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     void getAnalysisUsage(AnalysisUsage &AU) const override { }
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 
64 FunctionPass *llvm::createDwarfEHPass(const TargetMachine *TM) {
65   return new DwarfEHPrepare(TM);
66 }
67 
68 /// GetExceptionObject - Return the exception object from the value passed into
69 /// the 'resume' instruction (typically an aggregate). Clean up any dead
70 /// instructions, including the 'resume' instruction.
71 Value *DwarfEHPrepare::GetExceptionObject(ResumeInst *RI) {
72   Value *V = RI->getOperand(0);
73   Value *ExnObj = nullptr;
74   InsertValueInst *SelIVI = dyn_cast<InsertValueInst>(V);
75   LoadInst *SelLoad = nullptr;
76   InsertValueInst *ExcIVI = nullptr;
77   bool EraseIVIs = false;
78 
79   if (SelIVI) {
80     if (SelIVI->getNumIndices() == 1 && *SelIVI->idx_begin() == 1) {
81       ExcIVI = dyn_cast<InsertValueInst>(SelIVI->getOperand(0));
82       if (ExcIVI && isa<UndefValue>(ExcIVI->getOperand(0)) &&
83           ExcIVI->getNumIndices() == 1 && *ExcIVI->idx_begin() == 0) {
84         ExnObj = ExcIVI->getOperand(1);
85         SelLoad = dyn_cast<LoadInst>(SelIVI->getOperand(1));
86         EraseIVIs = true;
87       }
88     }
89   }
90 
91   if (!ExnObj)
92     ExnObj = ExtractValueInst::Create(RI->getOperand(0), 0, "exn.obj", RI);
93 
94   RI->eraseFromParent();
95 
96   if (EraseIVIs) {
97     if (SelIVI->getNumUses() == 0)
98       SelIVI->eraseFromParent();
99     if (ExcIVI->getNumUses() == 0)
100       ExcIVI->eraseFromParent();
101     if (SelLoad && SelLoad->getNumUses() == 0)
102       SelLoad->eraseFromParent();
103   }
104 
105   return ExnObj;
106 }
107 
108 /// InsertUnwindResumeCalls - Convert the ResumeInsts that are still present
109 /// into calls to the appropriate _Unwind_Resume function.
110 bool DwarfEHPrepare::InsertUnwindResumeCalls(Function &Fn) {
111   SmallVector<ResumeInst*, 16> Resumes;
112   for (Function::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I) {
113     TerminatorInst *TI = I->getTerminator();
114     if (ResumeInst *RI = dyn_cast<ResumeInst>(TI))
115       Resumes.push_back(RI);
116   }
117 
118   if (Resumes.empty())
119     return false;
120 
121   // Find the rewind function if we didn't already.
122   const TargetLowering *TLI = TM->getSubtargetImpl()->getTargetLowering();
123   if (!RewindFunction) {
124     LLVMContext &Ctx = Resumes[0]->getContext();
125     FunctionType *FTy = FunctionType::get(Type::getVoidTy(Ctx),
126                                           Type::getInt8PtrTy(Ctx), false);
127     const char *RewindName = TLI->getLibcallName(RTLIB::UNWIND_RESUME);
128     RewindFunction = Fn.getParent()->getOrInsertFunction(RewindName, FTy);
129   }
130 
131   // Create the basic block where the _Unwind_Resume call will live.
132   LLVMContext &Ctx = Fn.getContext();
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 (SmallVectorImpl<ResumeInst*>::iterator
158          I = Resumes.begin(), E = Resumes.end(); I != E; ++I) {
159     ResumeInst *RI = *I;
160     BasicBlock *Parent = RI->getParent();
161     BranchInst::Create(UnwindBB, Parent);
162 
163     Value *ExnObj = GetExceptionObject(RI);
164     PN->addIncoming(ExnObj, Parent);
165 
166     ++NumResumesLowered;
167   }
168 
169   // Call the function.
170   CallInst *CI = CallInst::Create(RewindFunction, PN, "", UnwindBB);
171   CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
172 
173   // We never expect _Unwind_Resume to return.
174   new UnreachableInst(Ctx, UnwindBB);
175   return true;
176 }
177 
178 bool DwarfEHPrepare::runOnFunction(Function &Fn) {
179   bool Changed = InsertUnwindResumeCalls(Fn);
180   return Changed;
181 }
182