DwarfEHPrepare.cpp revision 24fd31656faf1f70c5e4ade5b6462b2e66465748
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. 111/// Abnormal landing pads are fixed up by redirecting all unwind edges to 112/// a new basic block which falls through to the original. 113bool DwarfEHPrepare::NormalizeLandingPads() { 114 bool Changed = false; 115 116 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) { 117 TerminatorInst *TI = I->getTerminator(); 118 if (!isa<InvokeInst>(TI)) 119 continue; 120 BasicBlock *LPad = TI->getSuccessor(1); 121 // Skip landing pads that have already been normalized. 122 if (LandingPads.count(LPad)) 123 continue; 124 125 // Check that only invoke unwind edges end at the landing pad. 126 bool OnlyUnwoundTo = true; 127 for (pred_iterator PI = pred_begin(LPad), PE = pred_end(LPad); 128 PI != PE; ++PI) { 129 TerminatorInst *PT = (*PI)->getTerminator(); 130 if (!isa<InvokeInst>(PT) || LPad == PT->getSuccessor(0)) { 131 OnlyUnwoundTo = false; 132 break; 133 } 134 } 135 if (OnlyUnwoundTo) { 136 // Only unwind edges lead to the landing pad. Remember the landing pad. 137 LandingPads.insert(LPad); 138 continue; 139 } 140 141 // At least one normal edge ends at the landing pad. Redirect the unwind 142 // edges to a new basic block which falls through into this one. 143 144 // Create the new basic block. 145 BasicBlock *NewBB = BasicBlock::Create(F->getContext(), 146 LPad->getName() + "_unwind_edge"); 147 148 // Insert it into the function right before the original landing pad. 149 LPad->getParent()->getBasicBlockList().insert(LPad, NewBB); 150 151 // Redirect unwind edges from the original landing pad to NewBB. 152 for (pred_iterator PI = pred_begin(LPad), PE = pred_end(LPad); PI != PE; ) { 153 TerminatorInst *PT = (*PI++)->getTerminator(); 154 if (isa<InvokeInst>(PT) && PT->getSuccessor(1) == LPad) 155 // Unwind to the new block. 156 PT->setSuccessor(1, NewBB); 157 } 158 159 // If there are any PHI nodes in LPad, we need to update them so that they 160 // merge incoming values from NewBB instead. 161 for (BasicBlock::iterator II = LPad->begin(); isa<PHINode>(II); ++II) { 162 PHINode *PN = cast<PHINode>(II); 163 pred_iterator PB = pred_begin(NewBB), PE = pred_end(NewBB); 164 165 // Check to see if all of the values coming in via unwind edges are the 166 // same. If so, we don't need to create a new PHI node. 167 Value *InVal = PN->getIncomingValueForBlock(*PB); 168 for (pred_iterator PI = PB; PI != PE; ++PI) { 169 if (PI != PB && InVal != PN->getIncomingValueForBlock(*PI)) { 170 InVal = 0; 171 break; 172 } 173 } 174 175 if (InVal == 0) { 176 // Different unwind edges have different values. Create a new PHI node 177 // in NewBB. 178 PHINode *NewPN = PHINode::Create(PN->getType(), PN->getName()+".unwind", 179 NewBB); 180 // Add an entry for each unwind edge, using the value from the old PHI. 181 for (pred_iterator PI = PB; PI != PE; ++PI) 182 NewPN->addIncoming(PN->getIncomingValueForBlock(*PI), *PI); 183 184 // Now use this new PHI as the common incoming value for NewBB in PN. 185 InVal = NewPN; 186 } 187 188 // Revector exactly one entry in the PHI node to come from NewBB 189 // and delete all other entries that come from unwind edges. If 190 // there are both normal and unwind edges from the same predecessor, 191 // this leaves an entry for the normal edge. 192 for (pred_iterator PI = PB; PI != PE; ++PI) 193 PN->removeIncomingValue(*PI); 194 PN->addIncoming(InVal, NewBB); 195 } 196 197 // Add a fallthrough from NewBB to the original landing pad. 198 BranchInst::Create(LPad, NewBB); 199 200 // Now update DominatorTree and DominanceFrontier analysis information. 201 if (DT) 202 DT->splitBlock(NewBB); 203 if (DF) 204 DF->splitBlock(NewBB); 205 206 // Remember the newly constructed landing pad. The original landing pad 207 // LPad is no longer a landing pad now that all unwind edges have been 208 // revectored to NewBB. 209 LandingPads.insert(NewBB); 210 ++NumLandingPadsSplit; 211 Changed = true; 212 } 213 214 return Changed; 215} 216 217/// LowerUnwinds - Turn unwind instructions into calls to _Unwind_Resume, 218/// rethrowing any previously caught exception. This will crash horribly 219/// at runtime if there is no such exception: using unwind to throw a new 220/// exception is currently not supported. 221bool DwarfEHPrepare::LowerUnwinds() { 222 bool Changed = false; 223 224 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) { 225 TerminatorInst *TI = I->getTerminator(); 226 if (!isa<UnwindInst>(TI)) 227 continue; 228 229 // Replace the unwind instruction with a call to _Unwind_Resume (or the 230 // appropriate target equivalent) followed by an UnreachableInst. 231 232 // Find the rewind function if we didn't already. 233 if (!RewindFunction) { 234 std::vector<const Type*> Params(1, 235 PointerType::getUnqual(Type::getInt8Ty(TI->getContext()))); 236 FunctionType *FTy = FunctionType::get(Type::getVoidTy(TI->getContext()), 237 Params, false); 238 const char *RewindName = TLI->getLibcallName(RTLIB::UNWIND_RESUME); 239 RewindFunction = F->getParent()->getOrInsertFunction(RewindName, FTy); 240 } 241 242 // Create the call... 243 CallInst *CI = CallInst::Create(RewindFunction, 244 CreateReadOfExceptionValue(I), "", TI); 245 CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME)); 246 // ...followed by an UnreachableInst. 247 new UnreachableInst(TI->getContext(), TI); 248 249 // Nuke the unwind instruction. 250 TI->eraseFromParent(); 251 ++NumUnwindsLowered; 252 Changed = true; 253 } 254 255 return Changed; 256} 257 258/// MoveExceptionValueCalls - Ensure that eh.exception is only ever called from 259/// landing pads by replacing calls outside of landing pads with loads from a 260/// stack temporary. Move eh.exception calls inside landing pads to the start 261/// of the landing pad (optional, but may make things simpler for later passes). 262bool DwarfEHPrepare::MoveExceptionValueCalls() { 263 // If the eh.exception intrinsic is not declared in the module then there is 264 // nothing to do. Speed up compilation by checking for this common case. 265 if (!ExceptionValueIntrinsic && 266 !F->getParent()->getFunction(Intrinsic::getName(Intrinsic::eh_exception))) 267 return false; 268 269 bool Changed = false; 270 271 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) { 272 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) 273 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) 274 if (CI->getIntrinsicID() == Intrinsic::eh_exception) { 275 if (!CI->use_empty()) { 276 Value *ExceptionValue = CreateReadOfExceptionValue(BB); 277 if (CI == ExceptionValue) { 278 // The call was at the start of a landing pad - leave it alone. 279 assert(LandingPads.count(BB) && 280 "Created eh.exception call outside landing pad!"); 281 continue; 282 } 283 CI->replaceAllUsesWith(ExceptionValue); 284 } 285 CI->eraseFromParent(); 286 ++NumExceptionValuesMoved; 287 Changed = true; 288 } 289 } 290 291 return Changed; 292} 293 294/// FinishStackTemporaries - If we introduced a stack variable to hold the 295/// exception value then initialize it in each landing pad. 296bool DwarfEHPrepare::FinishStackTemporaries() { 297 if (!ExceptionValueVar) 298 // Nothing to do. 299 return false; 300 301 bool Changed = false; 302 303 // Make sure that there is a store of the exception value at the start of 304 // each landing pad. 305 for (BBSet::iterator LI = LandingPads.begin(), LE = LandingPads.end(); 306 LI != LE; ++LI) { 307 Instruction *ExceptionValue = CreateReadOfExceptionValue(*LI); 308 Instruction *Store = new StoreInst(ExceptionValue, ExceptionValueVar); 309 Store->insertAfter(ExceptionValue); 310 Changed = true; 311 } 312 313 return Changed; 314} 315 316/// PromoteStackTemporaries - Turn any stack temporaries we introduced into 317/// registers if possible. 318bool DwarfEHPrepare::PromoteStackTemporaries() { 319 if (ExceptionValueVar && DT && DF && isAllocaPromotable(ExceptionValueVar)) { 320 // Turn the exception temporary into registers and phi nodes if possible. 321 std::vector<AllocaInst*> Allocas(1, ExceptionValueVar); 322 PromoteMemToReg(Allocas, *DT, *DF, ExceptionValueVar->getContext()); 323 return true; 324 } 325 return false; 326} 327 328/// CreateExceptionValueCall - Insert a call to the eh.exception intrinsic at 329/// the start of the basic block (unless there already is one, in which case 330/// the existing call is returned). 331Instruction *DwarfEHPrepare::CreateExceptionValueCall(BasicBlock *BB) { 332 Instruction *Start = BB->getFirstNonPHI(); 333 // Is this a call to eh.exception? 334 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(Start)) 335 if (CI->getIntrinsicID() == Intrinsic::eh_exception) 336 // Reuse the existing call. 337 return Start; 338 339 // Find the eh.exception intrinsic if we didn't already. 340 if (!ExceptionValueIntrinsic) 341 ExceptionValueIntrinsic = Intrinsic::getDeclaration(F->getParent(), 342 Intrinsic::eh_exception); 343 344 // Create the call. 345 return CallInst::Create(ExceptionValueIntrinsic, "eh.value.call", Start); 346} 347 348/// CreateValueLoad - Insert a load of the exception value stack variable 349/// (creating it if necessary) at the start of the basic block (unless 350/// there already is a load, in which case the existing load is returned). 351Instruction *DwarfEHPrepare::CreateValueLoad(BasicBlock *BB) { 352 Instruction *Start = BB->getFirstNonPHI(); 353 // Is this a load of the exception temporary? 354 if (ExceptionValueVar) 355 if (LoadInst* LI = dyn_cast<LoadInst>(Start)) 356 if (LI->getPointerOperand() == ExceptionValueVar) 357 // Reuse the existing load. 358 return Start; 359 360 // Create the temporary if we didn't already. 361 if (!ExceptionValueVar) { 362 ExceptionValueVar = new AllocaInst(PointerType::getUnqual( 363 Type::getInt8Ty(BB->getContext())), "eh.value", F->begin()->begin()); 364 ++NumStackTempsIntroduced; 365 } 366 367 // Load the value. 368 return new LoadInst(ExceptionValueVar, "eh.value.load", Start); 369} 370 371bool DwarfEHPrepare::runOnFunction(Function &Fn) { 372 bool Changed = false; 373 374 // Initialize internal state. 375 DT = getAnalysisIfAvailable<DominatorTree>(); 376 DF = getAnalysisIfAvailable<DominanceFrontier>(); 377 ExceptionValueVar = 0; 378 F = &Fn; 379 380 // Ensure that only unwind edges end at landing pads (a landing pad is a 381 // basic block where an invoke unwind edge ends). 382 Changed |= NormalizeLandingPads(); 383 384 // Turn unwind instructions into libcalls. 385 Changed |= LowerUnwinds(); 386 387 // TODO: Move eh.selector calls to landing pads and combine them. 388 389 // Move eh.exception calls to landing pads. 390 Changed |= MoveExceptionValueCalls(); 391 392 // Initialize any stack temporaries we introduced. 393 Changed |= FinishStackTemporaries(); 394 395 // Turn any stack temporaries into registers if possible. 396 if (!CompileFast) 397 Changed |= PromoteStackTemporaries(); 398 399 LandingPads.clear(); 400 401 return Changed; 402} 403