Dominators.cpp

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//===- Dominators.cpp - Dominator Calculation -----------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements simple dominator construction algorithms for finding
// forward dominators.  Postdominators are available in libanalysis, but are not
// included in libvmcore, because it's not needed.  Forward dominators are
// needed to support the Verifier pass.
//
//===----------------------------------------------------------------------===//

#include "llvm/Analysis/Dominators.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/DominatorInternals.h"
#include "llvm/Assembly/Writer.h"
#include "llvm/IR/Instructions.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
using namespace llvm;

// Always verify dominfo if expensive checking is enabled.
#ifdef XDEBUG
static bool VerifyDomInfo = true;
#else
static bool VerifyDomInfo = false;
#endif
static cl::opt<bool,true>
VerifyDomInfoX("verify-dom-info", cl::location(VerifyDomInfo),
               cl::desc("Verify dominator info (time consuming)"));

bool BasicBlockEdge::isSingleEdge() const {
  const TerminatorInst *TI = Start->getTerminator();
  unsigned NumEdgesToEnd = 0;
  for (unsigned int i = 0, n = TI->getNumSuccessors(); i < n; ++i) {
    if (TI->getSuccessor(i) == End)
      ++NumEdgesToEnd;
    if (NumEdgesToEnd >= 2)
      return false;
  }
  assert(NumEdgesToEnd == 1);
  return true;
}

//===----------------------------------------------------------------------===//
//  DominatorTree Implementation
//===----------------------------------------------------------------------===//
//
// Provide public access to DominatorTree information.  Implementation details
// can be found in DominatorInternals.h.
//
//===----------------------------------------------------------------------===//

TEMPLATE_INSTANTIATION(class llvm::DomTreeNodeBase<BasicBlock>);
TEMPLATE_INSTANTIATION(class llvm::DominatorTreeBase<BasicBlock>);

char DominatorTree::ID = 0;
INITIALIZE_PASS(DominatorTree, "domtree",
                "Dominator Tree Construction", true, true)

bool DominatorTree::runOnFunction(Function &F) {
  DT->recalculate(F);
  return false;
}

void DominatorTree::verifyAnalysis() const {
  if (!VerifyDomInfo) return;

  Function &F = *getRoot()->getParent();

  DominatorTree OtherDT;
  OtherDT.getBase().recalculate(F);
  if (compare(OtherDT)) {
    errs() << "DominatorTree is not up to date!\nComputed:\n";
    print(errs());
    errs() << "\nActual:\n";
    OtherDT.print(errs());
    abort();
  }
}

void DominatorTree::print(raw_ostream &OS, const Module *) const {
  DT->print(OS);
}

// dominates - Return true if Def dominates a use in User. This performs
// the special checks necessary if Def and User are in the same basic block.
// Note that Def doesn't dominate a use in Def itself!
bool DominatorTree::dominates(const Instruction *Def,
                              const Instruction *User) const {
  const BasicBlock *UseBB = User->getParent();
  const BasicBlock *DefBB = Def->getParent();

  // Any unreachable use is dominated, even if Def == User.
  if (!isReachableFromEntry(UseBB))
    return true;

  // Unreachable definitions don't dominate anything.
  if (!isReachableFromEntry(DefBB))
    return false;

  // An instruction doesn't dominate a use in itself.
  if (Def == User)
    return false;

  // The value defined by an invoke dominates an instruction only if
  // it dominates every instruction in UseBB.
  // A PHI is dominated only if the instruction dominates every possible use
  // in the UseBB.
  if (isa<InvokeInst>(Def) || isa<PHINode>(User))
    return dominates(Def, UseBB);

  if (DefBB != UseBB)
    return dominates(DefBB, UseBB);

  // Loop through the basic block until we find Def or User.
  BasicBlock::const_iterator I = DefBB->begin();
  for (; &*I != Def && &*I != User; ++I)
    /*empty*/;

  return &*I == Def;
}

// true if Def would dominate a use in any instruction in UseBB.
// note that dominates(Def, Def->getParent()) is false.
bool DominatorTree::dominates(const Instruction *Def,
                              const BasicBlock *UseBB) const {
  const BasicBlock *DefBB = Def->getParent();

  // Any unreachable use is dominated, even if DefBB == UseBB.
  if (!isReachableFromEntry(UseBB))
    return true;

  // Unreachable definitions don't dominate anything.
  if (!isReachableFromEntry(DefBB))
    return false;

  if (DefBB == UseBB)
    return false;

  const InvokeInst *II = dyn_cast<InvokeInst>(Def);
  if (!II)
    return dominates(DefBB, UseBB);

  // Invoke results are only usable in the normal destination, not in the
  // exceptional destination.
  BasicBlock *NormalDest = II->getNormalDest();
  BasicBlockEdge E(DefBB, NormalDest);
  return dominates(E, UseBB);
}

bool DominatorTree::dominates(const BasicBlockEdge &BBE,
                              const BasicBlock *UseBB) const {
  // Assert that we have a single edge. We could handle them by simply
  // returning false, but since isSingleEdge is linear on the number of
  // edges, the callers can normally handle them more efficiently.
  assert(BBE.isSingleEdge());

  // If the BB the edge ends in doesn't dominate the use BB, then the
  // edge also doesn't.
  const BasicBlock *Start = BBE.getStart();
  const BasicBlock *End = BBE.getEnd();
  if (!dominates(End, UseBB))
    return false;

  // Simple case: if the end BB has a single predecessor, the fact that it
  // dominates the use block implies that the edge also does.
  if (End->getSinglePredecessor())
    return true;

  // The normal edge from the invoke is critical. Conceptually, what we would
  // like to do is split it and check if the new block dominates the use.
  // With X being the new block, the graph would look like:
  //
  //        DefBB
  //          /\      .  .
  //         /  \     .  .
  //        /    \    .  .
  //       /      \   |  |
  //      A        X  B  C
  //      |         \ | /
  //      .          \|/
  //      .      NormalDest
  //      .
  //
  // Given the definition of dominance, NormalDest is dominated by X iff X
  // dominates all of NormalDest's predecessors (X, B, C in the example). X
  // trivially dominates itself, so we only have to find if it dominates the
  // other predecessors. Since the only way out of X is via NormalDest, X can
  // only properly dominate a node if NormalDest dominates that node too.
  for (const_pred_iterator PI = pred_begin(End), E = pred_end(End);
       PI != E; ++PI) {
    const BasicBlock *BB = *PI;
    if (BB == Start)
      continue;

    if (!dominates(End, BB))
      return false;
  }
  return true;
}

bool DominatorTree::dominates(const BasicBlockEdge &BBE,
                              const Use &U) const {
  // Assert that we have a single edge. We could handle them by simply
  // returning false, but since isSingleEdge is linear on the number of
  // edges, the callers can normally handle them more efficiently.
  assert(BBE.isSingleEdge());

  Instruction *UserInst = cast<Instruction>(U.getUser());
  // A PHI in the end of the edge is dominated by it.
  PHINode *PN = dyn_cast<PHINode>(UserInst);
  if (PN && PN->getParent() == BBE.getEnd() &&
      PN->getIncomingBlock(U) == BBE.getStart())
    return true;

  // Otherwise use the edge-dominates-block query, which
  // handles the crazy critical edge cases properly.
  const BasicBlock *UseBB;
  if (PN)
    UseBB = PN->getIncomingBlock(U);
  else
    UseBB = UserInst->getParent();
  return dominates(BBE, UseBB);
}

bool DominatorTree::dominates(const Instruction *Def,
                              const Use &U) const {
  Instruction *UserInst = cast<Instruction>(U.getUser());
  const BasicBlock *DefBB = Def->getParent();

  // Determine the block in which the use happens. PHI nodes use
  // their operands on edges; simulate this by thinking of the use
  // happening at the end of the predecessor block.
  const BasicBlock *UseBB;
  if (PHINode *PN = dyn_cast<PHINode>(UserInst))
    UseBB = PN->getIncomingBlock(U);
  else
    UseBB = UserInst->getParent();

  // Any unreachable use is dominated, even if Def == User.
  if (!isReachableFromEntry(UseBB))
    return true;

  // Unreachable definitions don't dominate anything.
  if (!isReachableFromEntry(DefBB))
    return false;

  // Invoke instructions define their return values on the edges
  // to their normal successors, so we have to handle them specially.
  // Among other things, this means they don't dominate anything in
  // their own block, except possibly a phi, so we don't need to
  // walk the block in any case.
  if (const InvokeInst *II = dyn_cast<InvokeInst>(Def)) {
    BasicBlock *NormalDest = II->getNormalDest();
    BasicBlockEdge E(DefBB, NormalDest);
    return dominates(E, U);
  }

  // If the def and use are in different blocks, do a simple CFG dominator
  // tree query.
  if (DefBB != UseBB)
    return dominates(DefBB, UseBB);

  // Ok, def and use are in the same block. If the def is an invoke, it
  // doesn't dominate anything in the block. If it's a PHI, it dominates
  // everything in the block.
  if (isa<PHINode>(UserInst))
    return true;

  // Otherwise, just loop through the basic block until we find Def or User.
  BasicBlock::const_iterator I = DefBB->begin();
  for (; &*I != Def && &*I != UserInst; ++I)
    /*empty*/;

  return &*I != UserInst;
}

bool DominatorTree::isReachableFromEntry(const Use &U) const {
  Instruction *I = dyn_cast<Instruction>(U.getUser());

  // ConstantExprs aren't really reachable from the entry block, but they
  // don't need to be treated like unreachable code either.
  if (!I) return true;

  // PHI nodes use their operands on their incoming edges.
  if (PHINode *PN = dyn_cast<PHINode>(I))
    return isReachableFromEntry(PN->getIncomingBlock(U));

  // Everything else uses their operands in their own block.
  return isReachableFromEntry(I->getParent());
}