xref: /freebsd/contrib/llvm-project/llvm/lib/CodeGen/DwarfEHPrepare.cpp (revision 62cfcf62f627e5093fb37026a6d8c98e4d2ef04c) 
1 //===- DwarfEHPrepare - Prepare exception handling for code generation ----===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This pass mulches exception handling code into a form adapted to code
10 // generation. Required if using dwarf exception handling.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/ADT/BitVector.h"
15 #include "llvm/ADT/SmallVector.h"
16 #include "llvm/ADT/Statistic.h"
17 #include "llvm/Analysis/CFG.h"
18 #include "llvm/Analysis/EHPersonalities.h"
19 #include "llvm/Analysis/TargetTransformInfo.h"
20 #include "llvm/CodeGen/RuntimeLibcalls.h"
21 #include "llvm/CodeGen/TargetLowering.h"
22 #include "llvm/CodeGen/TargetPassConfig.h"
23 #include "llvm/CodeGen/TargetSubtargetInfo.h"
24 #include "llvm/IR/BasicBlock.h"
25 #include "llvm/IR/Constants.h"
26 #include "llvm/IR/DerivedTypes.h"
27 #include "llvm/IR/Dominators.h"
28 #include "llvm/IR/Function.h"
29 #include "llvm/IR/Instructions.h"
30 #include "llvm/IR/Module.h"
31 #include "llvm/IR/Type.h"
32 #include "llvm/InitializePasses.h"
33 #include "llvm/Pass.h"
34 #include "llvm/Support/Casting.h"
35 #include "llvm/Target/TargetMachine.h"
36 #include "llvm/Transforms/Utils/Local.h"
37 #include <cstddef>
38 
39 using namespace llvm;
40 
41 #define DEBUG_TYPE "dwarfehprepare"
42 
43 STATISTIC(NumResumesLowered, "Number of resume calls lowered");
44 
45 namespace {
46 
47   class DwarfEHPrepare : public FunctionPass {
48     // RewindFunction - _Unwind_Resume or the target equivalent.
49     FunctionCallee RewindFunction = nullptr;
50 
51     DominatorTree *DT = nullptr;
52     const TargetLowering *TLI = nullptr;
53 
54     bool InsertUnwindResumeCalls(Function &Fn);
55     Value *GetExceptionObject(ResumeInst *RI);
56     size_t
57     pruneUnreachableResumes(Function &Fn,
58                             SmallVectorImpl<ResumeInst *> &Resumes,
59                             SmallVectorImpl<LandingPadInst *> &CleanupLPads);
60 
61   public:
62     static char ID; // Pass identification, replacement for typeid.
63 
64     DwarfEHPrepare() : FunctionPass(ID) {}
65 
66     bool runOnFunction(Function &Fn) override;
67 
68     bool doFinalization(Module &M) override {
69       RewindFunction = nullptr;
70       return false;
71     }
72 
73     void getAnalysisUsage(AnalysisUsage &AU) const override;
74 
75     StringRef getPassName() const override {
76       return "Exception handling preparation";
77     }
78   };
79 
80 } // end anonymous namespace
81 
82 char DwarfEHPrepare::ID = 0;
83 
84 INITIALIZE_PASS_BEGIN(DwarfEHPrepare, DEBUG_TYPE,
85                       "Prepare DWARF exceptions", false, false)
86 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
87 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
88 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
89 INITIALIZE_PASS_END(DwarfEHPrepare, DEBUG_TYPE,
90                     "Prepare DWARF exceptions", false, false)
91 
92 FunctionPass *llvm::createDwarfEHPass() { return new DwarfEHPrepare(); }
93 
94 void DwarfEHPrepare::getAnalysisUsage(AnalysisUsage &AU) const {
95   AU.addRequired<TargetPassConfig>();
96   AU.addRequired<TargetTransformInfoWrapperPass>();
97   AU.addRequired<DominatorTreeWrapperPass>();
98 }
99 
100 /// GetExceptionObject - Return the exception object from the value passed into
101 /// the 'resume' instruction (typically an aggregate). Clean up any dead
102 /// instructions, including the 'resume' instruction.
103 Value *DwarfEHPrepare::GetExceptionObject(ResumeInst *RI) {
104   Value *V = RI->getOperand(0);
105   Value *ExnObj = nullptr;
106   InsertValueInst *SelIVI = dyn_cast<InsertValueInst>(V);
107   LoadInst *SelLoad = nullptr;
108   InsertValueInst *ExcIVI = nullptr;
109   bool EraseIVIs = false;
110 
111   if (SelIVI) {
112     if (SelIVI->getNumIndices() == 1 && *SelIVI->idx_begin() == 1) {
113       ExcIVI = dyn_cast<InsertValueInst>(SelIVI->getOperand(0));
114       if (ExcIVI && isa<UndefValue>(ExcIVI->getOperand(0)) &&
115           ExcIVI->getNumIndices() == 1 && *ExcIVI->idx_begin() == 0) {
116         ExnObj = ExcIVI->getOperand(1);
117         SelLoad = dyn_cast<LoadInst>(SelIVI->getOperand(1));
118         EraseIVIs = true;
119       }
120     }
121   }
122 
123   if (!ExnObj)
124     ExnObj = ExtractValueInst::Create(RI->getOperand(0), 0, "exn.obj", RI);
125 
126   RI->eraseFromParent();
127 
128   if (EraseIVIs) {
129     if (SelIVI->use_empty())
130       SelIVI->eraseFromParent();
131     if (ExcIVI->use_empty())
132       ExcIVI->eraseFromParent();
133     if (SelLoad && SelLoad->use_empty())
134       SelLoad->eraseFromParent();
135   }
136 
137   return ExnObj;
138 }
139 
140 /// Replace resumes that are not reachable from a cleanup landing pad with
141 /// unreachable and then simplify those blocks.
142 size_t DwarfEHPrepare::pruneUnreachableResumes(
143     Function &Fn, SmallVectorImpl<ResumeInst *> &Resumes,
144     SmallVectorImpl<LandingPadInst *> &CleanupLPads) {
145   BitVector ResumeReachable(Resumes.size());
146   size_t ResumeIndex = 0;
147   for (auto *RI : Resumes) {
148     for (auto *LP : CleanupLPads) {
149       if (isPotentiallyReachable(LP, RI, nullptr, DT)) {
150         ResumeReachable.set(ResumeIndex);
151         break;
152       }
153     }
154     ++ResumeIndex;
155   }
156 
157   // If everything is reachable, there is no change.
158   if (ResumeReachable.all())
159     return Resumes.size();
160 
161   const TargetTransformInfo &TTI =
162       getAnalysis<TargetTransformInfoWrapperPass>().getTTI(Fn);
163   LLVMContext &Ctx = Fn.getContext();
164 
165   // Otherwise, insert unreachable instructions and call simplifycfg.
166   size_t ResumesLeft = 0;
167   for (size_t I = 0, E = Resumes.size(); I < E; ++I) {
168     ResumeInst *RI = Resumes[I];
169     if (ResumeReachable[I]) {
170       Resumes[ResumesLeft++] = RI;
171     } else {
172       BasicBlock *BB = RI->getParent();
173       new UnreachableInst(Ctx, RI);
174       RI->eraseFromParent();
175       simplifyCFG(BB, TTI);
176     }
177   }
178   Resumes.resize(ResumesLeft);
179   return ResumesLeft;
180 }
181 
182 /// InsertUnwindResumeCalls - Convert the ResumeInsts that are still present
183 /// into calls to the appropriate _Unwind_Resume function.
184 bool DwarfEHPrepare::InsertUnwindResumeCalls(Function &Fn) {
185   SmallVector<ResumeInst*, 16> Resumes;
186   SmallVector<LandingPadInst*, 16> CleanupLPads;
187   for (BasicBlock &BB : Fn) {
188     if (auto *RI = dyn_cast<ResumeInst>(BB.getTerminator()))
189       Resumes.push_back(RI);
190     if (auto *LP = BB.getLandingPadInst())
191       if (LP->isCleanup())
192         CleanupLPads.push_back(LP);
193   }
194 
195   if (Resumes.empty())
196     return false;
197 
198   // Check the personality, don't do anything if it's scope-based.
199   EHPersonality Pers = classifyEHPersonality(Fn.getPersonalityFn());
200   if (isScopedEHPersonality(Pers))
201     return false;
202 
203   LLVMContext &Ctx = Fn.getContext();
204 
205   size_t ResumesLeft = pruneUnreachableResumes(Fn, Resumes, CleanupLPads);
206   if (ResumesLeft == 0)
207     return true; // We pruned them all.
208 
209   // Find the rewind function if we didn't already.
210   if (!RewindFunction) {
211     FunctionType *FTy = FunctionType::get(Type::getVoidTy(Ctx),
212                                           Type::getInt8PtrTy(Ctx), false);
213     const char *RewindName = TLI->getLibcallName(RTLIB::UNWIND_RESUME);
214     RewindFunction = Fn.getParent()->getOrInsertFunction(RewindName, FTy);
215   }
216 
217   // Create the basic block where the _Unwind_Resume call will live.
218   if (ResumesLeft == 1) {
219     // Instead of creating a new BB and PHI node, just append the call to
220     // _Unwind_Resume to the end of the single resume block.
221     ResumeInst *RI = Resumes.front();
222     BasicBlock *UnwindBB = RI->getParent();
223     Value *ExnObj = GetExceptionObject(RI);
224 
225     // Call the _Unwind_Resume function.
226     CallInst *CI = CallInst::Create(RewindFunction, ExnObj, "", UnwindBB);
227     CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
228 
229     // We never expect _Unwind_Resume to return.
230     new UnreachableInst(Ctx, UnwindBB);
231     return true;
232   }
233 
234   BasicBlock *UnwindBB = BasicBlock::Create(Ctx, "unwind_resume", &Fn);
235   PHINode *PN = PHINode::Create(Type::getInt8PtrTy(Ctx), ResumesLeft,
236                                 "exn.obj", UnwindBB);
237 
238   // Extract the exception object from the ResumeInst and add it to the PHI node
239   // that feeds the _Unwind_Resume call.
240   for (ResumeInst *RI : Resumes) {
241     BasicBlock *Parent = RI->getParent();
242     BranchInst::Create(UnwindBB, Parent);
243 
244     Value *ExnObj = GetExceptionObject(RI);
245     PN->addIncoming(ExnObj, Parent);
246 
247     ++NumResumesLowered;
248   }
249 
250   // Call the function.
251   CallInst *CI = CallInst::Create(RewindFunction, PN, "", UnwindBB);
252   CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
253 
254   // We never expect _Unwind_Resume to return.
255   new UnreachableInst(Ctx, UnwindBB);
256   return true;
257 }
258 
259 bool DwarfEHPrepare::runOnFunction(Function &Fn) {
260   const TargetMachine &TM =
261       getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
262   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
263   TLI = TM.getSubtargetImpl(Fn)->getTargetLowering();
264   bool Changed = InsertUnwindResumeCalls(Fn);
265   DT = nullptr;
266   TLI = nullptr;
267   return Changed;
268 }
269