DwarfEHPrepare.cpp revision 360784 
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    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
82char DwarfEHPrepare::ID = 0;
83
84INITIALIZE_PASS_BEGIN(DwarfEHPrepare, DEBUG_TYPE,
85                      "Prepare DWARF exceptions", false, false)
86INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
87INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
88INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
89INITIALIZE_PASS_END(DwarfEHPrepare, DEBUG_TYPE,
90                    "Prepare DWARF exceptions", false, false)
91
92FunctionPass *llvm::createDwarfEHPass() { return new DwarfEHPrepare(); }
93
94void 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.
103Value *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.
142size_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.
184bool 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
259bool 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