DwarfEHPrepare.cpp revision f2519d6193d736ee7f2b9cc24e0143cfa933f79e
1//===-- DwarfEHPrepare - Prepare exception handling for code generation ---===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This pass mulches exception handling code into a form adapted to code 11// generation. Required if using dwarf exception handling. 12// 13//===----------------------------------------------------------------------===// 14 15#define DEBUG_TYPE "dwarfehprepare" 16#include "llvm/ADT/Statistic.h" 17#include "llvm/Analysis/Dominators.h" 18#include "llvm/CodeGen/Passes.h" 19#include "llvm/Function.h" 20#include "llvm/Instructions.h" 21#include "llvm/IntrinsicInst.h" 22#include "llvm/Module.h" 23#include "llvm/Pass.h" 24#include "llvm/Support/Compiler.h" 25#include "llvm/Target/TargetLowering.h" 26#include "llvm/Transforms/Utils/BasicBlockUtils.h" 27#include "llvm/Transforms/Utils/PromoteMemToReg.h" 28using namespace llvm; 29 30STATISTIC(NumLandingPadsSplit, "Number of landing pads split"); 31STATISTIC(NumUnwindsLowered, "Number of unwind instructions lowered"); 32STATISTIC(NumExceptionValuesMoved, "Number of eh.exception calls moved"); 33STATISTIC(NumStackTempsIntroduced, "Number of stack temporaries introduced"); 34 35namespace { 36 class VISIBILITY_HIDDEN DwarfEHPrepare : public FunctionPass { 37 const TargetLowering *TLI; 38 bool CompileFast; 39 40 // The eh.exception intrinsic. 41 Function *ExceptionValueIntrinsic; 42 43 // _Unwind_Resume or the target equivalent. 44 Constant *RewindFunction; 45 46 // Dominator info is used when turning stack temporaries into registers. 47 DominatorTree *DT; 48 DominanceFrontier *DF; 49 50 // The function we are running on. 51 Function *F; 52 53 // The landing pads for this function. 54 typedef SmallPtrSet<BasicBlock*, 8> BBSet; 55 BBSet LandingPads; 56 57 // Stack temporary used to hold eh.exception values. 58 AllocaInst *ExceptionValueVar; 59 60 bool NormalizeLandingPads(); 61 bool LowerUnwinds(); 62 bool MoveExceptionValueCalls(); 63 bool FinishStackTemporaries(); 64 bool PromoteStackTemporaries(); 65 66 Instruction *CreateExceptionValueCall(BasicBlock *BB); 67 Instruction *CreateValueLoad(BasicBlock *BB); 68 69 /// CreateReadOfExceptionValue - Return the result of the eh.exception 70 /// intrinsic by calling the intrinsic if in a landing pad, or loading 71 /// it from the exception value variable otherwise. 72 Instruction *CreateReadOfExceptionValue(BasicBlock *BB) { 73 return LandingPads.count(BB) ? 74 CreateExceptionValueCall(BB) : CreateValueLoad(BB); 75 } 76 77 public: 78 static char ID; // Pass identification, replacement for typeid. 79 DwarfEHPrepare(const TargetLowering *tli, bool fast) : 80 FunctionPass(&ID), TLI(tli), CompileFast(fast), 81 ExceptionValueIntrinsic(0), RewindFunction(0) {} 82 83 virtual bool runOnFunction(Function &Fn); 84 85 // getAnalysisUsage - We need dominance frontiers for memory promotion. 86 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 87 if (!CompileFast) 88 AU.addRequired<DominatorTree>(); 89 AU.addPreserved<DominatorTree>(); 90 if (!CompileFast) 91 AU.addRequired<DominanceFrontier>(); 92 AU.addPreserved<DominanceFrontier>(); 93 } 94 95 const char *getPassName() const { 96 return "Exception handling preparation"; 97 } 98 99 }; 100} // end anonymous namespace 101 102char DwarfEHPrepare::ID = 0; 103 104FunctionPass *llvm::createDwarfEHPass(const TargetLowering *tli, bool fast) { 105 return new DwarfEHPrepare(tli, fast); 106} 107 108/// NormalizeLandingPads - Normalize and discover landing pads, noting them 109/// in the LandingPads set. A landing pad is normal if the only CFG edges 110/// that end at it are unwind edges from invoke instructions. If we inlined 111/// through an invoke we could have a normal branch from the previous 112/// unwind block through to the landing pad for the original invoke. 113/// Abnormal landing pads are fixed up by redirecting all unwind edges to 114/// a new basic block which falls through to the original. 115bool DwarfEHPrepare::NormalizeLandingPads() { 116 bool Changed = false; 117 118 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) { 119 TerminatorInst *TI = I->getTerminator(); 120 if (!isa<InvokeInst>(TI)) 121 continue; 122 BasicBlock *LPad = TI->getSuccessor(1); 123 // Skip landing pads that have already been normalized. 124 if (LandingPads.count(LPad)) 125 continue; 126 127 // Check that only invoke unwind edges end at the landing pad. 128 bool OnlyUnwoundTo = true; 129 for (pred_iterator PI = pred_begin(LPad), PE = pred_end(LPad); 130 PI != PE; ++PI) { 131 TerminatorInst *PT = (*PI)->getTerminator(); 132 if (!isa<InvokeInst>(PT) || LPad == PT->getSuccessor(0)) { 133 OnlyUnwoundTo = false; 134 break; 135 } 136 } 137 138 if (OnlyUnwoundTo) { 139 // Only unwind edges lead to the landing pad. Remember the landing pad. 140 LandingPads.insert(LPad); 141 continue; 142 } 143 144 // At least one normal edge ends at the landing pad. Redirect the unwind 145 // edges to a new basic block which falls through into this one. 146 147 // Create the new basic block. 148 BasicBlock *NewBB = BasicBlock::Create(F->getContext(), 149 LPad->getName() + "_unwind_edge"); 150 151 // Insert it into the function right before the original landing pad. 152 LPad->getParent()->getBasicBlockList().insert(LPad, NewBB); 153 154 // Redirect unwind edges from the original landing pad to NewBB. 155 for (pred_iterator PI = pred_begin(LPad), PE = pred_end(LPad); PI != PE; ) { 156 TerminatorInst *PT = (*PI++)->getTerminator(); 157 if (isa<InvokeInst>(PT) && PT->getSuccessor(1) == LPad) 158 // Unwind to the new block. 159 PT->setSuccessor(1, NewBB); 160 } 161 162 // If there are any PHI nodes in LPad, we need to update them so that they 163 // merge incoming values from NewBB instead. 164 for (BasicBlock::iterator II = LPad->begin(); isa<PHINode>(II); ++II) { 165 PHINode *PN = cast<PHINode>(II); 166 pred_iterator PB = pred_begin(NewBB), PE = pred_end(NewBB); 167 168 // Check to see if all of the values coming in via unwind edges are the 169 // same. If so, we don't need to create a new PHI node. 170 Value *InVal = PN->getIncomingValueForBlock(*PB); 171 for (pred_iterator PI = PB; PI != PE; ++PI) { 172 if (PI != PB && InVal != PN->getIncomingValueForBlock(*PI)) { 173 InVal = 0; 174 break; 175 } 176 } 177 178 if (InVal == 0) { 179 // Different unwind edges have different values. Create a new PHI node 180 // in NewBB. 181 PHINode *NewPN = PHINode::Create(PN->getType(), PN->getName()+".unwind", 182 NewBB); 183 // Add an entry for each unwind edge, using the value from the old PHI. 184 for (pred_iterator PI = PB; PI != PE; ++PI) 185 NewPN->addIncoming(PN->getIncomingValueForBlock(*PI), *PI); 186 187 // Now use this new PHI as the common incoming value for NewBB in PN. 188 InVal = NewPN; 189 } 190 191 // Revector exactly one entry in the PHI node to come from NewBB 192 // and delete all other entries that come from unwind edges. If 193 // there are both normal and unwind edges from the same predecessor, 194 // this leaves an entry for the normal edge. 195 for (pred_iterator PI = PB; PI != PE; ++PI) 196 PN->removeIncomingValue(*PI); 197 PN->addIncoming(InVal, NewBB); 198 } 199 200 // Add a fallthrough from NewBB to the original landing pad. 201 BranchInst::Create(LPad, NewBB); 202 203 // Now update DominatorTree and DominanceFrontier analysis information. 204 if (DT) 205 DT->splitBlock(NewBB); 206 if (DF) 207 DF->splitBlock(NewBB); 208 209 // Remember the newly constructed landing pad. The original landing pad 210 // LPad is no longer a landing pad now that all unwind edges have been 211 // revectored to NewBB. 212 LandingPads.insert(NewBB); 213 ++NumLandingPadsSplit; 214 Changed = true; 215 } 216 217 return Changed; 218} 219 220/// LowerUnwinds - Turn unwind instructions into calls to _Unwind_Resume, 221/// rethrowing any previously caught exception. This will crash horribly 222/// at runtime if there is no such exception: using unwind to throw a new 223/// exception is currently not supported. 224bool DwarfEHPrepare::LowerUnwinds() { 225 SmallVector<TerminatorInst*, 16> UnwindInsts; 226 227 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) { 228 TerminatorInst *TI = I->getTerminator(); 229 if (isa<UnwindInst>(TI)) 230 UnwindInsts.push_back(TI); 231 } 232 233 if (UnwindInsts.empty()) return false; 234 235 // Find the rewind function if we didn't already. 236 if (!RewindFunction) { 237 LLVMContext &Ctx = UnwindInsts[0]->getContext(); 238 std::vector<const Type*> 239 Params(1, Type::getInt8PtrTy(Ctx)); 240 FunctionType *FTy = FunctionType::get(Type::getVoidTy(Ctx), 241 Params, false); 242 const char *RewindName = TLI->getLibcallName(RTLIB::UNWIND_RESUME); 243 RewindFunction = F->getParent()->getOrInsertFunction(RewindName, FTy); 244 } 245 246 bool Changed = false; 247 248 for (SmallVectorImpl<TerminatorInst*>::iterator 249 I = UnwindInsts.begin(), E = UnwindInsts.end(); I != E; ++I) { 250 TerminatorInst *TI = *I; 251 252 // Replace the unwind instruction with a call to _Unwind_Resume (or the 253 // appropriate target equivalent) followed by an UnreachableInst. 254 255 // Create the call... 256 CallInst *CI = CallInst::Create(RewindFunction, 257 CreateReadOfExceptionValue(TI->getParent()), 258 "", TI); 259 CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME)); 260 // ...followed by an UnreachableInst. 261 new UnreachableInst(TI->getContext(), TI); 262 263 // Nuke the unwind instruction. 264 TI->eraseFromParent(); 265 ++NumUnwindsLowered; 266 Changed = true; 267 } 268 269 return Changed; 270} 271 272/// MoveExceptionValueCalls - Ensure that eh.exception is only ever called from 273/// landing pads by replacing calls outside of landing pads with loads from a 274/// stack temporary. Move eh.exception calls inside landing pads to the start 275/// of the landing pad (optional, but may make things simpler for later passes). 276bool DwarfEHPrepare::MoveExceptionValueCalls() { 277 // If the eh.exception intrinsic is not declared in the module then there is 278 // nothing to do. Speed up compilation by checking for this common case. 279 if (!ExceptionValueIntrinsic && 280 !F->getParent()->getFunction(Intrinsic::getName(Intrinsic::eh_exception))) 281 return false; 282 283 bool Changed = false; 284 285 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) { 286 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) 287 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) 288 if (CI->getIntrinsicID() == Intrinsic::eh_exception) { 289 if (!CI->use_empty()) { 290 Value *ExceptionValue = CreateReadOfExceptionValue(BB); 291 if (CI == ExceptionValue) { 292 // The call was at the start of a landing pad - leave it alone. 293 assert(LandingPads.count(BB) && 294 "Created eh.exception call outside landing pad!"); 295 continue; 296 } 297 CI->replaceAllUsesWith(ExceptionValue); 298 } 299 CI->eraseFromParent(); 300 ++NumExceptionValuesMoved; 301 Changed = true; 302 } 303 } 304 305 return Changed; 306} 307 308/// FinishStackTemporaries - If we introduced a stack variable to hold the 309/// exception value then initialize it in each landing pad. 310bool DwarfEHPrepare::FinishStackTemporaries() { 311 if (!ExceptionValueVar) 312 // Nothing to do. 313 return false; 314 315 bool Changed = false; 316 317 // Make sure that there is a store of the exception value at the start of 318 // each landing pad. 319 for (BBSet::iterator LI = LandingPads.begin(), LE = LandingPads.end(); 320 LI != LE; ++LI) { 321 Instruction *ExceptionValue = CreateReadOfExceptionValue(*LI); 322 Instruction *Store = new StoreInst(ExceptionValue, ExceptionValueVar); 323 Store->insertAfter(ExceptionValue); 324 Changed = true; 325 } 326 327 return Changed; 328} 329 330/// PromoteStackTemporaries - Turn any stack temporaries we introduced into 331/// registers if possible. 332bool DwarfEHPrepare::PromoteStackTemporaries() { 333 if (ExceptionValueVar && DT && DF && isAllocaPromotable(ExceptionValueVar)) { 334 // Turn the exception temporary into registers and phi nodes if possible. 335 std::vector<AllocaInst*> Allocas(1, ExceptionValueVar); 336 PromoteMemToReg(Allocas, *DT, *DF, ExceptionValueVar->getContext()); 337 return true; 338 } 339 return false; 340} 341 342/// CreateExceptionValueCall - Insert a call to the eh.exception intrinsic at 343/// the start of the basic block (unless there already is one, in which case 344/// the existing call is returned). 345Instruction *DwarfEHPrepare::CreateExceptionValueCall(BasicBlock *BB) { 346 Instruction *Start = BB->getFirstNonPHI(); 347 // Is this a call to eh.exception? 348 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(Start)) 349 if (CI->getIntrinsicID() == Intrinsic::eh_exception) 350 // Reuse the existing call. 351 return Start; 352 353 // Find the eh.exception intrinsic if we didn't already. 354 if (!ExceptionValueIntrinsic) 355 ExceptionValueIntrinsic = Intrinsic::getDeclaration(F->getParent(), 356 Intrinsic::eh_exception); 357 358 // Create the call. 359 return CallInst::Create(ExceptionValueIntrinsic, "eh.value.call", Start); 360} 361 362/// CreateValueLoad - Insert a load of the exception value stack variable 363/// (creating it if necessary) at the start of the basic block (unless 364/// there already is a load, in which case the existing load is returned). 365Instruction *DwarfEHPrepare::CreateValueLoad(BasicBlock *BB) { 366 Instruction *Start = BB->getFirstNonPHI(); 367 // Is this a load of the exception temporary? 368 if (ExceptionValueVar) 369 if (LoadInst* LI = dyn_cast<LoadInst>(Start)) 370 if (LI->getPointerOperand() == ExceptionValueVar) 371 // Reuse the existing load. 372 return Start; 373 374 // Create the temporary if we didn't already. 375 if (!ExceptionValueVar) { 376 ExceptionValueVar = new AllocaInst(PointerType::getUnqual( 377 Type::getInt8Ty(BB->getContext())), "eh.value", F->begin()->begin()); 378 ++NumStackTempsIntroduced; 379 } 380 381 // Load the value. 382 return new LoadInst(ExceptionValueVar, "eh.value.load", Start); 383} 384 385bool DwarfEHPrepare::runOnFunction(Function &Fn) { 386 bool Changed = false; 387 388 // Initialize internal state. 389 DT = getAnalysisIfAvailable<DominatorTree>(); 390 DF = getAnalysisIfAvailable<DominanceFrontier>(); 391 ExceptionValueVar = 0; 392 F = &Fn; 393 394 // Ensure that only unwind edges end at landing pads (a landing pad is a 395 // basic block where an invoke unwind edge ends). 396 Changed |= NormalizeLandingPads(); 397 398 // Turn unwind instructions into libcalls. 399 Changed |= LowerUnwinds(); 400 401 // TODO: Move eh.selector calls to landing pads and combine them. 402 403 // Move eh.exception calls to landing pads. 404 Changed |= MoveExceptionValueCalls(); 405 406 // Initialize any stack temporaries we introduced. 407 Changed |= FinishStackTemporaries(); 408 409 // Turn any stack temporaries into registers if possible. 410 if (!CompileFast) 411 Changed |= PromoteStackTemporaries(); 412 413 LandingPads.clear(); 414 415 return Changed; 416} 417