xref: /llvm-project/llvm/lib/CodeGen/DwarfEHPrepare.cpp (revision 735ab61ac828bd61398e6847d60e308fdf2b54ec) 
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/CodeGen/DwarfEHPrepare.h"
15 #include "llvm/ADT/BitVector.h"
16 #include "llvm/ADT/SmallVector.h"
17 #include "llvm/ADT/Statistic.h"
18 #include "llvm/Analysis/CFG.h"
19 #include "llvm/Analysis/DomTreeUpdater.h"
20 #include "llvm/Analysis/TargetTransformInfo.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/DebugInfoMetadata.h"
27 #include "llvm/IR/DerivedTypes.h"
28 #include "llvm/IR/Dominators.h"
29 #include "llvm/IR/EHPersonalities.h"
30 #include "llvm/IR/Function.h"
31 #include "llvm/IR/Instructions.h"
32 #include "llvm/IR/Module.h"
33 #include "llvm/IR/Type.h"
34 #include "llvm/InitializePasses.h"
35 #include "llvm/Pass.h"
36 #include "llvm/Support/Casting.h"
37 #include "llvm/Target/TargetMachine.h"
38 #include "llvm/TargetParser/Triple.h"
39 #include "llvm/Transforms/Utils/Local.h"
40 #include <cstddef>
41 
42 using namespace llvm;
43 
44 #define DEBUG_TYPE "dwarf-eh-prepare"
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   CodeGenOptLevel 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(CodeGenOptLevel 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",
114                                       RI->getIterator());
115 
116   RI->eraseFromParent();
117 
118   if (EraseIVIs) {
119     if (SelIVI->use_empty())
120       SelIVI->eraseFromParent();
121     if (ExcIVI->use_empty())
122       ExcIVI->eraseFromParent();
123     if (SelLoad && SelLoad->use_empty())
124       SelLoad->eraseFromParent();
125   }
126 
127   return ExnObj;
128 }
129 
130 size_t DwarfEHPrepare::pruneUnreachableResumes(
131     SmallVectorImpl<ResumeInst *> &Resumes,
132     SmallVectorImpl<LandingPadInst *> &CleanupLPads) {
133   assert(DTU && "Should have DomTreeUpdater here.");
134 
135   BitVector ResumeReachable(Resumes.size());
136   size_t ResumeIndex = 0;
137   for (auto *RI : Resumes) {
138     for (auto *LP : CleanupLPads) {
139       if (isPotentiallyReachable(LP, RI, nullptr, &DTU->getDomTree())) {
140         ResumeReachable.set(ResumeIndex);
141         break;
142       }
143     }
144     ++ResumeIndex;
145   }
146 
147   // If everything is reachable, there is no change.
148   if (ResumeReachable.all())
149     return Resumes.size();
150 
151   LLVMContext &Ctx = F.getContext();
152 
153   // Otherwise, insert unreachable instructions and call simplifycfg.
154   size_t ResumesLeft = 0;
155   for (size_t I = 0, E = Resumes.size(); I < E; ++I) {
156     ResumeInst *RI = Resumes[I];
157     if (ResumeReachable[I]) {
158       Resumes[ResumesLeft++] = RI;
159     } else {
160       BasicBlock *BB = RI->getParent();
161       new UnreachableInst(Ctx, RI->getIterator());
162       RI->eraseFromParent();
163       simplifyCFG(BB, *TTI, DTU);
164     }
165   }
166   Resumes.resize(ResumesLeft);
167   return ResumesLeft;
168 }
169 
170 bool DwarfEHPrepare::InsertUnwindResumeCalls() {
171   SmallVector<ResumeInst *, 16> Resumes;
172   SmallVector<LandingPadInst *, 16> CleanupLPads;
173   if (F.doesNotThrow())
174     NumNoUnwind++;
175   else
176     NumUnwind++;
177   for (BasicBlock &BB : F) {
178     if (auto *RI = dyn_cast<ResumeInst>(BB.getTerminator()))
179       Resumes.push_back(RI);
180     if (auto *LP = BB.getLandingPadInst())
181       if (LP->isCleanup())
182         CleanupLPads.push_back(LP);
183   }
184 
185   NumCleanupLandingPadsRemaining += CleanupLPads.size();
186 
187   if (Resumes.empty())
188     return false;
189 
190   // Check the personality, don't do anything if it's scope-based.
191   EHPersonality Pers = classifyEHPersonality(F.getPersonalityFn());
192   if (isScopedEHPersonality(Pers))
193     return false;
194 
195   LLVMContext &Ctx = F.getContext();
196 
197   size_t ResumesLeft = Resumes.size();
198   if (OptLevel != CodeGenOptLevel::None) {
199     ResumesLeft = pruneUnreachableResumes(Resumes, CleanupLPads);
200 #if LLVM_ENABLE_STATS
201     unsigned NumRemainingLPs = 0;
202     for (BasicBlock &BB : F) {
203       if (auto *LP = BB.getLandingPadInst())
204         if (LP->isCleanup())
205           NumRemainingLPs++;
206     }
207     NumCleanupLandingPadsUnreachable += CleanupLPads.size() - NumRemainingLPs;
208     NumCleanupLandingPadsRemaining -= CleanupLPads.size() - NumRemainingLPs;
209 #endif
210   }
211 
212   if (ResumesLeft == 0)
213     return true; // We pruned them all.
214 
215   // RewindFunction - _Unwind_Resume or the target equivalent.
216   FunctionCallee RewindFunction;
217   CallingConv::ID RewindFunctionCallingConv;
218   FunctionType *FTy;
219   const char *RewindName;
220   bool DoesRewindFunctionNeedExceptionObject;
221 
222   if ((Pers == EHPersonality::GNU_CXX || Pers == EHPersonality::GNU_CXX_SjLj) &&
223       TargetTriple.isTargetEHABICompatible()) {
224     RewindName = TLI.getLibcallName(RTLIB::CXA_END_CLEANUP);
225     FTy = FunctionType::get(Type::getVoidTy(Ctx), false);
226     RewindFunctionCallingConv =
227         TLI.getLibcallCallingConv(RTLIB::CXA_END_CLEANUP);
228     DoesRewindFunctionNeedExceptionObject = false;
229   } else {
230     RewindName = TLI.getLibcallName(RTLIB::UNWIND_RESUME);
231     FTy = FunctionType::get(Type::getVoidTy(Ctx), PointerType::getUnqual(Ctx),
232                             false);
233     RewindFunctionCallingConv = TLI.getLibcallCallingConv(RTLIB::UNWIND_RESUME);
234     DoesRewindFunctionNeedExceptionObject = true;
235   }
236   RewindFunction = F.getParent()->getOrInsertFunction(RewindName, FTy);
237 
238   // Create the basic block where the _Unwind_Resume call will live.
239   if (ResumesLeft == 1) {
240     // Instead of creating a new BB and PHI node, just append the call to
241     // _Unwind_Resume to the end of the single resume block.
242     ResumeInst *RI = Resumes.front();
243     BasicBlock *UnwindBB = RI->getParent();
244     Value *ExnObj = GetExceptionObject(RI);
245     llvm::SmallVector<Value *, 1> RewindFunctionArgs;
246     if (DoesRewindFunctionNeedExceptionObject)
247       RewindFunctionArgs.push_back(ExnObj);
248 
249     // Call the rewind function.
250     CallInst *CI =
251         CallInst::Create(RewindFunction, RewindFunctionArgs, "", UnwindBB);
252     // The verifier requires that all calls of debug-info-bearing functions
253     // from debug-info-bearing functions have a debug location (for inlining
254     // purposes). Assign a dummy location to satisfy the constraint.
255     Function *RewindFn = dyn_cast<Function>(RewindFunction.getCallee());
256     if (RewindFn && RewindFn->getSubprogram())
257       if (DISubprogram *SP = F.getSubprogram())
258         CI->setDebugLoc(DILocation::get(SP->getContext(), 0, 0, SP));
259     CI->setCallingConv(RewindFunctionCallingConv);
260 
261     // We never expect _Unwind_Resume to return.
262     CI->setDoesNotReturn();
263     new UnreachableInst(Ctx, UnwindBB);
264     return true;
265   }
266 
267   std::vector<DominatorTree::UpdateType> Updates;
268   Updates.reserve(Resumes.size());
269 
270   llvm::SmallVector<Value *, 1> RewindFunctionArgs;
271 
272   BasicBlock *UnwindBB = BasicBlock::Create(Ctx, "unwind_resume", &F);
273   PHINode *PN = PHINode::Create(PointerType::getUnqual(Ctx), ResumesLeft,
274                                 "exn.obj", UnwindBB);
275 
276   // Extract the exception object from the ResumeInst and add it to the PHI node
277   // that feeds the _Unwind_Resume call.
278   for (ResumeInst *RI : Resumes) {
279     BasicBlock *Parent = RI->getParent();
280     BranchInst::Create(UnwindBB, Parent);
281     Updates.push_back({DominatorTree::Insert, Parent, UnwindBB});
282 
283     Value *ExnObj = GetExceptionObject(RI);
284     PN->addIncoming(ExnObj, Parent);
285 
286     ++NumResumesLowered;
287   }
288 
289   if (DoesRewindFunctionNeedExceptionObject)
290     RewindFunctionArgs.push_back(PN);
291 
292   // Call the function.
293   CallInst *CI =
294       CallInst::Create(RewindFunction, RewindFunctionArgs, "", UnwindBB);
295   // The verifier requires that all calls of debug-info-bearing functions
296   // from debug-info-bearing functions have a debug location (for inlining
297   // purposes). Assign a dummy location to satisfy the constraint.
298   Function *RewindFn = dyn_cast<Function>(RewindFunction.getCallee());
299   if (RewindFn && RewindFn->getSubprogram())
300     if (DISubprogram *SP = F.getSubprogram())
301       CI->setDebugLoc(DILocation::get(SP->getContext(), 0, 0, SP));
302   CI->setCallingConv(RewindFunctionCallingConv);
303 
304   // We never expect _Unwind_Resume to return.
305   CI->setDoesNotReturn();
306   new UnreachableInst(Ctx, UnwindBB);
307 
308   if (DTU)
309     DTU->applyUpdates(Updates);
310 
311   return true;
312 }
313 
314 bool DwarfEHPrepare::run() {
315   bool Changed = InsertUnwindResumeCalls();
316 
317   return Changed;
318 }
319 
320 static bool prepareDwarfEH(CodeGenOptLevel OptLevel, Function &F,
321                            const TargetLowering &TLI, DominatorTree *DT,
322                            const TargetTransformInfo *TTI,
323                            const Triple &TargetTriple) {
324   DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
325 
326   return DwarfEHPrepare(OptLevel, F, TLI, DT ? &DTU : nullptr, TTI,
327                         TargetTriple)
328       .run();
329 }
330 
331 namespace {
332 
333 class DwarfEHPrepareLegacyPass : public FunctionPass {
334 
335   CodeGenOptLevel OptLevel;
336 
337 public:
338   static char ID; // Pass identification, replacement for typeid.
339 
340   DwarfEHPrepareLegacyPass(CodeGenOptLevel OptLevel = CodeGenOptLevel::Default)
341       : FunctionPass(ID), OptLevel(OptLevel) {}
342 
343   bool runOnFunction(Function &F) override {
344     const TargetMachine &TM =
345         getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
346     const TargetLowering &TLI = *TM.getSubtargetImpl(F)->getTargetLowering();
347     DominatorTree *DT = nullptr;
348     const TargetTransformInfo *TTI = nullptr;
349     if (auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>())
350       DT = &DTWP->getDomTree();
351     if (OptLevel != CodeGenOptLevel::None) {
352       if (!DT)
353         DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
354       TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
355     }
356     return prepareDwarfEH(OptLevel, F, TLI, DT, TTI, TM.getTargetTriple());
357   }
358 
359   void getAnalysisUsage(AnalysisUsage &AU) const override {
360     AU.addRequired<TargetPassConfig>();
361     AU.addRequired<TargetTransformInfoWrapperPass>();
362     if (OptLevel != CodeGenOptLevel::None) {
363       AU.addRequired<DominatorTreeWrapperPass>();
364       AU.addRequired<TargetTransformInfoWrapperPass>();
365     }
366     AU.addPreserved<DominatorTreeWrapperPass>();
367   }
368 
369   StringRef getPassName() const override {
370     return "Exception handling preparation";
371   }
372 };
373 
374 } // end anonymous namespace
375 
376 PreservedAnalyses DwarfEHPreparePass::run(Function &F,
377                                           FunctionAnalysisManager &FAM) {
378   const auto &TLI = *TM->getSubtargetImpl(F)->getTargetLowering();
379   auto *DT = FAM.getCachedResult<DominatorTreeAnalysis>(F);
380   const TargetTransformInfo *TTI = nullptr;
381   auto OptLevel = TM->getOptLevel();
382   if (OptLevel != CodeGenOptLevel::None) {
383     if (!DT)
384       DT = &FAM.getResult<DominatorTreeAnalysis>(F);
385     TTI = &FAM.getResult<TargetIRAnalysis>(F);
386   }
387   bool Changed =
388       prepareDwarfEH(OptLevel, F, TLI, DT, TTI, TM->getTargetTriple());
389 
390   if (!Changed)
391     return PreservedAnalyses::all();
392   PreservedAnalyses PA;
393   PA.preserve<DominatorTreeAnalysis>();
394   return PA;
395 }
396 
397 char DwarfEHPrepareLegacyPass::ID = 0;
398 
399 INITIALIZE_PASS_BEGIN(DwarfEHPrepareLegacyPass, DEBUG_TYPE,
400                       "Prepare DWARF exceptions", false, false)
401 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
402 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
403 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
404 INITIALIZE_PASS_END(DwarfEHPrepareLegacyPass, DEBUG_TYPE,
405                     "Prepare DWARF exceptions", false, false)
406 
407 FunctionPass *llvm::createDwarfEHPass(CodeGenOptLevel OptLevel) {
408   return new DwarfEHPrepareLegacyPass(OptLevel);
409 }
410