C++ PHINode::hasOneUse方法代码示例

// SimplifyPredecessors(switch) - We know that SI is switch based on a PHI node
// defined in this block.  If the phi node contains constant operands, then the
// blocks corresponding to those operands can be modified to jump directly to
// the destination instead of going through this block.
void CondProp::SimplifyPredecessors(SwitchInst *SI) {
  // TODO: We currently only handle the most trival case, where the PHI node has
  // one use (the branch), and is the only instruction besides the branch in the
  // block.
  PHINode *PN = cast<PHINode>(SI->getCondition());
  if (!PN->hasOneUse()) return;

  BasicBlock *BB = SI->getParent();
  if (&*BB->begin() != PN || &*next(BB->begin()) != SI)
    return;

  bool RemovedPreds = false;

  // Ok, we have this really simple case, walk the PHI operands, looking for
  // constants.  Walk from the end to remove operands from the end when
  // possible, and to avoid invalidating "i".
  for (unsigned i = PN->getNumIncomingValues(); i != 0; --i)
    if (ConstantInt *CI = dyn_cast<ConstantInt>(PN->getIncomingValue(i-1))) {
      // If we have a constant, forward the edge from its current to its
      // ultimate destination.
      unsigned DestCase = SI->findCaseValue(CI);
      RevectorBlockTo(PN->getIncomingBlock(i-1),
                      SI->getSuccessor(DestCase));
      ++NumSwThread;
      RemovedPreds = true;

      // If there were two predecessors before this simplification, or if the
      // PHI node contained all the same value except for the one we just
      // substituted, the PHI node may be deleted.  Don't iterate through it the
      // last time.
      if (SI->getCondition() != PN) return;
    }
}

/// Connect the unrolling epilog code to the original loop.
/// The unrolling epilog code contains code to execute the
/// 'extra' iterations if the run-time trip count modulo the
/// unroll count is non-zero.
///
/// This function performs the following:
/// - Update PHI nodes at the unrolling loop exit and epilog loop exit
/// - Create PHI nodes at the unrolling loop exit to combine
///   values that exit the unrolling loop code and jump around it.
/// - Update PHI operands in the epilog loop by the new PHI nodes
/// - Branch around the epilog loop if extra iters (ModVal) is zero.
///
static void ConnectEpilog(Loop *L, Value *ModVal, BasicBlock *NewExit,
                          BasicBlock *Exit, BasicBlock *PreHeader,
                          BasicBlock *EpilogPreHeader, BasicBlock *NewPreHeader,
                          ValueToValueMapTy &VMap, DominatorTree *DT,
                          LoopInfo *LI, bool PreserveLCSSA)  {
  BasicBlock *Latch = L->getLoopLatch();
  assert(Latch && "Loop must have a latch");
  BasicBlock *EpilogLatch = cast<BasicBlock>(VMap[Latch]);

  // Loop structure should be the following:
  //
  // PreHeader
  // NewPreHeader
  //   Header
  //   ...
  //   Latch
  // NewExit (PN)
  // EpilogPreHeader
  //   EpilogHeader
  //   ...
  //   EpilogLatch
  // Exit (EpilogPN)

  // Update PHI nodes at NewExit and Exit.
  for (Instruction &BBI : *NewExit) {
    PHINode *PN = dyn_cast<PHINode>(&BBI);
    // Exit when we passed all PHI nodes.
    if (!PN)
      break;
    // PN should be used in another PHI located in Exit block as
    // Exit was split by SplitBlockPredecessors into Exit and NewExit
    // Basicaly it should look like:
    // NewExit:
    //   PN = PHI [I, Latch]
    // ...
    // Exit:
    //   EpilogPN = PHI [PN, EpilogPreHeader]
    //
    // There is EpilogPreHeader incoming block instead of NewExit as
    // NewExit was spilt 1 more time to get EpilogPreHeader.
    assert(PN->hasOneUse() && "The phi should have 1 use");
    PHINode *EpilogPN = cast<PHINode> (PN->use_begin()->getUser());
    assert(EpilogPN->getParent() == Exit && "EpilogPN should be in Exit block");

    // Add incoming PreHeader from branch around the Loop
    PN->addIncoming(UndefValue::get(PN->getType()), PreHeader);

    Value *V = PN->getIncomingValueForBlock(Latch);
    Instruction *I = dyn_cast<Instruction>(V);
    if (I && L->contains(I))
      // If value comes from an instruction in the loop add VMap value.
      V = VMap.lookup(I);
    // For the instruction out of the loop, constant or undefined value
    // insert value itself.
    EpilogPN->addIncoming(V, EpilogLatch);

    assert(EpilogPN->getBasicBlockIndex(EpilogPreHeader) >= 0 &&
          "EpilogPN should have EpilogPreHeader incoming block");
    // Change EpilogPreHeader incoming block to NewExit.
    EpilogPN->setIncomingBlock(EpilogPN->getBasicBlockIndex(EpilogPreHeader),
                               NewExit);
    // Now PHIs should look like:
    // NewExit:
    //   PN = PHI [I, Latch], [undef, PreHeader]
    // ...
    // Exit:
    //   EpilogPN = PHI [PN, NewExit], [VMap[I], EpilogLatch]
  }

  // Create PHI nodes at NewExit (from the unrolling loop Latch and PreHeader).
  // Update corresponding PHI nodes in epilog loop.
  for (BasicBlock *Succ : successors(Latch)) {
    // Skip this as we already updated phis in exit blocks.
    if (!L->contains(Succ))
      continue;
    for (Instruction &BBI : *Succ) {
      PHINode *PN = dyn_cast<PHINode>(&BBI);
      // Exit when we passed all PHI nodes.
      if (!PN)
        break;
      // Add new PHI nodes to the loop exit block and update epilog
      // PHIs with the new PHI values.
      PHINode *NewPN = PHINode::Create(PN->getType(), 2, PN->getName() + ".unr",
                                       NewExit->getFirstNonPHI());
      // Adding a value to the new PHI node from the unrolling loop preheader.
      NewPN->addIncoming(PN->getIncomingValueForBlock(NewPreHeader), PreHeader);
      // Adding a value to the new PHI node from the unrolling loop latch.
      NewPN->addIncoming(PN->getIncomingValueForBlock(Latch), Latch);
//.........这里部分代码省略.........