xref: /freebsd/contrib/llvm-project/llvm/lib/CodeGen/DwarfEHPrepare.cpp (revision 7fdf597e96a02165cfe22ff357b857d5fa15ed8a) 
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/RuntimeLibcallUtil.h"
22 #include "llvm/CodeGen/TargetLowering.h"
23 #include "llvm/CodeGen/TargetPassConfig.h"
24 #include "llvm/CodeGen/TargetSubtargetInfo.h"
25 #include "llvm/IR/BasicBlock.h"
26 #include "llvm/IR/Constants.h"
27 #include "llvm/IR/DebugInfoMetadata.h"
28 #include "llvm/IR/DerivedTypes.h"
29 #include "llvm/IR/Dominators.h"
30 #include "llvm/IR/EHPersonalities.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/TargetParser/Triple.h"
40 #include "llvm/Transforms/Utils/Local.h"
41 #include <cstddef>
42 
43 using namespace llvm;
44 
45 #define DEBUG_TYPE "dwarf-eh-prepare"
46 
47 STATISTIC(NumResumesLowered, "Number of resume calls lowered");
48 STATISTIC(NumCleanupLandingPadsUnreachable,
49           "Number of cleanup landing pads found unreachable");
50 STATISTIC(NumCleanupLandingPadsRemaining,
51           "Number of cleanup landing pads remaining");
52 STATISTIC(NumNoUnwind, "Number of functions with nounwind");
53 STATISTIC(NumUnwind, "Number of functions with unwind");
54 
55 namespace {
56 
57 class DwarfEHPrepare {
58   CodeGenOptLevel OptLevel;
59 
60   Function &F;
61   const TargetLowering &TLI;
62   DomTreeUpdater *DTU;
63   const TargetTransformInfo *TTI;
64   const Triple &TargetTriple;
65 
66   /// Return the exception object from the value passed into
67   /// the 'resume' instruction (typically an aggregate). Clean up any dead
68   /// instructions, including the 'resume' instruction.
69   Value *GetExceptionObject(ResumeInst *RI);
70 
71   /// Replace resumes that are not reachable from a cleanup landing pad with
72   /// unreachable and then simplify those blocks.
73   size_t
74   pruneUnreachableResumes(SmallVectorImpl<ResumeInst *> &Resumes,
75                           SmallVectorImpl<LandingPadInst *> &CleanupLPads);
76 
77   /// Convert the ResumeInsts that are still present
78   /// into calls to the appropriate _Unwind_Resume function.
79   bool InsertUnwindResumeCalls();
80 
81 public:
82   DwarfEHPrepare(CodeGenOptLevel OptLevel_, Function &F_,
83                  const TargetLowering &TLI_, DomTreeUpdater *DTU_,
84                  const TargetTransformInfo *TTI_, const Triple &TargetTriple_)
85       : OptLevel(OptLevel_), F(F_), TLI(TLI_), DTU(DTU_), TTI(TTI_),
86         TargetTriple(TargetTriple_) {}
87 
88   bool run();
89 };
90 
91 } // namespace
92 
93 Value *DwarfEHPrepare::GetExceptionObject(ResumeInst *RI) {
94   Value *V = RI->getOperand(0);
95   Value *ExnObj = nullptr;
96   InsertValueInst *SelIVI = dyn_cast<InsertValueInst>(V);
97   LoadInst *SelLoad = nullptr;
98   InsertValueInst *ExcIVI = nullptr;
99   bool EraseIVIs = false;
100 
101   if (SelIVI) {
102     if (SelIVI->getNumIndices() == 1 && *SelIVI->idx_begin() == 1) {
103       ExcIVI = dyn_cast<InsertValueInst>(SelIVI->getOperand(0));
104       if (ExcIVI && isa<UndefValue>(ExcIVI->getOperand(0)) &&
105           ExcIVI->getNumIndices() == 1 && *ExcIVI->idx_begin() == 0) {
106         ExnObj = ExcIVI->getOperand(1);
107         SelLoad = dyn_cast<LoadInst>(SelIVI->getOperand(1));
108         EraseIVIs = true;
109       }
110     }
111   }
112 
113   if (!ExnObj)
114     ExnObj = ExtractValueInst::Create(RI->getOperand(0), 0, "exn.obj",
115                                       RI->getIterator());
116 
117   RI->eraseFromParent();
118 
119   if (EraseIVIs) {
120     if (SelIVI->use_empty())
121       SelIVI->eraseFromParent();
122     if (ExcIVI->use_empty())
123       ExcIVI->eraseFromParent();
124     if (SelLoad && SelLoad->use_empty())
125       SelLoad->eraseFromParent();
126   }
127 
128   return ExnObj;
129 }
130 
131 size_t DwarfEHPrepare::pruneUnreachableResumes(
132     SmallVectorImpl<ResumeInst *> &Resumes,
133     SmallVectorImpl<LandingPadInst *> &CleanupLPads) {
134   assert(DTU && "Should have DomTreeUpdater here.");
135 
136   BitVector ResumeReachable(Resumes.size());
137   size_t ResumeIndex = 0;
138   for (auto *RI : Resumes) {
139     for (auto *LP : CleanupLPads) {
140       if (isPotentiallyReachable(LP, RI, nullptr, &DTU->getDomTree())) {
141         ResumeReachable.set(ResumeIndex);
142         break;
143       }
144     }
145     ++ResumeIndex;
146   }
147 
148   // If everything is reachable, there is no change.
149   if (ResumeReachable.all())
150     return Resumes.size();
151 
152   LLVMContext &Ctx = F.getContext();
153 
154   // Otherwise, insert unreachable instructions and call simplifycfg.
155   size_t ResumesLeft = 0;
156   for (size_t I = 0, E = Resumes.size(); I < E; ++I) {
157     ResumeInst *RI = Resumes[I];
158     if (ResumeReachable[I]) {
159       Resumes[ResumesLeft++] = RI;
160     } else {
161       BasicBlock *BB = RI->getParent();
162       new UnreachableInst(Ctx, RI->getIterator());
163       RI->eraseFromParent();
164       simplifyCFG(BB, *TTI, DTU);
165     }
166   }
167   Resumes.resize(ResumesLeft);
168   return ResumesLeft;
169 }
170 
171 bool DwarfEHPrepare::InsertUnwindResumeCalls() {
172   SmallVector<ResumeInst *, 16> Resumes;
173   SmallVector<LandingPadInst *, 16> CleanupLPads;
174   if (F.doesNotThrow())
175     NumNoUnwind++;
176   else
177     NumUnwind++;
178   for (BasicBlock &BB : F) {
179     if (auto *RI = dyn_cast<ResumeInst>(BB.getTerminator()))
180       Resumes.push_back(RI);
181     if (auto *LP = BB.getLandingPadInst())
182       if (LP->isCleanup())
183         CleanupLPads.push_back(LP);
184   }
185 
186   NumCleanupLandingPadsRemaining += CleanupLPads.size();
187 
188   if (Resumes.empty())
189     return false;
190 
191   // Check the personality, don't do anything if it's scope-based.
192   EHPersonality Pers = classifyEHPersonality(F.getPersonalityFn());
193   if (isScopedEHPersonality(Pers))
194     return false;
195 
196   LLVMContext &Ctx = F.getContext();
197 
198   size_t ResumesLeft = Resumes.size();
199   if (OptLevel != CodeGenOptLevel::None) {
200     ResumesLeft = pruneUnreachableResumes(Resumes, CleanupLPads);
201 #if LLVM_ENABLE_STATS
202     unsigned NumRemainingLPs = 0;
203     for (BasicBlock &BB : F) {
204       if (auto *LP = BB.getLandingPadInst())
205         if (LP->isCleanup())
206           NumRemainingLPs++;
207     }
208     NumCleanupLandingPadsUnreachable += CleanupLPads.size() - NumRemainingLPs;
209     NumCleanupLandingPadsRemaining -= CleanupLPads.size() - NumRemainingLPs;
210 #endif
211   }
212 
213   if (ResumesLeft == 0)
214     return true; // We pruned them all.
215 
216   // RewindFunction - _Unwind_Resume or the target equivalent.
217   FunctionCallee RewindFunction;
218   CallingConv::ID RewindFunctionCallingConv;
219   FunctionType *FTy;
220   const char *RewindName;
221   bool DoesRewindFunctionNeedExceptionObject;
222 
223   if ((Pers == EHPersonality::GNU_CXX || Pers == EHPersonality::GNU_CXX_SjLj) &&
224       TargetTriple.isTargetEHABICompatible()) {
225     RewindName = TLI.getLibcallName(RTLIB::CXA_END_CLEANUP);
226     FTy = FunctionType::get(Type::getVoidTy(Ctx), false);
227     RewindFunctionCallingConv =
228         TLI.getLibcallCallingConv(RTLIB::CXA_END_CLEANUP);
229     DoesRewindFunctionNeedExceptionObject = false;
230   } else {
231     RewindName = TLI.getLibcallName(RTLIB::UNWIND_RESUME);
232     FTy = FunctionType::get(Type::getVoidTy(Ctx), PointerType::getUnqual(Ctx),
233                             false);
234     RewindFunctionCallingConv = TLI.getLibcallCallingConv(RTLIB::UNWIND_RESUME);
235     DoesRewindFunctionNeedExceptionObject = true;
236   }
237   RewindFunction = F.getParent()->getOrInsertFunction(RewindName, FTy);
238 
239   // Create the basic block where the _Unwind_Resume call will live.
240   if (ResumesLeft == 1) {
241     // Instead of creating a new BB and PHI node, just append the call to
242     // _Unwind_Resume to the end of the single resume block.
243     ResumeInst *RI = Resumes.front();
244     BasicBlock *UnwindBB = RI->getParent();
245     Value *ExnObj = GetExceptionObject(RI);
246     llvm::SmallVector<Value *, 1> RewindFunctionArgs;
247     if (DoesRewindFunctionNeedExceptionObject)
248       RewindFunctionArgs.push_back(ExnObj);
249 
250     // Call the rewind function.
251     CallInst *CI =
252         CallInst::Create(RewindFunction, RewindFunctionArgs, "", UnwindBB);
253     // The verifier requires that all calls of debug-info-bearing functions
254     // from debug-info-bearing functions have a debug location (for inlining
255     // purposes). Assign a dummy location to satisfy the constraint.
256     Function *RewindFn = dyn_cast<Function>(RewindFunction.getCallee());
257     if (RewindFn && RewindFn->getSubprogram())
258       if (DISubprogram *SP = F.getSubprogram())
259         CI->setDebugLoc(DILocation::get(SP->getContext(), 0, 0, SP));
260     CI->setCallingConv(RewindFunctionCallingConv);
261 
262     // We never expect _Unwind_Resume to return.
263     CI->setDoesNotReturn();
264     new UnreachableInst(Ctx, UnwindBB);
265     return true;
266   }
267 
268   std::vector<DominatorTree::UpdateType> Updates;
269   Updates.reserve(Resumes.size());
270 
271   llvm::SmallVector<Value *, 1> RewindFunctionArgs;
272 
273   BasicBlock *UnwindBB = BasicBlock::Create(Ctx, "unwind_resume", &F);
274   PHINode *PN = PHINode::Create(PointerType::getUnqual(Ctx), ResumesLeft,
275                                 "exn.obj", UnwindBB);
276 
277   // Extract the exception object from the ResumeInst and add it to the PHI node
278   // that feeds the _Unwind_Resume call.
279   for (ResumeInst *RI : Resumes) {
280     BasicBlock *Parent = RI->getParent();
281     BranchInst::Create(UnwindBB, Parent);
282     Updates.push_back({DominatorTree::Insert, Parent, UnwindBB});
283 
284     Value *ExnObj = GetExceptionObject(RI);
285     PN->addIncoming(ExnObj, Parent);
286 
287     ++NumResumesLowered;
288   }
289 
290   if (DoesRewindFunctionNeedExceptionObject)
291     RewindFunctionArgs.push_back(PN);
292 
293   // Call the function.
294   CallInst *CI =
295       CallInst::Create(RewindFunction, RewindFunctionArgs, "", UnwindBB);
296   // The verifier requires that all calls of debug-info-bearing functions
297   // from debug-info-bearing functions have a debug location (for inlining
298   // purposes). Assign a dummy location to satisfy the constraint.
299   Function *RewindFn = dyn_cast<Function>(RewindFunction.getCallee());
300   if (RewindFn && RewindFn->getSubprogram())
301     if (DISubprogram *SP = F.getSubprogram())
302       CI->setDebugLoc(DILocation::get(SP->getContext(), 0, 0, SP));
303   CI->setCallingConv(RewindFunctionCallingConv);
304 
305   // We never expect _Unwind_Resume to return.
306   CI->setDoesNotReturn();
307   new UnreachableInst(Ctx, UnwindBB);
308 
309   if (DTU)
310     DTU->applyUpdates(Updates);
311 
312   return true;
313 }
314 
315 bool DwarfEHPrepare::run() {
316   bool Changed = InsertUnwindResumeCalls();
317 
318   return Changed;
319 }
320 
321 static bool prepareDwarfEH(CodeGenOptLevel OptLevel, Function &F,
322                            const TargetLowering &TLI, DominatorTree *DT,
323                            const TargetTransformInfo *TTI,
324                            const Triple &TargetTriple) {
325   DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
326 
327   return DwarfEHPrepare(OptLevel, F, TLI, DT ? &DTU : nullptr, TTI,
328                         TargetTriple)
329       .run();
330 }
331 
332 namespace {
333 
334 class DwarfEHPrepareLegacyPass : public FunctionPass {
335 
336   CodeGenOptLevel OptLevel;
337 
338 public:
339   static char ID; // Pass identification, replacement for typeid.
340 
341   DwarfEHPrepareLegacyPass(CodeGenOptLevel OptLevel = CodeGenOptLevel::Default)
342       : FunctionPass(ID), OptLevel(OptLevel) {}
343 
344   bool runOnFunction(Function &F) override {
345     const TargetMachine &TM =
346         getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
347     const TargetLowering &TLI = *TM.getSubtargetImpl(F)->getTargetLowering();
348     DominatorTree *DT = nullptr;
349     const TargetTransformInfo *TTI = nullptr;
350     if (auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>())
351       DT = &DTWP->getDomTree();
352     if (OptLevel != CodeGenOptLevel::None) {
353       if (!DT)
354         DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
355       TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
356     }
357     return prepareDwarfEH(OptLevel, F, TLI, DT, TTI, TM.getTargetTriple());
358   }
359 
360   void getAnalysisUsage(AnalysisUsage &AU) const override {
361     AU.addRequired<TargetPassConfig>();
362     AU.addRequired<TargetTransformInfoWrapperPass>();
363     if (OptLevel != CodeGenOptLevel::None) {
364       AU.addRequired<DominatorTreeWrapperPass>();
365       AU.addRequired<TargetTransformInfoWrapperPass>();
366     }
367     AU.addPreserved<DominatorTreeWrapperPass>();
368   }
369 
370   StringRef getPassName() const override {
371     return "Exception handling preparation";
372   }
373 };
374 
375 } // end anonymous namespace
376 
377 PreservedAnalyses DwarfEHPreparePass::run(Function &F,
378                                           FunctionAnalysisManager &FAM) {
379   const auto &TLI = *TM->getSubtargetImpl(F)->getTargetLowering();
380   auto *DT = FAM.getCachedResult<DominatorTreeAnalysis>(F);
381   const TargetTransformInfo *TTI = nullptr;
382   auto OptLevel = TM->getOptLevel();
383   if (OptLevel != CodeGenOptLevel::None) {
384     if (!DT)
385       DT = &FAM.getResult<DominatorTreeAnalysis>(F);
386     TTI = &FAM.getResult<TargetIRAnalysis>(F);
387   }
388   bool Changed =
389       prepareDwarfEH(OptLevel, F, TLI, DT, TTI, TM->getTargetTriple());
390 
391   if (!Changed)
392     return PreservedAnalyses::all();
393   PreservedAnalyses PA;
394   PA.preserve<DominatorTreeAnalysis>();
395   return PA;
396 }
397 
398 char DwarfEHPrepareLegacyPass::ID = 0;
399 
400 INITIALIZE_PASS_BEGIN(DwarfEHPrepareLegacyPass, DEBUG_TYPE,
401                       "Prepare DWARF exceptions", false, false)
402 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
403 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
404 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
405 INITIALIZE_PASS_END(DwarfEHPrepareLegacyPass, DEBUG_TYPE,
406                     "Prepare DWARF exceptions", false, false)
407 
408 FunctionPass *llvm::createDwarfEHPass(CodeGenOptLevel OptLevel) {
409   return new DwarfEHPrepareLegacyPass(OptLevel);
410 }
411