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
39using namespace llvm;
40
41#define DEBUG_TYPE "dwarfehprepare"
42
43STATISTIC(NumResumesLowered, "Number of resume calls lowered");
44
45namespace {
46
47  class DwarfEHPrepare : public FunctionPass {
48    // RewindFunction - _Unwind_Resume or the target equivalent.
49    FunctionCallee RewindFunction = nullptr;
50
51    CodeGenOpt::Level OptLevel;
52    DominatorTree *DT = nullptr;
53    const TargetLowering *TLI = nullptr;
54
55    bool InsertUnwindResumeCalls(Function &Fn);
56    Value *GetExceptionObject(ResumeInst *RI);
57    size_t
58    pruneUnreachableResumes(Function &Fn,
59                            SmallVectorImpl<ResumeInst *> &Resumes,
60                            SmallVectorImpl<LandingPadInst *> &CleanupLPads);
61
62  public:
63    static char ID; // Pass identification, replacement for typeid.
64
65    DwarfEHPrepare(CodeGenOpt::Level OptLevel = CodeGenOpt::Default)
66      : FunctionPass(ID), OptLevel(OptLevel) {}
67
68    bool runOnFunction(Function &Fn) override;
69
70    bool doFinalization(Module &M) override {
71      RewindFunction = nullptr;
72      return false;
73    }
74
75    void getAnalysisUsage(AnalysisUsage &AU) const override;
76
77    StringRef getPassName() const override {
78      return "Exception handling preparation";
79    }
80  };
81
82} // end anonymous namespace
83
84char DwarfEHPrepare::ID = 0;
85
86INITIALIZE_PASS_BEGIN(DwarfEHPrepare, DEBUG_TYPE,
87                      "Prepare DWARF exceptions", false, false)
88INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
89INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
90INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
91INITIALIZE_PASS_END(DwarfEHPrepare, DEBUG_TYPE,
92                    "Prepare DWARF exceptions", false, false)
93
94FunctionPass *llvm::createDwarfEHPass(CodeGenOpt::Level OptLevel) {
95  return new DwarfEHPrepare(OptLevel);
96}
97
98void DwarfEHPrepare::getAnalysisUsage(AnalysisUsage &AU) const {
99  AU.addRequired<TargetPassConfig>();
100  AU.addRequired<TargetTransformInfoWrapperPass>();
101  if (OptLevel != CodeGenOpt::None)
102    AU.addRequired<DominatorTreeWrapperPass>();
103}
104
105/// GetExceptionObject - Return the exception object from the value passed into
106/// the 'resume' instruction (typically an aggregate). Clean up any dead
107/// instructions, including the 'resume' instruction.
108Value *DwarfEHPrepare::GetExceptionObject(ResumeInst *RI) {
109  Value *V = RI->getOperand(0);
110  Value *ExnObj = nullptr;
111  InsertValueInst *SelIVI = dyn_cast<InsertValueInst>(V);
112  LoadInst *SelLoad = nullptr;
113  InsertValueInst *ExcIVI = nullptr;
114  bool EraseIVIs = false;
115
116  if (SelIVI) {
117    if (SelIVI->getNumIndices() == 1 && *SelIVI->idx_begin() == 1) {
118      ExcIVI = dyn_cast<InsertValueInst>(SelIVI->getOperand(0));
119      if (ExcIVI && isa<UndefValue>(ExcIVI->getOperand(0)) &&
120          ExcIVI->getNumIndices() == 1 && *ExcIVI->idx_begin() == 0) {
121        ExnObj = ExcIVI->getOperand(1);
122        SelLoad = dyn_cast<LoadInst>(SelIVI->getOperand(1));
123        EraseIVIs = true;
124      }
125    }
126  }
127
128  if (!ExnObj)
129    ExnObj = ExtractValueInst::Create(RI->getOperand(0), 0, "exn.obj", RI);
130
131  RI->eraseFromParent();
132
133  if (EraseIVIs) {
134    if (SelIVI->use_empty())
135      SelIVI->eraseFromParent();
136    if (ExcIVI->use_empty())
137      ExcIVI->eraseFromParent();
138    if (SelLoad && SelLoad->use_empty())
139      SelLoad->eraseFromParent();
140  }
141
142  return ExnObj;
143}
144
145/// Replace resumes that are not reachable from a cleanup landing pad with
146/// unreachable and then simplify those blocks.
147size_t DwarfEHPrepare::pruneUnreachableResumes(
148    Function &Fn, SmallVectorImpl<ResumeInst *> &Resumes,
149    SmallVectorImpl<LandingPadInst *> &CleanupLPads) {
150  BitVector ResumeReachable(Resumes.size());
151  size_t ResumeIndex = 0;
152  for (auto *RI : Resumes) {
153    for (auto *LP : CleanupLPads) {
154      if (isPotentiallyReachable(LP, RI, nullptr, DT)) {
155        ResumeReachable.set(ResumeIndex);
156        break;
157      }
158    }
159    ++ResumeIndex;
160  }
161
162  // If everything is reachable, there is no change.
163  if (ResumeReachable.all())
164    return Resumes.size();
165
166  const TargetTransformInfo &TTI =
167      getAnalysis<TargetTransformInfoWrapperPass>().getTTI(Fn);
168  LLVMContext &Ctx = Fn.getContext();
169
170  // Otherwise, insert unreachable instructions and call simplifycfg.
171  size_t ResumesLeft = 0;
172  for (size_t I = 0, E = Resumes.size(); I < E; ++I) {
173    ResumeInst *RI = Resumes[I];
174    if (ResumeReachable[I]) {
175      Resumes[ResumesLeft++] = RI;
176    } else {
177      BasicBlock *BB = RI->getParent();
178      new UnreachableInst(Ctx, RI);
179      RI->eraseFromParent();
180      simplifyCFG(BB, TTI);
181    }
182  }
183  Resumes.resize(ResumesLeft);
184  return ResumesLeft;
185}
186
187/// InsertUnwindResumeCalls - Convert the ResumeInsts that are still present
188/// into calls to the appropriate _Unwind_Resume function.
189bool DwarfEHPrepare::InsertUnwindResumeCalls(Function &Fn) {
190  SmallVector<ResumeInst*, 16> Resumes;
191  SmallVector<LandingPadInst*, 16> CleanupLPads;
192  for (BasicBlock &BB : Fn) {
193    if (auto *RI = dyn_cast<ResumeInst>(BB.getTerminator()))
194      Resumes.push_back(RI);
195    if (auto *LP = BB.getLandingPadInst())
196      if (LP->isCleanup())
197        CleanupLPads.push_back(LP);
198  }
199
200  if (Resumes.empty())
201    return false;
202
203  // Check the personality, don't do anything if it's scope-based.
204  EHPersonality Pers = classifyEHPersonality(Fn.getPersonalityFn());
205  if (isScopedEHPersonality(Pers))
206    return false;
207
208  LLVMContext &Ctx = Fn.getContext();
209
210  size_t ResumesLeft = Resumes.size();
211  if (OptLevel != CodeGenOpt::None)
212    ResumesLeft = pruneUnreachableResumes(Fn, Resumes, CleanupLPads);
213
214  if (ResumesLeft == 0)
215    return true; // We pruned them all.
216
217  // Find the rewind function if we didn't already.
218  if (!RewindFunction) {
219    FunctionType *FTy = FunctionType::get(Type::getVoidTy(Ctx),
220                                          Type::getInt8PtrTy(Ctx), false);
221    const char *RewindName = TLI->getLibcallName(RTLIB::UNWIND_RESUME);
222    RewindFunction = Fn.getParent()->getOrInsertFunction(RewindName, FTy);
223  }
224
225  // Create the basic block where the _Unwind_Resume call will live.
226  if (ResumesLeft == 1) {
227    // Instead of creating a new BB and PHI node, just append the call to
228    // _Unwind_Resume to the end of the single resume block.
229    ResumeInst *RI = Resumes.front();
230    BasicBlock *UnwindBB = RI->getParent();
231    Value *ExnObj = GetExceptionObject(RI);
232
233    // Call the _Unwind_Resume function.
234    CallInst *CI = CallInst::Create(RewindFunction, ExnObj, "", UnwindBB);
235    CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
236
237    // We never expect _Unwind_Resume to return.
238    new UnreachableInst(Ctx, UnwindBB);
239    return true;
240  }
241
242  BasicBlock *UnwindBB = BasicBlock::Create(Ctx, "unwind_resume", &Fn);
243  PHINode *PN = PHINode::Create(Type::getInt8PtrTy(Ctx), ResumesLeft,
244                                "exn.obj", UnwindBB);
245
246  // Extract the exception object from the ResumeInst and add it to the PHI node
247  // that feeds the _Unwind_Resume call.
248  for (ResumeInst *RI : Resumes) {
249    BasicBlock *Parent = RI->getParent();
250    BranchInst::Create(UnwindBB, Parent);
251
252    Value *ExnObj = GetExceptionObject(RI);
253    PN->addIncoming(ExnObj, Parent);
254
255    ++NumResumesLowered;
256  }
257
258  // Call the function.
259  CallInst *CI = CallInst::Create(RewindFunction, PN, "", UnwindBB);
260  CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
261
262  // We never expect _Unwind_Resume to return.
263  new UnreachableInst(Ctx, UnwindBB);
264  return true;
265}
266
267bool DwarfEHPrepare::runOnFunction(Function &Fn) {
268  const TargetMachine &TM =
269      getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
270  DT = OptLevel != CodeGenOpt::None
271      ? &getAnalysis<DominatorTreeWrapperPass>().getDomTree() : nullptr;
272  TLI = TM.getSubtargetImpl(Fn)->getTargetLowering();
273  bool Changed = InsertUnwindResumeCalls(Fn);
274  DT = nullptr;
275  TLI = nullptr;
276  return Changed;
277}
278