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/ADT/SmallVector.h" 21#include "llvm/IR/CFG.h" 22#include "llvm/IR/Instructions.h" 23#include "llvm/Support/CommandLine.h" 24#include "llvm/Support/Compiler.h" 25#include "llvm/Support/Debug.h" 26#include "llvm/Support/GenericDomTreeConstruction.h" 27#include "llvm/Support/raw_ostream.h" 28#include <algorithm> 29using namespace llvm; 30 31// Always verify dominfo if expensive checking is enabled. 32#ifdef XDEBUG 33static bool VerifyDomInfo = true; 34#else 35static bool VerifyDomInfo = false; 36#endif 37static cl::opt<bool,true> 38VerifyDomInfoX("verify-dom-info", cl::location(VerifyDomInfo), 39 cl::desc("Verify dominator info (time consuming)")); 40 41bool BasicBlockEdge::isSingleEdge() const { 42 const TerminatorInst *TI = Start->getTerminator(); 43 unsigned NumEdgesToEnd = 0; 44 for (unsigned int i = 0, n = TI->getNumSuccessors(); i < n; ++i) { 45 if (TI->getSuccessor(i) == End) 46 ++NumEdgesToEnd; 47 if (NumEdgesToEnd >= 2) 48 return false; 49 } 50 assert(NumEdgesToEnd == 1); 51 return true; 52} 53 54//===----------------------------------------------------------------------===// 55// DominatorTree Implementation 56//===----------------------------------------------------------------------===// 57// 58// Provide public access to DominatorTree information. Implementation details 59// can be found in Dominators.h, GenericDomTree.h, and 60// GenericDomTreeConstruction.h. 61// 62//===----------------------------------------------------------------------===// 63 64TEMPLATE_INSTANTIATION(class llvm::DomTreeNodeBase<BasicBlock>); 65TEMPLATE_INSTANTIATION(class llvm::DominatorTreeBase<BasicBlock>); 66 67#define LLVM_COMMA , 68TEMPLATE_INSTANTIATION(void llvm::Calculate<Function LLVM_COMMA BasicBlock *>( 69 DominatorTreeBase<GraphTraits<BasicBlock *>::NodeType> &DT LLVM_COMMA 70 Function &F)); 71TEMPLATE_INSTANTIATION( 72 void llvm::Calculate<Function LLVM_COMMA Inverse<BasicBlock *> >( 73 DominatorTreeBase<GraphTraits<Inverse<BasicBlock *> >::NodeType> &DT 74 LLVM_COMMA Function &F)); 75#undef LLVM_COMMA 76 77// dominates - Return true if Def dominates a use in User. This performs 78// the special checks necessary if Def and User are in the same basic block. 79// Note that Def doesn't dominate a use in Def itself! 80bool DominatorTree::dominates(const Instruction *Def, 81 const Instruction *User) const { 82 const BasicBlock *UseBB = User->getParent(); 83 const BasicBlock *DefBB = Def->getParent(); 84 85 // Any unreachable use is dominated, even if Def == User. 86 if (!isReachableFromEntry(UseBB)) 87 return true; 88 89 // Unreachable definitions don't dominate anything. 90 if (!isReachableFromEntry(DefBB)) 91 return false; 92 93 // An instruction doesn't dominate a use in itself. 94 if (Def == User) 95 return false; 96 97 // The value defined by an invoke dominates an instruction only if 98 // it dominates every instruction in UseBB. 99 // A PHI is dominated only if the instruction dominates every possible use 100 // in the UseBB. 101 if (isa<InvokeInst>(Def) || isa<PHINode>(User)) 102 return dominates(Def, UseBB); 103 104 if (DefBB != UseBB) 105 return dominates(DefBB, UseBB); 106 107 // Loop through the basic block until we find Def or User. 108 BasicBlock::const_iterator I = DefBB->begin(); 109 for (; &*I != Def && &*I != User; ++I) 110 /*empty*/; 111 112 return &*I == Def; 113} 114 115// true if Def would dominate a use in any instruction in UseBB. 116// note that dominates(Def, Def->getParent()) is false. 117bool DominatorTree::dominates(const Instruction *Def, 118 const BasicBlock *UseBB) const { 119 const BasicBlock *DefBB = Def->getParent(); 120 121 // Any unreachable use is dominated, even if DefBB == UseBB. 122 if (!isReachableFromEntry(UseBB)) 123 return true; 124 125 // Unreachable definitions don't dominate anything. 126 if (!isReachableFromEntry(DefBB)) 127 return false; 128 129 if (DefBB == UseBB) 130 return false; 131 132 const InvokeInst *II = dyn_cast<InvokeInst>(Def); 133 if (!II) 134 return dominates(DefBB, UseBB); 135 136 // Invoke results are only usable in the normal destination, not in the 137 // exceptional destination. 138 BasicBlock *NormalDest = II->getNormalDest(); 139 BasicBlockEdge E(DefBB, NormalDest); 140 return dominates(E, UseBB); 141} 142 143bool DominatorTree::dominates(const BasicBlockEdge &BBE, 144 const BasicBlock *UseBB) const { 145 // Assert that we have a single edge. We could handle them by simply 146 // returning false, but since isSingleEdge is linear on the number of 147 // edges, the callers can normally handle them more efficiently. 148 assert(BBE.isSingleEdge()); 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()); 199 200 Instruction *UserInst = cast<Instruction>(U.getUser()); 201 // A PHI in the end of the edge is dominated by it. 202 PHINode *PN = dyn_cast<PHINode>(UserInst); 203 if (PN && PN->getParent() == BBE.getEnd() && 204 PN->getIncomingBlock(U) == BBE.getStart()) 205 return true; 206 207 // Otherwise use the edge-dominates-block query, which 208 // handles the crazy critical edge cases properly. 209 const BasicBlock *UseBB; 210 if (PN) 211 UseBB = PN->getIncomingBlock(U); 212 else 213 UseBB = UserInst->getParent(); 214 return dominates(BBE, UseBB); 215} 216 217bool DominatorTree::dominates(const Instruction *Def, const Use &U) const { 218 Instruction *UserInst = cast<Instruction>(U.getUser()); 219 const BasicBlock *DefBB = Def->getParent(); 220 221 // Determine the block in which the use happens. PHI nodes use 222 // their operands on edges; simulate this by thinking of the use 223 // happening at the end of the predecessor block. 224 const BasicBlock *UseBB; 225 if (PHINode *PN = dyn_cast<PHINode>(UserInst)) 226 UseBB = PN->getIncomingBlock(U); 227 else 228 UseBB = UserInst->getParent(); 229 230 // Any unreachable use is dominated, even if Def == User. 231 if (!isReachableFromEntry(UseBB)) 232 return true; 233 234 // Unreachable definitions don't dominate anything. 235 if (!isReachableFromEntry(DefBB)) 236 return false; 237 238 // Invoke instructions define their return values on the edges 239 // to their normal successors, so we have to handle them specially. 240 // Among other things, this means they don't dominate anything in 241 // their own block, except possibly a phi, so we don't need to 242 // walk the block in any case. 243 if (const InvokeInst *II = dyn_cast<InvokeInst>(Def)) { 244 BasicBlock *NormalDest = II->getNormalDest(); 245 BasicBlockEdge E(DefBB, NormalDest); 246 return dominates(E, U); 247 } 248 249 // If the def and use are in different blocks, do a simple CFG dominator 250 // tree query. 251 if (DefBB != UseBB) 252 return dominates(DefBB, UseBB); 253 254 // Ok, def and use are in the same block. If the def is an invoke, it 255 // doesn't dominate anything in the block. If it's a PHI, it dominates 256 // everything in the block. 257 if (isa<PHINode>(UserInst)) 258 return true; 259 260 // Otherwise, just loop through the basic block until we find Def or User. 261 BasicBlock::const_iterator I = DefBB->begin(); 262 for (; &*I != Def && &*I != UserInst; ++I) 263 /*empty*/; 264 265 return &*I != UserInst; 266} 267 268bool DominatorTree::isReachableFromEntry(const Use &U) const { 269 Instruction *I = dyn_cast<Instruction>(U.getUser()); 270 271 // ConstantExprs aren't really reachable from the entry block, but they 272 // don't need to be treated like unreachable code either. 273 if (!I) return true; 274 275 // PHI nodes use their operands on their incoming edges. 276 if (PHINode *PN = dyn_cast<PHINode>(I)) 277 return isReachableFromEntry(PN->getIncomingBlock(U)); 278 279 // Everything else uses their operands in their own block. 280 return isReachableFromEntry(I->getParent()); 281} 282 283void DominatorTree::verifyDomTree() const { 284 if (!VerifyDomInfo) 285 return; 286 287 Function &F = *getRoot()->getParent(); 288 289 DominatorTree OtherDT; 290 OtherDT.recalculate(F); 291 if (compare(OtherDT)) { 292 errs() << "DominatorTree is not up to date!\nComputed:\n"; 293 print(errs()); 294 errs() << "\nActual:\n"; 295 OtherDT.print(errs()); 296 abort(); 297 } 298} 299 300//===----------------------------------------------------------------------===// 301// DominatorTreeWrapperPass Implementation 302//===----------------------------------------------------------------------===// 303// 304// The implementation details of the wrapper pass that holds a DominatorTree. 305// 306//===----------------------------------------------------------------------===// 307 308char DominatorTreeWrapperPass::ID = 0; 309INITIALIZE_PASS(DominatorTreeWrapperPass, "domtree", 310 "Dominator Tree Construction", true, true) 311 312bool DominatorTreeWrapperPass::runOnFunction(Function &F) { 313 DT.recalculate(F); 314 return false; 315} 316 317void DominatorTreeWrapperPass::verifyAnalysis() const { DT.verifyDomTree(); } 318 319void DominatorTreeWrapperPass::print(raw_ostream &OS, const Module *) const { 320 DT.print(OS); 321} 322 323