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