C++ BasicBlock::canSplitPredecessors方法代码示例

static bool canSplitCallSite(CallSite CS, TargetTransformInfo &TTI) {
  // FIXME: As of now we handle only CallInst. InvokeInst could be handled
  // without too much effort.
  Instruction *Instr = CS.getInstruction();
  if (!isa<CallInst>(Instr))
    return false;

  BasicBlock *CallSiteBB = Instr->getParent();
  // Allow splitting a call-site only when the CodeSize cost of the
  // instructions before the call is less then DuplicationThreshold. The
  // instructions before the call will be duplicated in the split blocks and
  // corresponding uses will be updated.
  unsigned Cost = 0;
  for (auto &InstBeforeCall :
       llvm::make_range(CallSiteBB->begin(), Instr->getIterator())) {
    Cost += TTI.getInstructionCost(&InstBeforeCall,
                                   TargetTransformInfo::TCK_CodeSize);
    if (Cost >= DuplicationThreshold)
      return false;
  }

  // Need 2 predecessors and cannot split an edge from an IndirectBrInst.
  SmallVector<BasicBlock *, 2> Preds(predecessors(CallSiteBB));
  if (Preds.size() != 2 || isa<IndirectBrInst>(Preds[0]->getTerminator()) ||
      isa<IndirectBrInst>(Preds[1]->getTerminator()))
    return false;

  return CallSiteBB->canSplitPredecessors();
}

/// \brief If this loop has multiple backedges, try to pull one of them out into
/// a nested loop.
///
/// This is important for code that looks like
/// this:
///
///  Loop:
///     ...
///     br cond, Loop, Next
///     ...
///     br cond2, Loop, Out
///
/// To identify this common case, we look at the PHI nodes in the header of the
/// loop.  PHI nodes with unchanging values on one backedge correspond to values
/// that change in the "outer" loop, but not in the "inner" loop.
///
/// If we are able to separate out a loop, return the new outer loop that was
/// created.
///
static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,
                                DominatorTree *DT, LoopInfo *LI,
                                ScalarEvolution *SE, Pass *PP,
                                AssumptionCache *AC) {
  // Don't try to separate loops without a preheader.
  if (!Preheader)
    return nullptr;

  // The header is not a landing pad; preheader insertion should ensure this.
  BasicBlock *Header = L->getHeader();
  assert(!Header->isLandingPad() && "Can't insert backedge to landing pad");
  if (!Header->canSplitPredecessors())
    return nullptr;

  PHINode *PN = findPHIToPartitionLoops(L, DT, AC);
  if (!PN) return nullptr;  // No known way to partition.

  // Pull out all predecessors that have varying values in the loop.  This
  // handles the case when a PHI node has multiple instances of itself as
  // arguments.
  SmallVector<BasicBlock*, 8> OuterLoopPreds;
  for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
    if (PN->getIncomingValue(i) != PN ||
        !L->contains(PN->getIncomingBlock(i))) {
      // We can't split indirectbr edges.
      if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator()))
        return nullptr;
      OuterLoopPreds.push_back(PN->getIncomingBlock(i));
    }
  }
  DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");

  // If ScalarEvolution is around and knows anything about values in
  // this loop, tell it to forget them, because we're about to
  // substantially change it.
  if (SE)
    SE->forgetLoop(L);

  bool PreserveLCSSA = PP->mustPreserveAnalysisID(LCSSAID);

  BasicBlock *NewBB = SplitBlockPredecessors(Header, OuterLoopPreds, ".outer",
                                             DT, LI, PreserveLCSSA);

  // Make sure that NewBB is put someplace intelligent, which doesn't mess up
  // code layout too horribly.
  placeSplitBlockCarefully(NewBB, OuterLoopPreds, L);

  // Create the new outer loop.
  Loop *NewOuter = new Loop();

  // Change the parent loop to use the outer loop as its child now.
  if (Loop *Parent = L->getParentLoop())
    Parent->replaceChildLoopWith(L, NewOuter);
  else
    LI->changeTopLevelLoop(L, NewOuter);

  // L is now a subloop of our outer loop.
  NewOuter->addChildLoop(L);

  for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
       I != E; ++I)
    NewOuter->addBlockEntry(*I);

  // Now reset the header in L, which had been moved by
  // SplitBlockPredecessors for the outer loop.
  L->moveToHeader(Header);

  // Determine which blocks should stay in L and which should be moved out to
  // the Outer loop now.
  std::set<BasicBlock*> BlocksInL;
  for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
    BasicBlock *P = *PI;
    if (DT->dominates(Header, P))
      addBlockAndPredsToSet(P, Header, BlocksInL);
  }

  // Scan all of the loop children of L, moving them to OuterLoop if they are
  // not part of the inner loop.
  const std::vector<Loop*> &SubLoops = L->getSubLoops();
  for (size_t I = 0; I != SubLoops.size(); )
    if (BlocksInL.count(SubLoops[I]->getHeader()))
//.........这里部分代码省略.........