DwarfEHPrepare.cpp revision 1d0be15f89cb5056e20e2d24faa8d6afb1573bca
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::Create(RewindFunction, CreateReadOfExceptionValue(I), "", TI); 244 // ...followed by an UnreachableInst. 245 new UnreachableInst(TI->getContext(), TI); 246 247 // Nuke the unwind instruction. 248 TI->eraseFromParent(); 249 ++NumUnwindsLowered; 250 Changed = true; 251 } 252 253 return Changed; 254} 255 256/// MoveExceptionValueCalls - Ensure that eh.exception is only ever called from 257/// landing pads by replacing calls outside of landing pads with loads from a 258/// stack temporary. Move eh.exception calls inside landing pads to the start 259/// of the landing pad (optional, but may make things simpler for later passes). 260bool DwarfEHPrepare::MoveExceptionValueCalls() { 261 // If the eh.exception intrinsic is not declared in the module then there is 262 // nothing to do. Speed up compilation by checking for this common case. 263 if (!ExceptionValueIntrinsic && 264 !F->getParent()->getFunction(Intrinsic::getName(Intrinsic::eh_exception))) 265 return false; 266 267 bool Changed = false; 268 269 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) { 270 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) 271 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) 272 if (CI->getIntrinsicID() == Intrinsic::eh_exception) { 273 if (!CI->use_empty()) { 274 Value *ExceptionValue = CreateReadOfExceptionValue(BB); 275 if (CI == ExceptionValue) { 276 // The call was at the start of a landing pad - leave it alone. 277 assert(LandingPads.count(BB) && 278 "Created eh.exception call outside landing pad!"); 279 continue; 280 } 281 CI->replaceAllUsesWith(ExceptionValue); 282 } 283 CI->eraseFromParent(); 284 ++NumExceptionValuesMoved; 285 Changed = true; 286 } 287 } 288 289 return Changed; 290} 291 292/// FinishStackTemporaries - If we introduced a stack variable to hold the 293/// exception value then initialize it in each landing pad. 294bool DwarfEHPrepare::FinishStackTemporaries() { 295 if (!ExceptionValueVar) 296 // Nothing to do. 297 return false; 298 299 bool Changed = false; 300 301 // Make sure that there is a store of the exception value at the start of 302 // each landing pad. 303 for (BBSet::iterator LI = LandingPads.begin(), LE = LandingPads.end(); 304 LI != LE; ++LI) { 305 Instruction *ExceptionValue = CreateReadOfExceptionValue(*LI); 306 Instruction *Store = new StoreInst(ExceptionValue, ExceptionValueVar); 307 Store->insertAfter(ExceptionValue); 308 Changed = true; 309 } 310 311 return Changed; 312} 313 314/// PromoteStackTemporaries - Turn any stack temporaries we introduced into 315/// registers if possible. 316bool DwarfEHPrepare::PromoteStackTemporaries() { 317 if (ExceptionValueVar && DT && DF && isAllocaPromotable(ExceptionValueVar)) { 318 // Turn the exception temporary into registers and phi nodes if possible. 319 std::vector<AllocaInst*> Allocas(1, ExceptionValueVar); 320 PromoteMemToReg(Allocas, *DT, *DF, ExceptionValueVar->getContext()); 321 return true; 322 } 323 return false; 324} 325 326/// CreateExceptionValueCall - Insert a call to the eh.exception intrinsic at 327/// the start of the basic block (unless there already is one, in which case 328/// the existing call is returned). 329Instruction *DwarfEHPrepare::CreateExceptionValueCall(BasicBlock *BB) { 330 Instruction *Start = BB->getFirstNonPHI(); 331 // Is this a call to eh.exception? 332 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(Start)) 333 if (CI->getIntrinsicID() == Intrinsic::eh_exception) 334 // Reuse the existing call. 335 return Start; 336 337 // Find the eh.exception intrinsic if we didn't already. 338 if (!ExceptionValueIntrinsic) 339 ExceptionValueIntrinsic = Intrinsic::getDeclaration(F->getParent(), 340 Intrinsic::eh_exception); 341 342 // Create the call. 343 return CallInst::Create(ExceptionValueIntrinsic, "eh.value.call", Start); 344} 345 346/// CreateValueLoad - Insert a load of the exception value stack variable 347/// (creating it if necessary) at the start of the basic block (unless 348/// there already is a load, in which case the existing load is returned). 349Instruction *DwarfEHPrepare::CreateValueLoad(BasicBlock *BB) { 350 Instruction *Start = BB->getFirstNonPHI(); 351 // Is this a load of the exception temporary? 352 if (ExceptionValueVar) 353 if (LoadInst* LI = dyn_cast<LoadInst>(Start)) 354 if (LI->getPointerOperand() == ExceptionValueVar) 355 // Reuse the existing load. 356 return Start; 357 358 // Create the temporary if we didn't already. 359 if (!ExceptionValueVar) { 360 ExceptionValueVar = new AllocaInst(PointerType::getUnqual( 361 Type::getInt8Ty(BB->getContext())), "eh.value", F->begin()->begin()); 362 ++NumStackTempsIntroduced; 363 } 364 365 // Load the value. 366 return new LoadInst(ExceptionValueVar, "eh.value.load", Start); 367} 368 369bool DwarfEHPrepare::runOnFunction(Function &Fn) { 370 bool Changed = false; 371 372 // Initialize internal state. 373 DT = getAnalysisIfAvailable<DominatorTree>(); 374 DF = getAnalysisIfAvailable<DominanceFrontier>(); 375 ExceptionValueVar = 0; 376 F = &Fn; 377 378 // Ensure that only unwind edges end at landing pads (a landing pad is a 379 // basic block where an invoke unwind edge ends). 380 Changed |= NormalizeLandingPads(); 381 382 // Turn unwind instructions into libcalls. 383 Changed |= LowerUnwinds(); 384 385 // TODO: Move eh.selector calls to landing pads and combine them. 386 387 // Move eh.exception calls to landing pads. 388 Changed |= MoveExceptionValueCalls(); 389 390 // Initialize any stack temporaries we introduced. 391 Changed |= FinishStackTemporaries(); 392 393 // Turn any stack temporaries into registers if possible. 394 if (!CompileFast) 395 Changed |= PromoteStackTemporaries(); 396 397 LandingPads.clear(); 398 399 return Changed; 400} 401