C++ PHINode::blocks方法代码示例

// Compute the unlikely successors to the block BB in the loop L, specifically
// those that are unlikely because this is a loop, and add them to the
// UnlikelyBlocks set.
static void
computeUnlikelySuccessors(const BasicBlock *BB, Loop *L,
                          SmallPtrSetImpl<const BasicBlock*> &UnlikelyBlocks) {
  // Sometimes in a loop we have a branch whose condition is made false by
  // taking it. This is typically something like
  //  int n = 0;
  //  while (...) {
  //    if (++n >= MAX) {
  //      n = 0;
  //    }
  //  }
  // In this sort of situation taking the branch means that at the very least it
  // won't be taken again in the next iteration of the loop, so we should
  // consider it less likely than a typical branch.
  //
  // We detect this by looking back through the graph of PHI nodes that sets the
  // value that the condition depends on, and seeing if we can reach a successor
  // block which can be determined to make the condition false.
  //
  // FIXME: We currently consider unlikely blocks to be half as likely as other
  // blocks, but if we consider the example above the likelyhood is actually
  // 1/MAX. We could therefore be more precise in how unlikely we consider
  // blocks to be, but it would require more careful examination of the form
  // of the comparison expression.
  const BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
  if (!BI || !BI->isConditional())
    return;

  // Check if the branch is based on an instruction compared with a constant
  CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
  if (!CI || !isa<Instruction>(CI->getOperand(0)) ||
      !isa<Constant>(CI->getOperand(1)))
    return;

  // Either the instruction must be a PHI, or a chain of operations involving
  // constants that ends in a PHI which we can then collapse into a single value
  // if the PHI value is known.
  Instruction *CmpLHS = dyn_cast<Instruction>(CI->getOperand(0));
  PHINode *CmpPHI = dyn_cast<PHINode>(CmpLHS);
  Constant *CmpConst = dyn_cast<Constant>(CI->getOperand(1));
  // Collect the instructions until we hit a PHI
  SmallVector<BinaryOperator *, 1> InstChain;
  while (!CmpPHI && CmpLHS && isa<BinaryOperator>(CmpLHS) &&
         isa<Constant>(CmpLHS->getOperand(1))) {
    // Stop if the chain extends outside of the loop
    if (!L->contains(CmpLHS))
      return;
    InstChain.push_back(cast<BinaryOperator>(CmpLHS));
    CmpLHS = dyn_cast<Instruction>(CmpLHS->getOperand(0));
    if (CmpLHS)
      CmpPHI = dyn_cast<PHINode>(CmpLHS);
  }
  if (!CmpPHI || !L->contains(CmpPHI))
    return;

  // Trace the phi node to find all values that come from successors of BB
  SmallPtrSet<PHINode*, 8> VisitedInsts;
  SmallVector<PHINode*, 8> WorkList;
  WorkList.push_back(CmpPHI);
  VisitedInsts.insert(CmpPHI);
  while (!WorkList.empty()) {
    PHINode *P = WorkList.back();
    WorkList.pop_back();
    for (BasicBlock *B : P->blocks()) {
      // Skip blocks that aren't part of the loop
      if (!L->contains(B))
        continue;
      Value *V = P->getIncomingValueForBlock(B);
      // If the source is a PHI add it to the work list if we haven't
      // already visited it.
      if (PHINode *PN = dyn_cast<PHINode>(V)) {
        if (VisitedInsts.insert(PN).second)
          WorkList.push_back(PN);
        continue;
      }
      // If this incoming value is a constant and B is a successor of BB, then
      // we can constant-evaluate the compare to see if it makes the branch be
      // taken or not.
      Constant *CmpLHSConst = dyn_cast<Constant>(V);
      if (!CmpLHSConst ||
          std::find(succ_begin(BB), succ_end(BB), B) == succ_end(BB))
        continue;
      // First collapse InstChain
      for (Instruction *I : llvm::reverse(InstChain)) {
        CmpLHSConst = ConstantExpr::get(I->getOpcode(), CmpLHSConst,
                                        cast<Constant>(I->getOperand(1)), true);
        if (!CmpLHSConst)
          break;
      }
      if (!CmpLHSConst)
        continue;
      // Now constant-evaluate the compare
      Constant *Result = ConstantExpr::getCompare(CI->getPredicate(),
                                                  CmpLHSConst, CmpConst, true);
      // If the result means we don't branch to the block then that block is
      // unlikely.
      if (Result &&
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