Dominators.cpp revision 314564
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> 28using namespace llvm; 29 30// Always verify dominfo if expensive checking is enabled. 31#ifdef EXPENSIVE_CHECKS 32static bool VerifyDomInfo = true; 33#else 34static bool VerifyDomInfo = false; 35#endif 36static cl::opt<bool,true> 37VerifyDomInfoX("verify-dom-info", cl::location(VerifyDomInfo), 38 cl::desc("Verify dominator info (time consuming)")); 39 40bool 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 63template class llvm::DomTreeNodeBase<BasicBlock>; 64template class llvm::DominatorTreeBase<BasicBlock>; 65 66template void llvm::Calculate<Function, BasicBlock *>( 67 DominatorTreeBase< 68 typename std::remove_pointer<GraphTraits<BasicBlock *>::NodeRef>::type> 69 &DT, 70 Function &F); 71template void llvm::Calculate<Function, Inverse<BasicBlock *>>( 72 DominatorTreeBase<typename std::remove_pointer< 73 GraphTraits<Inverse<BasicBlock *>>::NodeRef>::type> &DT, 74 Function &F); 75 76// dominates - Return true if Def dominates a use in User. This performs 77// the special checks necessary if Def and User are in the same basic block. 78// Note that Def doesn't dominate a use in Def itself! 79bool DominatorTree::dominates(const Instruction *Def, 80 const Instruction *User) const { 81 const BasicBlock *UseBB = User->getParent(); 82 const BasicBlock *DefBB = Def->getParent(); 83 84 // Any unreachable use is dominated, even if Def == User. 85 if (!isReachableFromEntry(UseBB)) 86 return true; 87 88 // Unreachable definitions don't dominate anything. 89 if (!isReachableFromEntry(DefBB)) 90 return false; 91 92 // An instruction doesn't dominate a use in itself. 93 if (Def == User) 94 return false; 95 96 // The value defined by an invoke dominates an instruction only if it 97 // dominates every instruction in UseBB. 98 // A PHI is dominated only if the instruction dominates every possible use in 99 // the UseBB. 100 if (isa<InvokeInst>(Def) || isa<PHINode>(User)) 101 return dominates(Def, UseBB); 102 103 if (DefBB != UseBB) 104 return dominates(DefBB, UseBB); 105 106 // Loop through the basic block until we find Def or User. 107 BasicBlock::const_iterator I = DefBB->begin(); 108 for (; &*I != Def && &*I != User; ++I) 109 /*empty*/; 110 111 return &*I == Def; 112} 113 114// true if Def would dominate a use in any instruction in UseBB. 115// note that dominates(Def, Def->getParent()) is false. 116bool DominatorTree::dominates(const Instruction *Def, 117 const BasicBlock *UseBB) const { 118 const BasicBlock *DefBB = Def->getParent(); 119 120 // Any unreachable use is dominated, even if DefBB == UseBB. 121 if (!isReachableFromEntry(UseBB)) 122 return true; 123 124 // Unreachable definitions don't dominate anything. 125 if (!isReachableFromEntry(DefBB)) 126 return false; 127 128 if (DefBB == UseBB) 129 return false; 130 131 // Invoke results are only usable in the normal destination, not in the 132 // exceptional destination. 133 if (const auto *II = dyn_cast<InvokeInst>(Def)) { 134 BasicBlock *NormalDest = II->getNormalDest(); 135 BasicBlockEdge E(DefBB, NormalDest); 136 return dominates(E, UseBB); 137 } 138 139 return dominates(DefBB, UseBB); 140} 141 142bool DominatorTree::dominates(const BasicBlockEdge &BBE, 143 const BasicBlock *UseBB) const { 144 // Assert that we have a single edge. We could handle them by simply 145 // returning false, but since isSingleEdge is linear on the number of 146 // edges, the callers can normally handle them more efficiently. 147 assert(BBE.isSingleEdge() && 148 "This function is not efficient in handling multiple edges"); 149 150 // If the BB the edge ends in doesn't dominate the use BB, then the 151 // edge also doesn't. 152 const BasicBlock *Start = BBE.getStart(); 153 const BasicBlock *End = BBE.getEnd(); 154 if (!dominates(End, UseBB)) 155 return false; 156 157 // Simple case: if the end BB has a single predecessor, the fact that it 158 // dominates the use block implies that the edge also does. 159 if (End->getSinglePredecessor()) 160 return true; 161 162 // The normal edge from the invoke is critical. Conceptually, what we would 163 // like to do is split it and check if the new block dominates the use. 164 // With X being the new block, the graph would look like: 165 // 166 // DefBB 167 // /\ . . 168 // / \ . . 169 // / \ . . 170 // / \ | | 171 // A X B C 172 // | \ | / 173 // . \|/ 174 // . NormalDest 175 // . 176 // 177 // Given the definition of dominance, NormalDest is dominated by X iff X 178 // dominates all of NormalDest's predecessors (X, B, C in the example). X 179 // trivially dominates itself, so we only have to find if it dominates the 180 // other predecessors. Since the only way out of X is via NormalDest, X can 181 // only properly dominate a node if NormalDest dominates that node too. 182 for (const_pred_iterator PI = pred_begin(End), E = pred_end(End); 183 PI != E; ++PI) { 184 const BasicBlock *BB = *PI; 185 if (BB == Start) 186 continue; 187 188 if (!dominates(End, BB)) 189 return false; 190 } 191 return true; 192} 193 194bool DominatorTree::dominates(const BasicBlockEdge &BBE, const Use &U) const { 195 // Assert that we have a single edge. We could handle them by simply 196 // returning false, but since isSingleEdge is linear on the number of 197 // edges, the callers can normally handle them more efficiently. 198 assert(BBE.isSingleEdge() && 199 "This function is not efficient in handling multiple edges"); 200 201 Instruction *UserInst = cast<Instruction>(U.getUser()); 202 // A PHI in the end of the edge is dominated by it. 203 PHINode *PN = dyn_cast<PHINode>(UserInst); 204 if (PN && PN->getParent() == BBE.getEnd() && 205 PN->getIncomingBlock(U) == BBE.getStart()) 206 return true; 207 208 // Otherwise use the edge-dominates-block query, which 209 // handles the crazy critical edge cases properly. 210 const BasicBlock *UseBB; 211 if (PN) 212 UseBB = PN->getIncomingBlock(U); 213 else 214 UseBB = UserInst->getParent(); 215 return dominates(BBE, UseBB); 216} 217 218bool DominatorTree::dominates(const Instruction *Def, const Use &U) const { 219 Instruction *UserInst = cast<Instruction>(U.getUser()); 220 const BasicBlock *DefBB = Def->getParent(); 221 222 // Determine the block in which the use happens. PHI nodes use 223 // their operands on edges; simulate this by thinking of the use 224 // happening at the end of the predecessor block. 225 const BasicBlock *UseBB; 226 if (PHINode *PN = dyn_cast<PHINode>(UserInst)) 227 UseBB = PN->getIncomingBlock(U); 228 else 229 UseBB = UserInst->getParent(); 230 231 // Any unreachable use is dominated, even if Def == User. 232 if (!isReachableFromEntry(UseBB)) 233 return true; 234 235 // Unreachable definitions don't dominate anything. 236 if (!isReachableFromEntry(DefBB)) 237 return false; 238 239 // Invoke instructions define their return values on the edges to their normal 240 // successors, so we have to handle them specially. 241 // Among other things, this means they don't dominate anything in 242 // their own block, except possibly a phi, so we don't need to 243 // walk the block in any case. 244 if (const InvokeInst *II = dyn_cast<InvokeInst>(Def)) { 245 BasicBlock *NormalDest = II->getNormalDest(); 246 BasicBlockEdge E(DefBB, NormalDest); 247 return dominates(E, U); 248 } 249 250 // If the def and use are in different blocks, do a simple CFG dominator 251 // tree query. 252 if (DefBB != UseBB) 253 return dominates(DefBB, UseBB); 254 255 // Ok, def and use are in the same block. If the def is an invoke, it 256 // doesn't dominate anything in the block. If it's a PHI, it dominates 257 // everything in the block. 258 if (isa<PHINode>(UserInst)) 259 return true; 260 261 // Otherwise, just loop through the basic block until we find Def or User. 262 BasicBlock::const_iterator I = DefBB->begin(); 263 for (; &*I != Def && &*I != UserInst; ++I) 264 /*empty*/; 265 266 return &*I != UserInst; 267} 268 269bool DominatorTree::isReachableFromEntry(const Use &U) const { 270 Instruction *I = dyn_cast<Instruction>(U.getUser()); 271 272 // ConstantExprs aren't really reachable from the entry block, but they 273 // don't need to be treated like unreachable code either. 274 if (!I) return true; 275 276 // PHI nodes use their operands on their incoming edges. 277 if (PHINode *PN = dyn_cast<PHINode>(I)) 278 return isReachableFromEntry(PN->getIncomingBlock(U)); 279 280 // Everything else uses their operands in their own block. 281 return isReachableFromEntry(I->getParent()); 282} 283 284void DominatorTree::verifyDomTree() const { 285 Function &F = *getRoot()->getParent(); 286 287 DominatorTree OtherDT; 288 OtherDT.recalculate(F); 289 if (compare(OtherDT)) { 290 errs() << "DominatorTree is not up to date!\nComputed:\n"; 291 print(errs()); 292 errs() << "\nActual:\n"; 293 OtherDT.print(errs()); 294 abort(); 295 } 296} 297 298//===----------------------------------------------------------------------===// 299// DominatorTreeAnalysis and related pass implementations 300//===----------------------------------------------------------------------===// 301// 302// This implements the DominatorTreeAnalysis which is used with the new pass 303// manager. It also implements some methods from utility passes. 304// 305//===----------------------------------------------------------------------===// 306 307DominatorTree DominatorTreeAnalysis::run(Function &F, 308 FunctionAnalysisManager &) { 309 DominatorTree DT; 310 DT.recalculate(F); 311 return DT; 312} 313 314AnalysisKey DominatorTreeAnalysis::Key; 315 316DominatorTreePrinterPass::DominatorTreePrinterPass(raw_ostream &OS) : OS(OS) {} 317 318PreservedAnalyses DominatorTreePrinterPass::run(Function &F, 319 FunctionAnalysisManager &AM) { 320 OS << "DominatorTree for function: " << F.getName() << "\n"; 321 AM.getResult<DominatorTreeAnalysis>(F).print(OS); 322 323 return PreservedAnalyses::all(); 324} 325 326PreservedAnalyses DominatorTreeVerifierPass::run(Function &F, 327 FunctionAnalysisManager &AM) { 328 AM.getResult<DominatorTreeAnalysis>(F).verifyDomTree(); 329 330 return PreservedAnalyses::all(); 331} 332 333//===----------------------------------------------------------------------===// 334// DominatorTreeWrapperPass Implementation 335//===----------------------------------------------------------------------===// 336// 337// The implementation details of the wrapper pass that holds a DominatorTree 338// suitable for use with the legacy pass manager. 339// 340//===----------------------------------------------------------------------===// 341 342char DominatorTreeWrapperPass::ID = 0; 343INITIALIZE_PASS(DominatorTreeWrapperPass, "domtree", 344 "Dominator Tree Construction", true, true) 345 346bool DominatorTreeWrapperPass::runOnFunction(Function &F) { 347 DT.recalculate(F); 348 return false; 349} 350 351void DominatorTreeWrapperPass::verifyAnalysis() const { 352 if (VerifyDomInfo) 353 DT.verifyDomTree(); 354} 355 356void DominatorTreeWrapperPass::print(raw_ostream &OS, const Module *) const { 357 DT.print(OS); 358} 359 360