src/llvm/lib/IR/Dominators.cpp

   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/IR/PassManager.h"
  24 #include "llvm/Support/CommandLine.h"
  25 #include "llvm/Support/Compiler.h"
  26 #include "llvm/Support/Debug.h"
  27 #include "llvm/Support/GenericDomTreeConstruction.h"
  28 #include "llvm/Support/raw_ostream.h"
  29 #include <algorithm>
  30 using namespace llvm;
  31
  32 // Always verify dominfo if expensive checking is enabled.
  33 #ifdef XDEBUG
  34 static bool VerifyDomInfo = true;
  35 #else
  36 static bool VerifyDomInfo = false;
  37 #endif
  38 static cl::opt<bool,true>
  39 VerifyDomInfoX("verify-dom-info", cl::location(VerifyDomInfo),
  40                cl::desc("Verify dominator info (time consuming)"));
  41
  42 bool 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 Dominators.h, GenericDomTree.h, and
  61 // GenericDomTreeConstruction.h.
  62 //
  63 //===----------------------------------------------------------------------===//
  64
  65 TEMPLATE_INSTANTIATION(class llvm::DomTreeNodeBase<BasicBlock>);
  66 TEMPLATE_INSTANTIATION(class llvm::DominatorTreeBase<BasicBlock>);
  67
  68 #define LLVM_COMMA ,
  69 TEMPLATE_INSTANTIATION(void llvm::Calculate<Function LLVM_COMMA BasicBlock *>(
  70     DominatorTreeBase<GraphTraits<BasicBlock *>::NodeType> &DT LLVM_COMMA
  71         Function &F));
  72 TEMPLATE_INSTANTIATION(
  73     void llvm::Calculate<Function LLVM_COMMA Inverse<BasicBlock *> >(
  74         DominatorTreeBase<GraphTraits<Inverse<BasicBlock *> >::NodeType> &DT
  75             LLVM_COMMA Function &F));
  76 #undef LLVM_COMMA
  77
  78 // dominates - Return true if Def dominates a use in User. This performs
  79 // the special checks necessary if Def and User are in the same basic block.
  80 // Note that Def doesn't dominate a use in Def itself!
  81 bool DominatorTree::dominates(const Instruction *Def,
  82                               const Instruction *User) const {
  83   const BasicBlock *UseBB = User->getParent();
  84   const BasicBlock *DefBB = Def->getParent();
  85
  86   // Any unreachable use is dominated, even if Def == User.
  87   if (!isReachableFromEntry(UseBB))
  88     return true;
  89
  90   // Unreachable definitions don't dominate anything.
  91   if (!isReachableFromEntry(DefBB))
  92     return false;
  93
  94   // An instruction doesn't dominate a use in itself.
  95   if (Def == User)
  96     return false;
  97
  98   // The value defined by an invoke dominates an instruction only if
  99   // it dominates every instruction in UseBB.
 100   // A PHI is dominated only if the instruction dominates every possible use
 101   // in the UseBB.
 102   if (isa<InvokeInst>(Def) || isa<PHINode>(User))
 103     return dominates(Def, UseBB);
 104
 105   if (DefBB != UseBB)
 106     return dominates(DefBB, UseBB);
 107
 108   // Loop through the basic block until we find Def or User.
 109   BasicBlock::const_iterator I = DefBB->begin();
 110   for (; &*I != Def && &*I != User; ++I)
 111     /*empty*/;
 112
 113   return &*I == Def;
 114 }
 115
 116 // true if Def would dominate a use in any instruction in UseBB.
 117 // note that dominates(Def, Def->getParent()) is false.
 118 bool DominatorTree::dominates(const Instruction *Def,
 119                               const BasicBlock *UseBB) const {
 120   const BasicBlock *DefBB = Def->getParent();
 121
 122   // Any unreachable use is dominated, even if DefBB == UseBB.
 123   if (!isReachableFromEntry(UseBB))
 124     return true;
 125
 126   // Unreachable definitions don't dominate anything.
 127   if (!isReachableFromEntry(DefBB))
 128     return false;
 129
 130   if (DefBB == UseBB)
 131     return false;
 132
 133   const InvokeInst *II = dyn_cast<InvokeInst>(Def);
 134   if (!II)
 135     return dominates(DefBB, UseBB);
 136
 137   // Invoke results are only usable in the normal destination, not in the
 138   // exceptional destination.
 139   BasicBlock *NormalDest = II->getNormalDest();
 140   BasicBlockEdge E(DefBB, NormalDest);
 141   return dominates(E, UseBB);
 142 }
 143
 144 bool DominatorTree::dominates(const BasicBlockEdge &BBE,
 145                               const BasicBlock *UseBB) const {
 146   // Assert that we have a single edge. We could handle them by simply
 147   // returning false, but since isSingleEdge is linear on the number of
 148   // edges, the callers can normally handle them more efficiently.
 149   assert(BBE.isSingleEdge());
 150
 151   // If the BB the edge ends in doesn't dominate the use BB, then the
 152   // edge also doesn't.
 153   const BasicBlock *Start = BBE.getStart();
 154   const BasicBlock *End = BBE.getEnd();
 155   if (!dominates(End, UseBB))
 156     return false;
 157
 158   // Simple case: if the end BB has a single predecessor, the fact that it
 159   // dominates the use block implies that the edge also does.
 160   if (End->getSinglePredecessor())
 161     return true;
 162
 163   // The normal edge from the invoke is critical. Conceptually, what we would
 164   // like to do is split it and check if the new block dominates the use.
 165   // With X being the new block, the graph would look like:
 166   //
 167   //        DefBB
 168   //          /\      .  .
 169   //         /  \     .  .
 170   //        /    \    .  .
 171   //       /      \   |  |
 172   //      A        X  B  C
 173   //      |         \ | /
 174   //      .          \|/
 175   //      .      NormalDest
 176   //      .
 177   //
 178   // Given the definition of dominance, NormalDest is dominated by X iff X
 179   // dominates all of NormalDest's predecessors (X, B, C in the example). X
 180   // trivially dominates itself, so we only have to find if it dominates the
 181   // other predecessors. Since the only way out of X is via NormalDest, X can
 182   // only properly dominate a node if NormalDest dominates that node too.
 183   for (const_pred_iterator PI = pred_begin(End), E = pred_end(End);
 184        PI != E; ++PI) {
 185     const BasicBlock *BB = *PI;
 186     if (BB == Start)
 187       continue;
 188
 189     if (!dominates(End, BB))
 190       return false;
 191   }
 192   return true;
 193 }
 194
 195 bool DominatorTree::dominates(const BasicBlockEdge &BBE, const Use &U) const {
 196   // Assert that we have a single edge. We could handle them by simply
 197   // returning false, but since isSingleEdge is linear on the number of
 198   // edges, the callers can normally handle them more efficiently.
 199   assert(BBE.isSingleEdge());
 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
 218 bool 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
 240   // to their normal 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
 269 bool 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
 284 void DominatorTree::verifyDomTree() const {
 285   if (!VerifyDomInfo)
 286     return;
 287
 288   Function &F = *getRoot()->getParent();
 289
 290   DominatorTree OtherDT;
 291   OtherDT.recalculate(F);
 292   if (compare(OtherDT)) {
 293     errs() << "DominatorTree is not up to date!\nComputed:\n";
 294     print(errs());
 295     errs() << "\nActual:\n";
 296     OtherDT.print(errs());
 297     abort();
 298   }
 299 }
 300
 301 //===----------------------------------------------------------------------===//
 302 //  DominatorTreeAnalysis and related pass implementations
 303 //===----------------------------------------------------------------------===//
 304 //
 305 // This implements the DominatorTreeAnalysis which is used with the new pass
 306 // manager. It also implements some methods from utility passes.
 307 //
 308 //===----------------------------------------------------------------------===//
 309
 310 DominatorTree DominatorTreeAnalysis::run(Function &F) {
 311   DominatorTree DT;
 312   DT.recalculate(F);
 313   return DT;
 314 }
 315
 316 char DominatorTreeAnalysis::PassID;
 317
 318 DominatorTreePrinterPass::DominatorTreePrinterPass(raw_ostream &OS) : OS(OS) {}
 319
 320 PreservedAnalyses DominatorTreePrinterPass::run(Function &F,
 321                                                 FunctionAnalysisManager *AM) {
 322   OS << "DominatorTree for function: " << F.getName() << "\n";
 323   AM->getResult<DominatorTreeAnalysis>(F).print(OS);
 324
 325   return PreservedAnalyses::all();
 326 }
 327
 328 PreservedAnalyses DominatorTreeVerifierPass::run(Function &F,
 329                                                  FunctionAnalysisManager *AM) {
 330   AM->getResult<DominatorTreeAnalysis>(F).verifyDomTree();
 331
 332   return PreservedAnalyses::all();
 333 }
 334
 335 //===----------------------------------------------------------------------===//
 336 //  DominatorTreeWrapperPass Implementation
 337 //===----------------------------------------------------------------------===//
 338 //
 339 // The implementation details of the wrapper pass that holds a DominatorTree
 340 // suitable for use with the legacy pass manager.
 341 //
 342 //===----------------------------------------------------------------------===//
 343
 344 char DominatorTreeWrapperPass::ID = 0;
 345 INITIALIZE_PASS(DominatorTreeWrapperPass, "domtree",
 346                 "Dominator Tree Construction", true, true)
 347
 348 bool DominatorTreeWrapperPass::runOnFunction(Function &F) {
 349   DT.recalculate(F);
 350   return false;
 351 }
 352
 353 void DominatorTreeWrapperPass::verifyAnalysis() const { DT.verifyDomTree(); }
 354
 355 void DominatorTreeWrapperPass::print(raw_ostream &OS, const Module *) const {
 356   DT.print(OS);
 357 }
 358