C++ PHINode::takeName方法代码示例

/// If we have a SelectInst that will likely profit from branch prediction,
/// turn it into a branch.
bool CodeGenPrepare::OptimizeSelectInst(SelectInst *SI) {
  bool VectorCond = !SI->getCondition()->getType()->isIntegerTy(1);

  // Can we convert the 'select' to CF ?
  if (DisableSelectToBranch || OptSize || !TLI || VectorCond)
    return false;

  TargetLowering::SelectSupportKind SelectKind;
  if (VectorCond)
    SelectKind = TargetLowering::VectorMaskSelect;
  else if (SI->getType()->isVectorTy())
    SelectKind = TargetLowering::ScalarCondVectorVal;
  else
    SelectKind = TargetLowering::ScalarValSelect;

  // Do we have efficient codegen support for this kind of 'selects' ?
  if (TLI->isSelectSupported(SelectKind)) {
    // We have efficient codegen support for the select instruction.
    // Check if it is profitable to keep this 'select'.
    if (!TLI->isPredictableSelectExpensive() ||
        !isFormingBranchFromSelectProfitable(SI))
      return false;
  }

  ModifiedDT = true;

  // First, we split the block containing the select into 2 blocks.
  BasicBlock *StartBlock = SI->getParent();
  BasicBlock::iterator SplitPt = ++(BasicBlock::iterator(SI));
  BasicBlock *NextBlock = StartBlock->splitBasicBlock(SplitPt, "select.end");

  // Create a new block serving as the landing pad for the branch.
  BasicBlock *SmallBlock = BasicBlock::Create(SI->getContext(), "select.mid",
                                             NextBlock->getParent(), NextBlock);

  // Move the unconditional branch from the block with the select in it into our
  // landing pad block.
  StartBlock->getTerminator()->eraseFromParent();
  BranchInst::Create(NextBlock, SmallBlock);

  // Insert the real conditional branch based on the original condition.
  BranchInst::Create(NextBlock, SmallBlock, SI->getCondition(), SI);

  // The select itself is replaced with a PHI Node.
  PHINode *PN = PHINode::Create(SI->getType(), 2, "", NextBlock->begin());
  PN->takeName(SI);
  PN->addIncoming(SI->getTrueValue(), StartBlock);
  PN->addIncoming(SI->getFalseValue(), SmallBlock);
  SI->replaceAllUsesWith(PN);
  SI->eraseFromParent();

  // Instruct OptimizeBlock to skip to the next block.
  CurInstIterator = StartBlock->end();
  ++NumSelectsExpanded;
  return true;
}

bool CodeGenPrepare::OptimizeSelectInst(SelectInst *SI) {
  // If we have a SelectInst that will likely profit from branch prediction,
  // turn it into a branch.
  if (DisableSelectToBranch || OptSize || !TLI ||
      !TLI->isPredictableSelectExpensive())
    return false;

  if (!SI->getCondition()->getType()->isIntegerTy(1) ||
      !isFormingBranchFromSelectProfitable(SI))
    return false;

  ModifiedDT = true;

  // First, we split the block containing the select into 2 blocks.
  BasicBlock *StartBlock = SI->getParent();
  BasicBlock::iterator SplitPt = ++(BasicBlock::iterator(SI));
  BasicBlock *NextBlock = StartBlock->splitBasicBlock(SplitPt, "select.end");

  // Create a new block serving as the landing pad for the branch.
  BasicBlock *SmallBlock = BasicBlock::Create(SI->getContext(), "select.mid",
                                             NextBlock->getParent(), NextBlock);

  // Move the unconditional branch from the block with the select in it into our
  // landing pad block.
  StartBlock->getTerminator()->eraseFromParent();
  BranchInst::Create(NextBlock, SmallBlock);

  // Insert the real conditional branch based on the original condition.
  BranchInst::Create(NextBlock, SmallBlock, SI->getCondition(), SI);

  // The select itself is replaced with a PHI Node.
  PHINode *PN = PHINode::Create(SI->getType(), 2, "", NextBlock->begin());
  PN->takeName(SI);
  PN->addIncoming(SI->getTrueValue(), StartBlock);
  PN->addIncoming(SI->getFalseValue(), SmallBlock);
  SI->replaceAllUsesWith(PN);
  SI->eraseFromParent();

  // Instruct OptimizeBlock to skip to the next block.
  CurInstIterator = StartBlock->end();
  ++NumSelectsExpanded;
  return true;
}

/// RewriteLoopExitValues - Check to see if this loop has a computable
/// loop-invariant execution count.  If so, this means that we can compute the
/// final value of any expressions that are recurrent in the loop, and
/// substitute the exit values from the loop into any instructions outside of
/// the loop that use the final values of the current expressions.
///
/// This is mostly redundant with the regular IndVarSimplify activities that
/// happen later, except that it's more powerful in some cases, because it's
/// able to brute-force evaluate arbitrary instructions as long as they have
/// constant operands at the beginning of the loop.
void IndVarSimplify::RewriteLoopExitValues(Loop *L,
                                           SCEVExpander &Rewriter) {
  // Verify the input to the pass in already in LCSSA form.
  assert(L->isLCSSAForm(*DT));

  SmallVector<BasicBlock*, 8> ExitBlocks;
  L->getUniqueExitBlocks(ExitBlocks);

  // Find all values that are computed inside the loop, but used outside of it.
  // Because of LCSSA, these values will only occur in LCSSA PHI Nodes.  Scan
  // the exit blocks of the loop to find them.
  for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
    BasicBlock *ExitBB = ExitBlocks[i];

    // If there are no PHI nodes in this exit block, then no values defined
    // inside the loop are used on this path, skip it.
    PHINode *PN = dyn_cast<PHINode>(ExitBB->begin());
    if (!PN) continue;

    unsigned NumPreds = PN->getNumIncomingValues();

    // Iterate over all of the PHI nodes.
    BasicBlock::iterator BBI = ExitBB->begin();
    while ((PN = dyn_cast<PHINode>(BBI++))) {
      if (PN->use_empty())
        continue; // dead use, don't replace it

      // SCEV only supports integer expressions for now.
      if (!PN->getType()->isIntegerTy() && !PN->getType()->isPointerTy())
        continue;

      // It's necessary to tell ScalarEvolution about this explicitly so that
      // it can walk the def-use list and forget all SCEVs, as it may not be
      // watching the PHI itself. Once the new exit value is in place, there
      // may not be a def-use connection between the loop and every instruction
      // which got a SCEVAddRecExpr for that loop.
      SE->forgetValue(PN);

      // Iterate over all of the values in all the PHI nodes.
      for (unsigned i = 0; i != NumPreds; ++i) {
        // If the value being merged in is not integer or is not defined
        // in the loop, skip it.
        Value *InVal = PN->getIncomingValue(i);
        if (!isa<Instruction>(InVal))
          continue;

        // If this pred is for a subloop, not L itself, skip it.
        if (LI->getLoopFor(PN->getIncomingBlock(i)) != L)
          continue; // The Block is in a subloop, skip it.

        // Check that InVal is defined in the loop.
        Instruction *Inst = cast<Instruction>(InVal);
        if (!L->contains(Inst))
          continue;

        // Okay, this instruction has a user outside of the current loop
        // and varies predictably *inside* the loop.  Evaluate the value it
        // contains when the loop exits, if possible.
        const SCEV *ExitValue = SE->getSCEVAtScope(Inst, L->getParentLoop());
        if (!ExitValue->isLoopInvariant(L))
          continue;

        Changed = true;
        ++NumReplaced;

        Value *ExitVal = Rewriter.expandCodeFor(ExitValue, PN->getType(), Inst);

        DEBUG(dbgs() << "INDVARS: RLEV: AfterLoopVal = " << *ExitVal << '\n'
                     << "  LoopVal = " << *Inst << "\n");

        PN->setIncomingValue(i, ExitVal);

        // If this instruction is dead now, delete it.
        RecursivelyDeleteTriviallyDeadInstructions(Inst);

        if (NumPreds == 1) {
          // Completely replace a single-pred PHI. This is safe, because the
          // NewVal won't be variant in the loop, so we don't need an LCSSA phi
          // node anymore.
          PN->replaceAllUsesWith(ExitVal);
          RecursivelyDeleteTriviallyDeadInstructions(PN);
        }
      }
      if (NumPreds != 1) {
        // Clone the PHI and delete the original one. This lets IVUsers and
        // any other maps purge the original user from their records.
        PHINode *NewPN = cast<PHINode>(PN->clone());
        NewPN->takeName(PN);
        NewPN->insertBefore(PN);
        PN->replaceAllUsesWith(NewPN);
//.........这里部分代码省略.........