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/BitVector.h"
17 #include "llvm/ADT/Statistic.h"
18 #include "llvm/Analysis/CFG.h"
19 #include "llvm/Analysis/EHPersonalities.h"
20 #include "llvm/Analysis/TargetTransformInfo.h"
21 #include "llvm/IR/Dominators.h"
22 #include "llvm/IR/Function.h"
23 #include "llvm/IR/Instructions.h"
24 #include "llvm/IR/Module.h"
25 #include "llvm/Pass.h"
26 #include "llvm/Target/TargetLowering.h"
27 #include "llvm/Target/TargetSubtargetInfo.h"
28 #include "llvm/Transforms/Utils/Local.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 DominatorTree *DT;
43 const TargetLowering *TLI;
44
45 bool InsertUnwindResumeCalls(Function &Fn);
46 Value *GetExceptionObject(ResumeInst *RI);
47 size_t
48 pruneUnreachableResumes(Function &Fn,
49 SmallVectorImpl<ResumeInst *> &Resumes,
50 SmallVectorImpl<LandingPadInst *> &CleanupLPads);
51
52 public:
53 static char ID; // Pass identification, replacement for typeid.
54
55 // INITIALIZE_TM_PASS requires a default constructor, but it isn't used in
56 // practice.
DwarfEHPrepare()57 DwarfEHPrepare()
58 : FunctionPass(ID), TM(nullptr), RewindFunction(nullptr), DT(nullptr),
59 TLI(nullptr) {}
60
DwarfEHPrepare(const TargetMachine * TM)61 DwarfEHPrepare(const TargetMachine *TM)
62 : FunctionPass(ID), TM(TM), RewindFunction(nullptr), DT(nullptr),
63 TLI(nullptr) {}
64
65 bool runOnFunction(Function &Fn) override;
66
doFinalization(Module & M)67 bool doFinalization(Module &M) override {
68 RewindFunction = nullptr;
69 return false;
70 }
71
72 void getAnalysisUsage(AnalysisUsage &AU) const override;
73
getPassName() const74 const char *getPassName() const override {
75 return "Exception handling preparation";
76 }
77 };
78 } // end anonymous namespace
79
80 char DwarfEHPrepare::ID = 0;
81 INITIALIZE_TM_PASS_BEGIN(DwarfEHPrepare, "dwarfehprepare",
82 "Prepare DWARF exceptions", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)83 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
84 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
85 INITIALIZE_TM_PASS_END(DwarfEHPrepare, "dwarfehprepare",
86 "Prepare DWARF exceptions", false, false)
87
88 FunctionPass *llvm::createDwarfEHPass(const TargetMachine *TM) {
89 return new DwarfEHPrepare(TM);
90 }
91
getAnalysisUsage(AnalysisUsage & AU) const92 void DwarfEHPrepare::getAnalysisUsage(AnalysisUsage &AU) const {
93 AU.addRequired<TargetTransformInfoWrapperPass>();
94 AU.addRequired<DominatorTreeWrapperPass>();
95 }
96
97 /// GetExceptionObject - Return the exception object from the value passed into
98 /// the 'resume' instruction (typically an aggregate). Clean up any dead
99 /// instructions, including the 'resume' instruction.
GetExceptionObject(ResumeInst * RI)100 Value *DwarfEHPrepare::GetExceptionObject(ResumeInst *RI) {
101 Value *V = RI->getOperand(0);
102 Value *ExnObj = nullptr;
103 InsertValueInst *SelIVI = dyn_cast<InsertValueInst>(V);
104 LoadInst *SelLoad = nullptr;
105 InsertValueInst *ExcIVI = nullptr;
106 bool EraseIVIs = false;
107
108 if (SelIVI) {
109 if (SelIVI->getNumIndices() == 1 && *SelIVI->idx_begin() == 1) {
110 ExcIVI = dyn_cast<InsertValueInst>(SelIVI->getOperand(0));
111 if (ExcIVI && isa<UndefValue>(ExcIVI->getOperand(0)) &&
112 ExcIVI->getNumIndices() == 1 && *ExcIVI->idx_begin() == 0) {
113 ExnObj = ExcIVI->getOperand(1);
114 SelLoad = dyn_cast<LoadInst>(SelIVI->getOperand(1));
115 EraseIVIs = true;
116 }
117 }
118 }
119
120 if (!ExnObj)
121 ExnObj = ExtractValueInst::Create(RI->getOperand(0), 0, "exn.obj", RI);
122
123 RI->eraseFromParent();
124
125 if (EraseIVIs) {
126 if (SelIVI->use_empty())
127 SelIVI->eraseFromParent();
128 if (ExcIVI->use_empty())
129 ExcIVI->eraseFromParent();
130 if (SelLoad && SelLoad->use_empty())
131 SelLoad->eraseFromParent();
132 }
133
134 return ExnObj;
135 }
136
137 /// Replace resumes that are not reachable from a cleanup landing pad with
138 /// unreachable and then simplify those blocks.
pruneUnreachableResumes(Function & Fn,SmallVectorImpl<ResumeInst * > & Resumes,SmallVectorImpl<LandingPadInst * > & CleanupLPads)139 size_t DwarfEHPrepare::pruneUnreachableResumes(
140 Function &Fn, SmallVectorImpl<ResumeInst *> &Resumes,
141 SmallVectorImpl<LandingPadInst *> &CleanupLPads) {
142 BitVector ResumeReachable(Resumes.size());
143 size_t ResumeIndex = 0;
144 for (auto *RI : Resumes) {
145 for (auto *LP : CleanupLPads) {
146 if (isPotentiallyReachable(LP, RI, DT)) {
147 ResumeReachable.set(ResumeIndex);
148 break;
149 }
150 }
151 ++ResumeIndex;
152 }
153
154 // If everything is reachable, there is no change.
155 if (ResumeReachable.all())
156 return Resumes.size();
157
158 const TargetTransformInfo &TTI =
159 getAnalysis<TargetTransformInfoWrapperPass>().getTTI(Fn);
160 LLVMContext &Ctx = Fn.getContext();
161
162 // Otherwise, insert unreachable instructions and call simplifycfg.
163 size_t ResumesLeft = 0;
164 for (size_t I = 0, E = Resumes.size(); I < E; ++I) {
165 ResumeInst *RI = Resumes[I];
166 if (ResumeReachable[I]) {
167 Resumes[ResumesLeft++] = RI;
168 } else {
169 BasicBlock *BB = RI->getParent();
170 new UnreachableInst(Ctx, RI);
171 RI->eraseFromParent();
172 SimplifyCFG(BB, TTI, 1);
173 }
174 }
175 Resumes.resize(ResumesLeft);
176 return ResumesLeft;
177 }
178
179 /// InsertUnwindResumeCalls - Convert the ResumeInsts that are still present
180 /// into calls to the appropriate _Unwind_Resume function.
InsertUnwindResumeCalls(Function & Fn)181 bool DwarfEHPrepare::InsertUnwindResumeCalls(Function &Fn) {
182 SmallVector<ResumeInst*, 16> Resumes;
183 SmallVector<LandingPadInst*, 16> CleanupLPads;
184 for (BasicBlock &BB : Fn) {
185 if (auto *RI = dyn_cast<ResumeInst>(BB.getTerminator()))
186 Resumes.push_back(RI);
187 if (auto *LP = BB.getLandingPadInst())
188 if (LP->isCleanup())
189 CleanupLPads.push_back(LP);
190 }
191
192 if (Resumes.empty())
193 return false;
194
195 // Check the personality, don't do anything if it's funclet-based.
196 EHPersonality Pers = classifyEHPersonality(Fn.getPersonalityFn());
197 if (isFuncletEHPersonality(Pers))
198 return false;
199
200 LLVMContext &Ctx = Fn.getContext();
201
202 size_t ResumesLeft = pruneUnreachableResumes(Fn, Resumes, CleanupLPads);
203 if (ResumesLeft == 0)
204 return true; // We pruned them all.
205
206 // Find the rewind function if we didn't already.
207 if (!RewindFunction) {
208 FunctionType *FTy = FunctionType::get(Type::getVoidTy(Ctx),
209 Type::getInt8PtrTy(Ctx), false);
210 const char *RewindName = TLI->getLibcallName(RTLIB::UNWIND_RESUME);
211 RewindFunction = Fn.getParent()->getOrInsertFunction(RewindName, FTy);
212 }
213
214 // Create the basic block where the _Unwind_Resume call will live.
215 if (ResumesLeft == 1) {
216 // Instead of creating a new BB and PHI node, just append the call to
217 // _Unwind_Resume to the end of the single resume block.
218 ResumeInst *RI = Resumes.front();
219 BasicBlock *UnwindBB = RI->getParent();
220 Value *ExnObj = GetExceptionObject(RI);
221
222 // Call the _Unwind_Resume function.
223 CallInst *CI = CallInst::Create(RewindFunction, ExnObj, "", UnwindBB);
224 CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
225
226 // We never expect _Unwind_Resume to return.
227 new UnreachableInst(Ctx, UnwindBB);
228 return true;
229 }
230
231 BasicBlock *UnwindBB = BasicBlock::Create(Ctx, "unwind_resume", &Fn);
232 PHINode *PN = PHINode::Create(Type::getInt8PtrTy(Ctx), ResumesLeft,
233 "exn.obj", UnwindBB);
234
235 // Extract the exception object from the ResumeInst and add it to the PHI node
236 // that feeds the _Unwind_Resume call.
237 for (ResumeInst *RI : Resumes) {
238 BasicBlock *Parent = RI->getParent();
239 BranchInst::Create(UnwindBB, Parent);
240
241 Value *ExnObj = GetExceptionObject(RI);
242 PN->addIncoming(ExnObj, Parent);
243
244 ++NumResumesLowered;
245 }
246
247 // Call the function.
248 CallInst *CI = CallInst::Create(RewindFunction, PN, "", UnwindBB);
249 CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
250
251 // We never expect _Unwind_Resume to return.
252 new UnreachableInst(Ctx, UnwindBB);
253 return true;
254 }
255
runOnFunction(Function & Fn)256 bool DwarfEHPrepare::runOnFunction(Function &Fn) {
257 assert(TM && "DWARF EH preparation requires a target machine");
258 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
259 TLI = TM->getSubtargetImpl(Fn)->getTargetLowering();
260 bool Changed = InsertUnwindResumeCalls(Fn);
261 DT = nullptr;
262 TLI = nullptr;
263 return Changed;
264 }
265