIZE_TM_PASS requires a default constructor, but it isn't used in
43     // practice.
44     DwarfEHPrepare() : FunctionPass(ID), TM(nullptr), RewindFunction(nullptr) {}
45 
46     DwarfEHPrepare(const TargetMachine *TM)
47         : FunctionPass(ID), TM(TM), RewindFunction(nullptr) {}
48 
49     bool runOnFunction(Function &Fn) override;
50 
51     bool doFinalization(Module &M) override {
52       RewindFunction = nullptr;
53       return false;
54     }
55 
56     const char *getPassName() const override {
57       return "Exception handling preparation";
58     }
59   };
60 } // end anonymous namespace
61 
62 char DwarfEHPrepare::ID = 0;
63 INITIALIZE_TM_PASS(DwarfEHPrepare, "dwarfehprepare", "Prepare DWARF exceptions",
64                    false, false)
65 
66 FunctionPass *llvm::createDwarfEHPass(const TargetMachine *TM) {
67   return new DwarfEHPrepare(TM);
68 }
69 
70 /// GetExceptionObject - Return the exception object from the value passed into
71 /// the 'resume' instruction (typically an aggregate). Clean up any dead
72 /// instructions, including the 'resume' instruction.
73 Value *DwarfEHPrepare::GetExceptionObject(ResumeInst *RI) {
74   Value *V = RI->getOperand(0);
75   Value *ExnObj = nullptr;
76   InsertValueInst *SelIVI = dyn_cast<InsertValueInst>(V);
77   LoadInst *SelLoad = nullptr;
78   InsertValueInst *ExcIVI = nullptr;
79   bool EraseIVIs = false;
80 
81   if (SelIVI) {
82     if (SelIVI->getNumIndices() == 1 && *SelIVI->idx_begin() == 1) {
83       ExcIVI = dyn_cast<InsertValueInst>(SelIVI->getOperand(0));
84       if (ExcIVI && isa<UndefValue>(ExcIVI->getOperand(0)) &&
85           ExcIVI->getNumIndices() == 1 && *ExcIVI->idx_begin() == 0) {
86         ExnObj = ExcIVI->getOperand(1);
87         SelLoad = dyn_cast<LoadInst>(SelIVI->getOperand(1));
88         EraseIVIs = true;
89       }
90     }
91   }
92 
93   if (!ExnObj)
94     ExnObj = ExtractValueInst::Create(RI->getOperand(0), 0, "exn.obj", RI);
95 
96   RI->eraseFromParent();
97 
98   if (EraseIVIs) {
99     if (SelIVI->use_empty())
100       SelIVI->eraseFromParent();
101     if (ExcIVI->use_empty())
102       ExcIVI->eraseFromParent();
103     if (SelLoad && SelLoad->use_empty())
104       SelLoad->eraseFromParent();
105   }
106 
107   return ExnObj;
108 }
109 
110 /// InsertUnwindResumeCalls - Convert the ResumeInsts that are still present
111 /// into calls to the appropriate _Unwind_Resume function.
112 bool DwarfEHPrepare::InsertUnwindResumeCalls(Function &Fn) {
113   SmallVector<ResumeInst*, 16> Resumes;
114   for (BasicBlock &BB : Fn) {
115     if (auto *RI = dyn_cast<ResumeInst>(BB.getTerminator()))
116       Resumes.push_back(RI);
117   }
118 
119   if (Resumes.empty())
120     return false;
121 
122   // Find the rewind function if we didn't already.
123   const TargetLowering *TLI = TM->getSubtargetImpl(Fn)->getTargetLowering();
124   LLVMContext &Ctx = Fn.getContext();
125   if (!RewindFunction) {
126     FunctionType *FTy = FunctionType::get(Type::getVoidTy(Ctx),
127                                           Type::getInt8PtrTy(Ctx), false);
128     const char *RewindName = TLI->getLibcallName(RTLIB::UNWIND_RESUME);
129     RewindFunction = Fn.getParent()->getOrInsertFunction(RewindName, FTy);
130   }
131 
132   // Create the basic block where the _Unwind_Resume call will live.
133   unsigned ResumesSize = Resumes.size();
134 
135   if (ResumesSize == 1) {
136     // Instead of creating a new BB and PHI node, just append the call to
137     // _Unwind_Resume to the end of the single resume block.
138     ResumeInst *RI = Resumes.front();
139     BasicBlock *UnwindBB = RI->getParent();
140     Value *ExnObj = GetExceptionObject(RI);
141 
142     // Call the _Unwind_Resume function.
143     CallInst *CI = CallInst::Create(RewindFunction, ExnObj, "", UnwindBB);
144     CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
145 
146     // We never expect _Unwind_Resume to return.
147     new UnreachableInst(Ctx, UnwindBB);
148     return true;
149   }
150 
151   BasicBlock *UnwindBB = BasicBlock::Create(Ctx, "unwind_resume", &Fn);
152   PHINode *PN = PHINode::Create(Type::getInt8PtrTy(Ctx), ResumesSize,
153                                 "exn.obj", UnwindBB);
154 
155   // Extract the exception object from the ResumeInst and add it to the PHI node
156   // that feeds the _Unwind_Resume call.
157   for (ResumeInst *RI : Resumes) {
158     BasicBlock *Parent = RI->getParent();
159     BranchInst::Create(UnwindBB, Parent);
160 
161     Value *ExnObj = GetExceptionObject(RI);
162     PN->addIncoming(ExnObj, Parent);
163 
164     ++NumResumesLowered;
165   }
166 
167   // Call the function.
168   CallInst *CI = CallInst::Create(RewindFunction, PN, "", UnwindBB);
169   CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
170 
171   // We never expect _Unwind_Resume to return.
172   new UnreachableInst(Ctx, UnwindBB);
173   return true;
174 }
175 
176 bool DwarfEHPrepare::runOnFunction(Function &Fn) {
177   assert(TM && "DWARF EH preparation requires a target machine");
178   bool Changed = InsertUnwindResumeCalls(Fn);
179   return Changed;
180 }
181