insert a statement before the execution of a loop

Question

In LLVM intermediate representation, how can I insert an instruction for a loop, which will be executed exactly once before the execution of that loop? Inserting the instruction to the preheader does not work, because preheader is NULL for some loops.

Answer 1

If the loop does not have a preheader, you can create new preheader.

Here is an example http://www.cs.ucla.edu/classes/spring08/cs259/llvm-2.2/lib/Transforms/Utils/LoopSimplify.cpp or http://www.opensource.apple.com/source/clang/clang-23/clang/lib/Transforms/Utils/LoopSimplify.cpp (find function InsertPreheaderForLoop and call to it)

/// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
/// preheader, this method is called to insert one.  This method has two phases:
/// preheader insertion and analysis updating.
///
void LoopSimplify::InsertPreheaderForLoop(Loop *L) {
  BasicBlock *Header = L->getHeader();

  // Compute the set of predecessors of the loop that are not in the loop.
  std::vector<BasicBlock*> OutsideBlocks;
  for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
       PI != PE; ++PI)
    if (!L->contains(*PI))           // Coming in from outside the loop?
      OutsideBlocks.push_back(*PI);  // Keep track of it...

  // Split out the loop pre-header.
  BasicBlock *NewBB =
    SplitBlockPredecessors(Header, ".preheader", OutsideBlocks);


  //===--------------------------------------------------------------------===//
  //  Update analysis results now that we have performed the transformation
  //

  // We know that we have loop information to update... update it now.
  if (Loop *Parent = L->getParentLoop())
    Parent->addBasicBlockToLoop(NewBB, LI->getBase());

  DT->splitBlock(NewBB);
  if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>())
    DF->splitBlock(NewBB);

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