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/ADT/Triple.h"
18 #include "llvm/Analysis/CFG.h"
19 #include "llvm/Analysis/DomTreeUpdater.h"
20 #include "llvm/Analysis/EHPersonalities.h"
21 #include "llvm/Analysis/TargetTransformInfo.h"
22 #include "llvm/CodeGen/RuntimeLibcalls.h"
23 #include "llvm/CodeGen/TargetLowering.h"
24 #include "llvm/CodeGen/TargetPassConfig.h"
25 #include "llvm/CodeGen/TargetSubtargetInfo.h"
26 #include "llvm/IR/BasicBlock.h"
27 #include "llvm/IR/Constants.h"
28 #include "llvm/IR/DebugInfoMetadata.h"
29 #include "llvm/IR/DerivedTypes.h"
30 #include "llvm/IR/Dominators.h"
31 #include "llvm/IR/Function.h"
32 #include "llvm/IR/Instructions.h"
33 #include "llvm/IR/Module.h"
34 #include "llvm/IR/Type.h"
35 #include "llvm/InitializePasses.h"
36 #include "llvm/Pass.h"
37 #include "llvm/Support/Casting.h"
38 #include "llvm/Target/TargetMachine.h"
39 #include "llvm/Transforms/Utils/Local.h"
40 #include <cstddef>
41 
42 using namespace llvm;
43 
44 #define DEBUG_TYPE "dwarfehprepare"
45 
46 STATISTIC(NumResumesLowered, "Number of resume calls lowered");
47 STATISTIC(NumCleanupLandingPadsUnreachable,
48           "Number of cleanup landing pads found unreachable");
49 STATISTIC(NumCleanupLandingPadsRemaining,
50           "Number of cleanup landing pads remaining");
51 STATISTIC(NumNoUnwind, "Number of functions with nounwind");
52 STATISTIC(NumUnwind, "Number of functions with unwind");
53 
54 namespace {
55 
56 class DwarfEHPrepare {
57   CodeGenOpt::Level OptLevel;
58 
59   Function &F;
60   const TargetLowering &TLI;
61   DomTreeUpdater *DTU;
62   const TargetTransformInfo *TTI;
63   const Triple &TargetTriple;
64 
65   /// Return the exception object from the value passed into
66   /// the 'resume' instruction (typically an aggregate). Clean up any dead
67   /// instructions, including the 'resume' instruction.
68   Value *GetExceptionObject(ResumeInst *RI);
69 
70   /// Replace resumes that are not reachable from a cleanup landing pad with
71   /// unreachable and then simplify those blocks.
72   size_t
73   pruneUnreachableResumes(SmallVectorImpl<ResumeInst *> &Resumes,
74                           SmallVectorImpl<LandingPadInst *> &CleanupLPads);
75 
76   /// Convert the ResumeInsts that are still present
77   /// into calls to the appropriate _Unwind_Resume function.
78   bool InsertUnwindResumeCalls();
79 
80 public:
81   DwarfEHPrepare(CodeGenOpt::Level OptLevel_, Function &F_,
82                  const TargetLowering &TLI_, DomTreeUpdater *DTU_,
83                  const TargetTransformInfo *TTI_, const Triple &TargetTriple_)
84       : OptLevel(OptLevel_), F(F_), TLI(TLI_), DTU(DTU_), TTI(TTI_),
85         TargetTriple(TargetTriple_) {}
86 
87   bool run();
88 };
89 
90 } // namespace
91 
92 Value *DwarfEHPrepare::GetExceptionObject(ResumeInst *RI) {
93   Value *V = RI->getOperand(0);
94   Value *ExnObj = nullptr;
95   InsertValueInst *SelIVI = dyn_cast<InsertValueInst>(V);
96   LoadInst *SelLoad = nullptr;
97   InsertValueInst *ExcIVI = nullptr;
98   bool EraseIVIs = false;
99 
100   if (SelIVI) {
101     if (SelIVI->getNumIndices() == 1 && *SelIVI->idx_begin() == 1) {
102       ExcIVI = dyn_cast<InsertValueInst>(SelIVI->getOperand(0));
103       if (ExcIVI && isa<UndefValue>(ExcIVI->getOperand(0)) &&
104           ExcIVI->getNumIndices() == 1 && *ExcIVI->idx_begin() == 0) {
105         ExnObj = ExcIVI->getOperand(1);
106         SelLoad = dyn_cast<LoadInst>(SelIVI->getOperand(1));
107         EraseIVIs = true;
108       }
109     }
110   }
111 
112   if (!ExnObj)
113     ExnObj = ExtractValueInst::Create(RI->getOperand(0), 0, "exn.obj", RI);
114 
115   RI->eraseFromParent();
116 
117   if (EraseIVIs) {
118     if (SelIVI->use_empty())
119       SelIVI->eraseFromParent();
120     if (ExcIVI->use_empty())
121       ExcIVI->eraseFromParent();
122     if (SelLoad && SelLoad->use_empty())
123       SelLoad->eraseFromParent();
124   }
125 
126   return ExnObj;
127 }
128 
129 size_t DwarfEHPrepare::pruneUnreachableResumes(
130     SmallVectorImpl<ResumeInst *> &Resumes,
131     SmallVectorImpl<LandingPadInst *> &CleanupLPads) {
132   assert(DTU && "Should have DomTreeUpdater here.");
133 
134   BitVector ResumeReachable(Resumes.size());
135   size_t ResumeIndex = 0;
136   for (auto *RI : Resumes) {
137     for (auto *LP : CleanupLPads) {
138       if (isPotentiallyReachable(LP, RI, nullptr, &DTU->getDomTree())) {
139         ResumeReachable.set(ResumeIndex);
140         break;
141       }
142     }
143     ++ResumeIndex;
144   }
145 
146   // If everything is reachable, there is no change.
147   if (ResumeReachable.all())
148     return Resumes.size();
149 
150   LLVMContext &Ctx = F.getContext();
151 
152   // Otherwise, insert unreachable instructions and call simplifycfg.
153   size_t ResumesLeft = 0;
154   for (size_t I = 0, E = Resumes.size(); I < E; ++I) {
155     ResumeInst *RI = Resumes[I];
156     if (ResumeReachable[I]) {
157       Resumes[ResumesLeft++] = RI;
158     } else {
159       BasicBlock *BB = RI->getParent();
160       new UnreachableInst(Ctx, RI);
161       RI->eraseFromParent();
162       simplifyCFG(BB, *TTI, DTU);
163     }
164   }
165   Resumes.resize(ResumesLeft);
166   return ResumesLeft;
167 }
168 
169 bool DwarfEHPrepare::InsertUnwindResumeCalls() {
170   SmallVector<ResumeInst *, 16> Resumes;
171   SmallVector<LandingPadInst *, 16> CleanupLPads;
172   if (F.doesNotThrow())
173     NumNoUnwind++;
174   else
175     NumUnwind++;
176   for (BasicBlock &BB : F) {
177     if (auto *RI = dyn_cast<ResumeInst>(BB.getTerminator()))
178       Resumes.push_back(RI);
179     if (auto *LP = BB.getLandingPadInst())
180       if (LP->isCleanup())
181         CleanupLPads.push_back(LP);
182   }
183 
184   NumCleanupLandingPadsRemaining += CleanupLPads.size();
185 
186   if (Resumes.empty())
187     return false;
188 
189   // Check the personality, don't do anything if it's scope-based.
190   EHPersonality Pers = classifyEHPersonality(F.getPersonalityFn());
191   if (isScopedEHPersonality(Pers))
192     return false;
193 
194   LLVMContext &Ctx = F.getContext();
195 
196   size_t ResumesLeft = Resumes.size();
197   if (OptLevel != CodeGenOpt::None) {
198     ResumesLeft = pruneUnreachableResumes(Resumes, CleanupLPads);
199 #if LLVM_ENABLE_STATS
200     unsigned NumRemainingLPs = 0;
201     for (BasicBlock &BB : F) {
202       if (auto *LP = BB.getLandingPadInst())
203         if (LP->isCleanup())
204           NumRemainingLPs++;
205     }
206     NumCleanupLandingPadsUnreachable += CleanupLPads.size() - NumRemainingLPs;
207     NumCleanupLandingPadsRemaining -= CleanupLPads.size() - NumRemainingLPs;
208 #endif
209   }
210 
211   if (ResumesLeft == 0)
212     return true; // We pruned them all.
213 
214   // RewindFunction - _Unwind_Resume or the target equivalent.
215   FunctionCallee RewindFunction;
216   CallingConv::ID RewindFunctionCallingConv;
217   FunctionType *FTy;
218   const char *RewindName;
219   bool DoesRewindFunctionNeedExceptionObject;
220 
221   if ((Pers == EHPersonality::GNU_CXX || Pers == EHPersonality::GNU_CXX_SjLj) &&
222       TargetTriple.isTargetEHABICompatible()) {
223     RewindName = TLI.getLibcallName(RTLIB::CXA_END_CLEANUP);
224     FTy = FunctionType::get(Type::getVoidTy(Ctx), false);
225     RewindFunctionCallingConv =
226         TLI.getLibcallCallingConv(RTLIB::CXA_END_CLEANUP);
227     DoesRewindFunctionNeedExceptionObject = false;
228   } else {
229     RewindName = TLI.getLibcallName(RTLIB::UNWIND_RESUME);
230     FTy =
231         FunctionType::get(Type::getVoidTy(Ctx), Type::getInt8PtrTy(Ctx), false);
232     RewindFunctionCallingConv = TLI.getLibcallCallingConv(RTLIB::UNWIND_RESUME);
233     DoesRewindFunctionNeedExceptionObject = true;
234   }
235   RewindFunction = F.getParent()->getOrInsertFunction(RewindName, FTy);
236 
237   // Create the basic block where the _Unwind_Resume call will live.
238   if (ResumesLeft == 1) {
239     // Instead of creating a new BB and PHI node, just append the call to
240     // _Unwind_Resume to the end of the single resume block.
241     ResumeInst *RI = Resumes.front();
242     BasicBlock *UnwindBB = RI->getParent();
243     Value *ExnObj = GetExceptionObject(RI);
244     llvm::SmallVector<Value *, 1> RewindFunctionArgs;
245     if (DoesRewindFunctionNeedExceptionObject)
246       RewindFunctionArgs.push_back(ExnObj);
247 
248     // Call the rewind function.
249     CallInst *CI =
250         CallInst::Create(RewindFunction, RewindFunctionArgs, "", UnwindBB);
251     // The verifier requires that all calls of debug-info-bearing functions
252     // from debug-info-bearing functions have a debug location (for inlining
253     // purposes). Assign a dummy location to satisfy the constraint.
254     Function *RewindFn = dyn_cast<Function>(RewindFunction.getCallee());
255     if (RewindFn && RewindFn->getSubprogram())
256       if (DISubprogram *SP = F.getSubprogram())
257         CI->setDebugLoc(DILocation::get(SP->getContext(), 0, 0, SP));
258     CI->setCallingConv(RewindFunctionCallingConv);
259 
260     // We never expect _Unwind_Resume to return.
261     CI->setDoesNotReturn();
262     new UnreachableInst(Ctx, UnwindBB);
263     return true;
264   }
265 
266   std::vector<DominatorTree::UpdateType> Updates;
267   Updates.reserve(Resumes.size());
268 
269   llvm::SmallVector<Value *, 1> RewindFunctionArgs;
270 
271   BasicBlock *UnwindBB = BasicBlock::Create(Ctx, "unwind_resume", &F);
272   PHINode *PN = PHINode::Create(Type::getInt8PtrTy(Ctx), ResumesLeft, "exn.obj",
273                                 UnwindBB);
274 
275   // Extract the exception object from the ResumeInst and add it to the PHI node
276   // that feeds the _Unwind_Resume call.
277   for (ResumeInst *RI : Resumes) {
278     BasicBlock *Parent = RI->getParent();
279     BranchInst::Create(UnwindBB, Parent);
280     Updates.push_back({DominatorTree::Insert, Parent, UnwindBB});
281 
282     Value *ExnObj = GetExceptionObject(RI);
283     PN->addIncoming(ExnObj, Parent);
284 
285     ++NumResumesLowered;
286   }
287 
288   if (DoesRewindFunctionNeedExceptionObject)
289     RewindFunctionArgs.push_back(PN);
290 
291   // Call the function.
292   CallInst *CI =
293       CallInst::Create(RewindFunction, RewindFunctionArgs, "", UnwindBB);
294   CI->setCallingConv(RewindFunctionCallingConv);
295 
296   // We never expect _Unwind_Resume to return.
297   CI->setDoesNotReturn();
298   new UnreachableInst(Ctx, UnwindBB);
299 
300   if (DTU)
301     DTU->applyUpdates(Updates);
302 
303   return true;
304 }
305 
306 bool DwarfEHPrepare::run() {
307   bool Changed = InsertUnwindResumeCalls();
308 
309   return Changed;
310 }
311 
312 static bool prepareDwarfEH(CodeGenOpt::Level OptLevel, Function &F,
313                            const TargetLowering &TLI, DominatorTree *DT,
314                            const TargetTransformInfo *TTI,
315                            const Triple &TargetTriple) {
316   DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
317 
318   return DwarfEHPrepare(OptLevel, F, TLI, DT ? &DTU : nullptr, TTI,
319                         TargetTriple)
320       .run();
321 }
322 
323 namespace {
324 
325 class DwarfEHPrepareLegacyPass : public FunctionPass {
326 
327   CodeGenOpt::Level OptLevel;
328 
329 public:
330   static char ID; // Pass identification, replacement for typeid.
331 
332   DwarfEHPrepareLegacyPass(CodeGenOpt::Level OptLevel = CodeGenOpt::Default)
333       : FunctionPass(ID), OptLevel(OptLevel) {}
334 
335   bool runOnFunction(Function &F) override {
336     const TargetMachine &TM =
337         getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
338     const TargetLowering &TLI = *TM.getSubtargetImpl(F)->getTargetLowering();
339     DominatorTree *DT = nullptr;
340     const TargetTransformInfo *TTI = nullptr;
341     if (auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>())
342       DT = &DTWP->getDomTree();
343     if (OptLevel != CodeGenOpt::None) {
344       if (!DT)
345         DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
346       TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
347     }
348     return prepareDwarfEH(OptLevel, F, TLI, DT, TTI, TM.getTargetTriple());
349   }
350 
351   void getAnalysisUsage(AnalysisUsage &AU) const override {
352     AU.addRequired<TargetPassConfig>();
353     AU.addRequired<TargetTransformInfoWrapperPass>();
354     if (OptLevel != CodeGenOpt::None) {
355       AU.addRequired<DominatorTreeWrapperPass>();
356       AU.addRequired<TargetTransformInfoWrapperPass>();
357     }
358     AU.addPreserved<DominatorTreeWrapperPass>();
359   }
360 
361   StringRef getPassName() const override {
362     return "Exception handling preparation";
363   }
364 };
365 
366 } // end anonymous namespace
367 
368 char DwarfEHPrepareLegacyPass::ID = 0;
369 
370 INITIALIZE_PASS_BEGIN(DwarfEHPrepareLegacyPass, DEBUG_TYPE,
371                       "Prepare DWARF exceptions", false, false)
372 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
373 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
374 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
375 INITIALIZE_PASS_END(DwarfEHPrepareLegacyPass, DEBUG_TYPE,
376                     "Prepare DWARF exceptions", false, false)
377 
378 FunctionPass *llvm::createDwarfEHPass(CodeGenOpt::Level OptLevel) {
379   return new DwarfEHPrepareLegacyPass(OptLevel);
380 }
381