1 //===- Dominators.cpp - Dominator Calculation -----------------------------===// 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 file implements simple dominator construction algorithms for finding 11 // forward dominators. Postdominators are available in libanalysis, but are not 12 // included in libvmcore, because it's not needed. Forward dominators are 13 // needed to support the Verifier pass. 14 // 15 //===----------------------------------------------------------------------===// 16 17 #include "llvm/IR/Dominators.h" 18 #include "llvm/ADT/DepthFirstIterator.h" 19 #include "llvm/ADT/SmallPtrSet.h" 20 #include "llvm/IR/CFG.h" 21 #include "llvm/IR/Instructions.h" 22 #include "llvm/IR/PassManager.h" 23 #include "llvm/Support/CommandLine.h" 24 #include "llvm/Support/Debug.h" 25 #include "llvm/Support/GenericDomTreeConstruction.h" 26 #include "llvm/Support/raw_ostream.h" 27 #include <algorithm> 28 using namespace llvm; 29 30 // Always verify dominfo if expensive checking is enabled. 31 #ifdef EXPENSIVE_CHECKS 32 static bool VerifyDomInfo = true; 33 #else 34 static bool VerifyDomInfo = false; 35 #endif 36 static cl::opt<bool,true> 37 VerifyDomInfoX("verify-dom-info", cl::location(VerifyDomInfo), 38 cl::desc("Verify dominator info (time consuming)")); 39 40 bool BasicBlockEdge::isSingleEdge() const { 41 const TerminatorInst *TI = Start->getTerminator(); 42 unsigned NumEdgesToEnd = 0; 43 for (unsigned int i = 0, n = TI->getNumSuccessors(); i < n; ++i) { 44 if (TI->getSuccessor(i) == End) 45 ++NumEdgesToEnd; 46 if (NumEdgesToEnd >= 2) 47 return false; 48 } 49 assert(NumEdgesToEnd == 1); 50 return true; 51 } 52 53 //===----------------------------------------------------------------------===// 54 // DominatorTree Implementation 55 //===----------------------------------------------------------------------===// 56 // 57 // Provide public access to DominatorTree information. Implementation details 58 // can be found in Dominators.h, GenericDomTree.h, and 59 // GenericDomTreeConstruction.h. 60 // 61 //===----------------------------------------------------------------------===// 62 63 template class llvm::DomTreeNodeBase<BasicBlock>; 64 template class llvm::DominatorTreeBase<BasicBlock>; 65 66 template void llvm::Calculate<Function, BasicBlock *>( 67 DominatorTreeBase<GraphTraits<BasicBlock *>::NodeType> &DT, Function &F); 68 template void llvm::Calculate<Function, Inverse<BasicBlock *>>( 69 DominatorTreeBase<GraphTraits<Inverse<BasicBlock *>>::NodeType> &DT, 70 Function &F); 71 72 // dominates - Return true if Def dominates a use in User. This performs 73 // the special checks necessary if Def and User are in the same basic block. 74 // Note that Def doesn't dominate a use in Def itself! 75 bool DominatorTree::dominates(const Instruction *Def, 76 const Instruction *User) const { 77 const BasicBlock *UseBB = User->getParent(); 78 const BasicBlock *DefBB = Def->getParent(); 79 80 // Any unreachable use is dominated, even if Def == User. 81 if (!isReachableFromEntry(UseBB)) 82 return true; 83 84 // Unreachable definitions don't dominate anything. 85 if (!isReachableFromEntry(DefBB)) 86 return false; 87 88 // An instruction doesn't dominate a use in itself. 89 if (Def == User) 90 return false; 91 92 // The value defined by an invoke dominates an instruction only if it 93 // dominates every instruction in UseBB. 94 // A PHI is dominated only if the instruction dominates every possible use in 95 // the UseBB. 96 if (isa<InvokeInst>(Def) || isa<PHINode>(User)) 97 return dominates(Def, UseBB); 98 99 if (DefBB != UseBB) 100 return dominates(DefBB, UseBB); 101 102 // Loop through the basic block until we find Def or User. 103 BasicBlock::const_iterator I = DefBB->begin(); 104 for (; &*I != Def && &*I != User; ++I) 105 /*empty*/; 106 107 return &*I == Def; 108 } 109 110 // true if Def would dominate a use in any instruction in UseBB. 111 // note that dominates(Def, Def->getParent()) is false. 112 bool DominatorTree::dominates(const Instruction *Def, 113 const BasicBlock *UseBB) const { 114 const BasicBlock *DefBB = Def->getParent(); 115 116 // Any unreachable use is dominated, even if DefBB == UseBB. 117 if (!isReachableFromEntry(UseBB)) 118 return true; 119 120 // Unreachable definitions don't dominate anything. 121 if (!isReachableFromEntry(DefBB)) 122 return false; 123 124 if (DefBB == UseBB) 125 return false; 126 127 // Invoke results are only usable in the normal destination, not in the 128 // exceptional destination. 129 if (const auto *II = dyn_cast<InvokeInst>(Def)) { 130 BasicBlock *NormalDest = II->getNormalDest(); 131 BasicBlockEdge E(DefBB, NormalDest); 132 return dominates(E, UseBB); 133 } 134 135 return dominates(DefBB, UseBB); 136 } 137 138 bool DominatorTree::dominates(const BasicBlockEdge &BBE, 139 const BasicBlock *UseBB) const { 140 // Assert that we have a single edge. We could handle them by simply 141 // returning false, but since isSingleEdge is linear on the number of 142 // edges, the callers can normally handle them more efficiently. 143 assert(BBE.isSingleEdge() && 144 "This function is not efficient in handling multiple edges"); 145 146 // If the BB the edge ends in doesn't dominate the use BB, then the 147 // edge also doesn't. 148 const BasicBlock *Start = BBE.getStart(); 149 const BasicBlock *End = BBE.getEnd(); 150 if (!dominates(End, UseBB)) 151 return false; 152 153 // Simple case: if the end BB has a single predecessor, the fact that it 154 // dominates the use block implies that the edge also does. 155 if (End->getSinglePredecessor()) 156 return true; 157 158 // The normal edge from the invoke is critical. Conceptually, what we would 159 // like to do is split it and check if the new block dominates the use. 160 // With X being the new block, the graph would look like: 161 // 162 // DefBB 163 // /\ . . 164 // / \ . . 165 // / \ . . 166 // / \ | | 167 // A X B C 168 // | \ | / 169 // . \|/ 170 // . NormalDest 171 // . 172 // 173 // Given the definition of dominance, NormalDest is dominated by X iff X 174 // dominates all of NormalDest's predecessors (X, B, C in the example). X 175 // trivially dominates itself, so we only have to find if it dominates the 176 // other predecessors. Since the only way out of X is via NormalDest, X can 177 // only properly dominate a node if NormalDest dominates that node too. 178 for (const_pred_iterator PI = pred_begin(End), E = pred_end(End); 179 PI != E; ++PI) { 180 const BasicBlock *BB = *PI; 181 if (BB == Start) 182 continue; 183 184 if (!dominates(End, BB)) 185 return false; 186 } 187 return true; 188 } 189 190 bool DominatorTree::dominates(const BasicBlockEdge &BBE, const Use &U) const { 191 // Assert that we have a single edge. We could handle them by simply 192 // returning false, but since isSingleEdge is linear on the number of 193 // edges, the callers can normally handle them more efficiently. 194 assert(BBE.isSingleEdge() && 195 "This function is not efficient in handling multiple edges"); 196 197 Instruction *UserInst = cast<Instruction>(U.getUser()); 198 // A PHI in the end of the edge is dominated by it. 199 PHINode *PN = dyn_cast<PHINode>(UserInst); 200 if (PN && PN->getParent() == BBE.getEnd() && 201 PN->getIncomingBlock(U) == BBE.getStart()) 202 return true; 203 204 // Otherwise use the edge-dominates-block query, which 205 // handles the crazy critical edge cases properly. 206 const BasicBlock *UseBB; 207 if (PN) 208 UseBB = PN->getIncomingBlock(U); 209 else 210 UseBB = UserInst->getParent(); 211 return dominates(BBE, UseBB); 212 } 213 214 bool DominatorTree::dominates(const Instruction *Def, const Use &U) const { 215 Instruction *UserInst = cast<Instruction>(U.getUser()); 216 const BasicBlock *DefBB = Def->getParent(); 217 218 // Determine the block in which the use happens. PHI nodes use 219 // their operands on edges; simulate this by thinking of the use 220 // happening at the end of the predecessor block. 221 const BasicBlock *UseBB; 222 if (PHINode *PN = dyn_cast<PHINode>(UserInst)) 223 UseBB = PN->getIncomingBlock(U); 224 else 225 UseBB = UserInst->getParent(); 226 227 // Any unreachable use is dominated, even if Def == User. 228 if (!isReachableFromEntry(UseBB)) 229 return true; 230 231 // Unreachable definitions don't dominate anything. 232 if (!isReachableFromEntry(DefBB)) 233 return false; 234 235 // Invoke instructions define their return values on the edges to their normal 236 // successors, so we have to handle them specially. 237 // Among other things, this means they don't dominate anything in 238 // their own block, except possibly a phi, so we don't need to 239 // walk the block in any case. 240 if (const InvokeInst *II = dyn_cast<InvokeInst>(Def)) { 241 BasicBlock *NormalDest = II->getNormalDest(); 242 BasicBlockEdge E(DefBB, NormalDest); 243 return dominates(E, U); 244 } 245 246 // If the def and use are in different blocks, do a simple CFG dominator 247 // tree query. 248 if (DefBB != UseBB) 249 return dominates(DefBB, UseBB); 250 251 // Ok, def and use are in the same block. If the def is an invoke, it 252 // doesn't dominate anything in the block. If it's a PHI, it dominates 253 // everything in the block. 254 if (isa<PHINode>(UserInst)) 255 return true; 256 257 // Otherwise, just loop through the basic block until we find Def or User. 258 BasicBlock::const_iterator I = DefBB->begin(); 259 for (; &*I != Def && &*I != UserInst; ++I) 260 /*empty*/; 261 262 return &*I != UserInst; 263 } 264 265 bool DominatorTree::isReachableFromEntry(const Use &U) const { 266 Instruction *I = dyn_cast<Instruction>(U.getUser()); 267 268 // ConstantExprs aren't really reachable from the entry block, but they 269 // don't need to be treated like unreachable code either. 270 if (!I) return true; 271 272 // PHI nodes use their operands on their incoming edges. 273 if (PHINode *PN = dyn_cast<PHINode>(I)) 274 return isReachableFromEntry(PN->getIncomingBlock(U)); 275 276 // Everything else uses their operands in their own block. 277 return isReachableFromEntry(I->getParent()); 278 } 279 280 void DominatorTree::verifyDomTree() const { 281 Function &F = *getRoot()->getParent(); 282 283 DominatorTree OtherDT; 284 OtherDT.recalculate(F); 285 if (compare(OtherDT)) { 286 errs() << "DominatorTree is not up to date!\nComputed:\n"; 287 print(errs()); 288 errs() << "\nActual:\n"; 289 OtherDT.print(errs()); 290 abort(); 291 } 292 } 293 294 //===----------------------------------------------------------------------===// 295 // DominatorTreeAnalysis and related pass implementations 296 //===----------------------------------------------------------------------===// 297 // 298 // This implements the DominatorTreeAnalysis which is used with the new pass 299 // manager. It also implements some methods from utility passes. 300 // 301 //===----------------------------------------------------------------------===// 302 303 DominatorTree DominatorTreeAnalysis::run(Function &F, 304 AnalysisManager<Function> &) { 305 DominatorTree DT; 306 DT.recalculate(F); 307 return DT; 308 } 309 310 char DominatorTreeAnalysis::PassID; 311 312 DominatorTreePrinterPass::DominatorTreePrinterPass(raw_ostream &OS) : OS(OS) {} 313 314 PreservedAnalyses DominatorTreePrinterPass::run(Function &F, 315 FunctionAnalysisManager &AM) { 316 OS << "DominatorTree for function: " << F.getName() << "\n"; 317 AM.getResult<DominatorTreeAnalysis>(F).print(OS); 318 319 return PreservedAnalyses::all(); 320 } 321 322 PreservedAnalyses DominatorTreeVerifierPass::run(Function &F, 323 FunctionAnalysisManager &AM) { 324 AM.getResult<DominatorTreeAnalysis>(F).verifyDomTree(); 325 326 return PreservedAnalyses::all(); 327 } 328 329 //===----------------------------------------------------------------------===// 330 // DominatorTreeWrapperPass Implementation 331 //===----------------------------------------------------------------------===// 332 // 333 // The implementation details of the wrapper pass that holds a DominatorTree 334 // suitable for use with the legacy pass manager. 335 // 336 //===----------------------------------------------------------------------===// 337 338 char DominatorTreeWrapperPass::ID = 0; 339 INITIALIZE_PASS(DominatorTreeWrapperPass, "domtree", 340 "Dominator Tree Construction", true, true) 341 342 bool DominatorTreeWrapperPass::runOnFunction(Function &F) { 343 DT.recalculate(F); 344 return false; 345 } 346 347 void DominatorTreeWrapperPass::verifyAnalysis() const { 348 if (VerifyDomInfo) 349 DT.verifyDomTree(); 350 } 351 352 void DominatorTreeWrapperPass::print(raw_ostream &OS, const Module *) const { 353 DT.print(OS); 354 } 355 356