C++ ScalarEvolution::getMaxBackedgeTakenCount方法代码示例

/// Remove dead loops, by which we mean loops that do not impact the observable
/// behavior of the program other than finite running time.  Note we do ensure
/// that this never remove a loop that might be infinite, as doing so could
/// change the halting/non-halting nature of a program. NOTE: This entire
/// process relies pretty heavily on LoopSimplify and LCSSA in order to make
/// various safety checks work.
bool LoopDeletionPass::runImpl(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
                               LoopInfo &loopInfo) {
  assert(L->isLCSSAForm(DT) && "Expected LCSSA!");

  // We can only remove the loop if there is a preheader that we can
  // branch from after removing it.
  BasicBlock *preheader = L->getLoopPreheader();
  if (!preheader)
    return false;

  // If LoopSimplify form is not available, stay out of trouble.
  if (!L->hasDedicatedExits())
    return false;

  // We can't remove loops that contain subloops.  If the subloops were dead,
  // they would already have been removed in earlier executions of this pass.
  if (L->begin() != L->end())
    return false;

  SmallVector<BasicBlock *, 4> exitingBlocks;
  L->getExitingBlocks(exitingBlocks);

  SmallVector<BasicBlock *, 4> exitBlocks;
  L->getUniqueExitBlocks(exitBlocks);

  // We require that the loop only have a single exit block.  Otherwise, we'd
  // be in the situation of needing to be able to solve statically which exit
  // block will be branched to, or trying to preserve the branching logic in
  // a loop invariant manner.
  if (exitBlocks.size() != 1)
    return false;

  // Finally, we have to check that the loop really is dead.
  bool Changed = false;
  if (!isLoopDead(L, SE, exitingBlocks, exitBlocks, Changed, preheader))
    return Changed;

  // Don't remove loops for which we can't solve the trip count.
  // They could be infinite, in which case we'd be changing program behavior.
  const SCEV *S = SE.getMaxBackedgeTakenCount(L);
  if (isa<SCEVCouldNotCompute>(S))
    return Changed;

  // Now that we know the removal is safe, remove the loop by changing the
  // branch from the preheader to go to the single exit block.
  BasicBlock *exitBlock = exitBlocks[0];

  // Because we're deleting a large chunk of code at once, the sequence in which
  // we remove things is very important to avoid invalidation issues.  Don't
  // mess with this unless you have good reason and know what you're doing.

  // Tell ScalarEvolution that the loop is deleted. Do this before
  // deleting the loop so that ScalarEvolution can look at the loop
  // to determine what it needs to clean up.
  SE.forgetLoop(L);

  // Connect the preheader directly to the exit block.
  TerminatorInst *TI = preheader->getTerminator();
  TI->replaceUsesOfWith(L->getHeader(), exitBlock);

  // Rewrite phis in the exit block to get their inputs from
  // the preheader instead of the exiting block.
  BasicBlock *exitingBlock = exitingBlocks[0];
  BasicBlock::iterator BI = exitBlock->begin();
  while (PHINode *P = dyn_cast<PHINode>(BI)) {
    int j = P->getBasicBlockIndex(exitingBlock);
    assert(j >= 0 && "Can't find exiting block in exit block's phi node!");
    P->setIncomingBlock(j, preheader);
    for (unsigned i = 1; i < exitingBlocks.size(); ++i)
      P->removeIncomingValue(exitingBlocks[i]);
    ++BI;
  }

  // Update the dominator tree and remove the instructions and blocks that will
  // be deleted from the reference counting scheme.
  SmallVector<DomTreeNode*, 8> ChildNodes;
  for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
       LI != LE; ++LI) {
    // Move all of the block's children to be children of the preheader, which
    // allows us to remove the domtree entry for the block.
    ChildNodes.insert(ChildNodes.begin(), DT[*LI]->begin(), DT[*LI]->end());
    for (DomTreeNode *ChildNode : ChildNodes) {
      DT.changeImmediateDominator(ChildNode, DT[preheader]);
    }

    ChildNodes.clear();
    DT.eraseNode(*LI);

    // Remove the block from the reference counting scheme, so that we can
    // delete it freely later.
    (*LI)->dropAllReferences();
  }

  // Erase the instructions and the blocks without having to worry
//.........这里部分代码省略.........

/// Remove a loop if it is dead.
///
/// A loop is considered dead if it does not impact the observable behavior of
/// the program other than finite running time. This never removes a loop that
/// might be infinite (unless it is never executed), as doing so could change
/// the halting/non-halting nature of a program.
///
/// This entire process relies pretty heavily on LoopSimplify form and LCSSA in
/// order to make various safety checks work.
///
/// \returns true if any changes were made. This may mutate the loop even if it
/// is unable to delete it due to hoisting trivially loop invariant
/// instructions out of the loop.
static LoopDeletionResult deleteLoopIfDead(Loop *L, DominatorTree &DT,
                                           ScalarEvolution &SE, LoopInfo &LI) {
  assert(L->isLCSSAForm(DT) && "Expected LCSSA!");

  // We can only remove the loop if there is a preheader that we can branch from
  // after removing it. Also, if LoopSimplify form is not available, stay out
  // of trouble.
  BasicBlock *Preheader = L->getLoopPreheader();
  if (!Preheader || !L->hasDedicatedExits()) {
    DEBUG(dbgs()
          << "Deletion requires Loop with preheader and dedicated exits.\n");
    return LoopDeletionResult::Unmodified;
  }
  // We can't remove loops that contain subloops.  If the subloops were dead,
  // they would already have been removed in earlier executions of this pass.
  if (L->begin() != L->end()) {
    DEBUG(dbgs() << "Loop contains subloops.\n");
    return LoopDeletionResult::Unmodified;
  }


  BasicBlock *ExitBlock = L->getUniqueExitBlock();

  if (ExitBlock && isLoopNeverExecuted(L)) {
    DEBUG(dbgs() << "Loop is proven to never execute, delete it!");
    // Set incoming value to undef for phi nodes in the exit block.
    BasicBlock::iterator BI = ExitBlock->begin();
    while (PHINode *P = dyn_cast<PHINode>(BI)) {
      for (unsigned i = 0; i < P->getNumIncomingValues(); i++)
        P->setIncomingValue(i, UndefValue::get(P->getType()));
      BI++;
    }
    deleteDeadLoop(L, &DT, &SE, &LI);
    ++NumDeleted;
    return LoopDeletionResult::Deleted;
  }

  // The remaining checks below are for a loop being dead because all statements
  // in the loop are invariant.
  SmallVector<BasicBlock *, 4> ExitingBlocks;
  L->getExitingBlocks(ExitingBlocks);

  // We require that the loop only have a single exit block.  Otherwise, we'd
  // be in the situation of needing to be able to solve statically which exit
  // block will be branched to, or trying to preserve the branching logic in
  // a loop invariant manner.
  if (!ExitBlock) {
    DEBUG(dbgs() << "Deletion requires single exit block\n");
    return LoopDeletionResult::Unmodified;
  }
  // Finally, we have to check that the loop really is dead.
  bool Changed = false;
  if (!isLoopDead(L, SE, ExitingBlocks, ExitBlock, Changed, Preheader)) {
    DEBUG(dbgs() << "Loop is not invariant, cannot delete.\n");
    return Changed ? LoopDeletionResult::Modified
                   : LoopDeletionResult::Unmodified;
  }

  // Don't remove loops for which we can't solve the trip count.
  // They could be infinite, in which case we'd be changing program behavior.
  const SCEV *S = SE.getMaxBackedgeTakenCount(L);
  if (isa<SCEVCouldNotCompute>(S)) {
    DEBUG(dbgs() << "Could not compute SCEV MaxBackedgeTakenCount.\n");
    return Changed ? LoopDeletionResult::Modified
                   : LoopDeletionResult::Unmodified;
  }

  DEBUG(dbgs() << "Loop is invariant, delete it!");
  deleteDeadLoop(L, &DT, &SE, &LI);
  ++NumDeleted;

  return LoopDeletionResult::Deleted;
}