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:
DwarfEHPrepare(CodeGenOptLevel OptLevel_,Function & F_,const TargetLowering & TLI_,DomTreeUpdater * DTU_,const TargetTransformInfo * TTI_,const Triple & TargetTriple_)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
GetExceptionObject(ResumeInst * RI)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
pruneUnreachableResumes(SmallVectorImpl<ResumeInst * > & Resumes,SmallVectorImpl<LandingPadInst * > & CleanupLPads)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
InsertUnwindResumeCalls()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
run()314 bool DwarfEHPrepare::run() {
315 bool Changed = InsertUnwindResumeCalls();
316
317 return Changed;
318 }
319
prepareDwarfEH(CodeGenOptLevel OptLevel,Function & F,const TargetLowering & TLI,DominatorTree * DT,const TargetTransformInfo * TTI,const Triple & TargetTriple)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
DwarfEHPrepareLegacyPass(CodeGenOptLevel OptLevel=CodeGenOptLevel::Default)340 DwarfEHPrepareLegacyPass(CodeGenOptLevel OptLevel = CodeGenOptLevel::Default)
341 : FunctionPass(ID), OptLevel(OptLevel) {}
342
runOnFunction(Function & F)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
getAnalysisUsage(AnalysisUsage & AU) const359 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
getPassName() const369 StringRef getPassName() const override {
370 return "Exception handling preparation";
371 }
372 };
373
374 } // end anonymous namespace
375
run(Function & F,FunctionAnalysisManager & FAM)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)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)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