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