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