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/CallSite.h"
18 #include "llvm/IR/Dominators.h"
19 #include "llvm/IR/Function.h"
20 #include "llvm/IR/Instructions.h"
21 #include "llvm/IR/IntrinsicInst.h"
22 #include "llvm/IR/Module.h"
23 #include "llvm/MC/MCAsmInfo.h"
24 #include "llvm/Pass.h"
25 #include "llvm/Target/TargetLowering.h"
26 #include "llvm/Target/TargetSubtargetInfo.h"
27 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
28 #include "llvm/Transforms/Utils/SSAUpdater.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     bool InsertUnwindResumeCalls(Function &Fn);
43     Value *GetExceptionObject(ResumeInst *RI);
44 
45   public:
46     static char ID; // Pass identification, replacement for typeid.
47     DwarfEHPrepare(const TargetMachine *TM)
48         : FunctionPass(ID), TM(TM), RewindFunction(nullptr) {
49       initializeDominatorTreeWrapperPassPass(*PassRegistry::getPassRegistry());
50     }
51 
52     bool runOnFunction(Function &Fn) override;
53 
54     bool doFinalization(Module &M) override {
55       RewindFunction = nullptr;
56       return false;
57     }
58 
59     void getAnalysisUsage(AnalysisUsage &AU) const override { }
60 
61     const char *getPassName() const override {
62       return "Exception handling preparation";
63     }
64   };
65 } // end anonymous namespace
66 
67 char DwarfEHPrepare::ID = 0;
68 
69 FunctionPass *llvm::createDwarfEHPass(const TargetMachine *TM) {
70   return new DwarfEHPrepare(TM);
71 }
72 
73 /// GetExceptionObject - Return the exception object from the value passed into
74 /// the 'resume' instruction (typically an aggregate). Clean up any dead
75 /// instructions, including the 'resume' instruction.
76 Value *DwarfEHPrepare::GetExceptionObject(ResumeInst *RI) {
77   Value *V = RI->getOperand(0);
78   Value *ExnObj = nullptr;
79   InsertValueInst *SelIVI = dyn_cast<InsertValueInst>(V);
80   LoadInst *SelLoad = nullptr;
81   InsertValueInst *ExcIVI = nullptr;
82   bool EraseIVIs = false;
83 
84   if (SelIVI) {
85     if (SelIVI->getNumIndices() == 1 && *SelIVI->idx_begin() == 1) {
86       ExcIVI = dyn_cast<InsertValueInst>(SelIVI->getOperand(0));
87       if (ExcIVI && isa<UndefValue>(ExcIVI->getOperand(0)) &&
88           ExcIVI->getNumIndices() == 1 && *ExcIVI->idx_begin() == 0) {
89         ExnObj = ExcIVI->getOperand(1);
90         SelLoad = dyn_cast<LoadInst>(SelIVI->getOperand(1));
91         EraseIVIs = true;
92       }
93     }
94   }
95 
96   if (!ExnObj)
97     ExnObj = ExtractValueInst::Create(RI->getOperand(0), 0, "exn.obj", RI);
98 
99   RI->eraseFromParent();
100 
101   if (EraseIVIs) {
102     if (SelIVI->getNumUses() == 0)
103       SelIVI->eraseFromParent();
104     if (ExcIVI->getNumUses() == 0)
105       ExcIVI->eraseFromParent();
106     if (SelLoad && SelLoad->getNumUses() == 0)
107       SelLoad->eraseFromParent();
108   }
109 
110   return ExnObj;
111 }
112 
113 /// InsertUnwindResumeCalls - Convert the ResumeInsts that are still present
114 /// into calls to the appropriate _Unwind_Resume function.
115 bool DwarfEHPrepare::InsertUnwindResumeCalls(Function &Fn) {
116   SmallVector<ResumeInst*, 16> Resumes;
117   for (Function::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I) {
118     TerminatorInst *TI = I->getTerminator();
119     if (ResumeInst *RI = dyn_cast<ResumeInst>(TI))
120       Resumes.push_back(RI);
121   }
122 
123   if (Resumes.empty())
124     return false;
125 
126   // Find the rewind function if we didn't already.
127   const TargetLowering *TLI = TM->getSubtargetImpl()->getTargetLowering();
128   if (!RewindFunction) {
129     LLVMContext &Ctx = Resumes[0]->getContext();
130     FunctionType *FTy = FunctionType::get(Type::getVoidTy(Ctx),
131                                           Type::getInt8PtrTy(Ctx), false);
132     const char *RewindName = TLI->getLibcallName(RTLIB::UNWIND_RESUME);
133     RewindFunction = Fn.getParent()->getOrInsertFunction(RewindName, FTy);
134   }
135 
136   // Create the basic block where the _Unwind_Resume call will live.
137   LLVMContext &Ctx = Fn.getContext();
138   unsigned ResumesSize = Resumes.size();
139 
140   if (ResumesSize == 1) {
141     // Instead of creating a new BB and PHI node, just append the call to
142     // _Unwind_Resume to the end of the single resume block.
143     ResumeInst *RI = Resumes.front();
144     BasicBlock *UnwindBB = RI->getParent();
145     Value *ExnObj = GetExceptionObject(RI);
146 
147     // Call the _Unwind_Resume function.
148     CallInst *CI = CallInst::Create(RewindFunction, ExnObj, "", UnwindBB);
149     CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
150 
151     // We never expect _Unwind_Resume to return.
152     new UnreachableInst(Ctx, UnwindBB);
153     return true;
154   }
155 
156   BasicBlock *UnwindBB = BasicBlock::Create(Ctx, "unwind_resume", &Fn);
157   PHINode *PN = PHINode::Create(Type::getInt8PtrTy(Ctx), ResumesSize,
158                                 "exn.obj", UnwindBB);
159 
160   // Extract the exception object from the ResumeInst and add it to the PHI node
161   // that feeds the _Unwind_Resume call.
162   for (SmallVectorImpl<ResumeInst*>::iterator
163          I = Resumes.begin(), E = Resumes.end(); I != E; ++I) {
164     ResumeInst *RI = *I;
165     BasicBlock *Parent = RI->getParent();
166     BranchInst::Create(UnwindBB, Parent);
167 
168     Value *ExnObj = GetExceptionObject(RI);
169     PN->addIncoming(ExnObj, Parent);
170 
171     ++NumResumesLowered;
172   }
173 
174   // Call the function.
175   CallInst *CI = CallInst::Create(RewindFunction, PN, "", UnwindBB);
176   CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
177 
178   // We never expect _Unwind_Resume to return.
179   new UnreachableInst(Ctx, UnwindBB);
180   return true;
181 }
182 
183 bool DwarfEHPrepare::runOnFunction(Function &Fn) {
184   bool Changed = InsertUnwindResumeCalls(Fn);
185   return Changed;
186 }
187