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//===--------- LoopIterator.h - Iterate over loop blocks --------*- C++ -*-===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This file defines iterators to visit the basic blocks within a loop.
// These iterators currently visit blocks within subloops as well.
// Unfortunately we have no efficient way of summarizing loop exits which would
// allow skipping subloops during traversal.
// If you want to visit all blocks in a loop and don't need an ordered traveral,
// use Loop::block_begin() instead.
// This is intentionally designed to work with ill-formed loops in which the
// backedge has been deleted. The only prerequisite is that all blocks
// contained within the loop according to the most recent LoopInfo analysis are
// reachable from the loop header.
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/Analysis/LoopInfo.h"
namespace llvm {
class LoopBlocksTraversal;
/// Store the result of a depth first search within basic blocks contained by a
/// single loop.
/// TODO: This could be generalized for any CFG region, or the entire CFG.
class LoopBlocksDFS {
/// Postorder list iterators.
typedef std::vector<BasicBlock*>::const_iterator POIterator;
typedef std::vector<BasicBlock*>::const_reverse_iterator RPOIterator;
friend class LoopBlocksTraversal;
Loop *L;
/// Map each block to its postorder number. A block is only mapped after it is
/// preorder visited by DFS. It's postorder number is initially zero and set
/// to nonzero after it is finished by postorder traversal.
DenseMap<BasicBlock*, unsigned> PostNumbers;
std::vector<BasicBlock*> PostBlocks;
LoopBlocksDFS(Loop *Container) :
L(Container), PostNumbers(NextPowerOf2(Container->getNumBlocks())) {
Loop *getLoop() const { return L; }
/// Traverse the loop blocks and store the DFS result.
void perform(LoopInfo *LI);
/// Return true if postorder numbers are assigned to all loop blocks.
bool isComplete() const { return PostBlocks.size() == L->getNumBlocks(); }
/// Iterate over the cached postorder blocks.
POIterator beginPostorder() const {
assert(isComplete() && "bad loop DFS");
return PostBlocks.begin();
POIterator endPostorder() const { return PostBlocks.end(); }
/// Reverse iterate over the cached postorder blocks.
RPOIterator beginRPO() const {
assert(isComplete() && "bad loop DFS");
return PostBlocks.rbegin();
RPOIterator endRPO() const { return PostBlocks.rend(); }
/// Return true if this block has been preorder visited.
bool hasPreorder(BasicBlock *BB) const { return PostNumbers.count(BB); }
/// Return true if this block has a postorder number.
bool hasPostorder(BasicBlock *BB) const {
DenseMap<BasicBlock*, unsigned>::const_iterator I = PostNumbers.find(BB);
return I != PostNumbers.end() && I->second;
/// Get a block's postorder number.
unsigned getPostorder(BasicBlock *BB) const {
DenseMap<BasicBlock*, unsigned>::const_iterator I = PostNumbers.find(BB);
assert(I != PostNumbers.end() && "block not visited by DFS");
assert(I->second && "block not finished by DFS");
return I->second;
/// Get a block's reverse postorder number.
unsigned getRPO(BasicBlock *BB) const {
return 1 + PostBlocks.size() - getPostorder(BB);
void clear() {
/// Specialize po_iterator_storage to record postorder numbers.
template<> class po_iterator_storage<LoopBlocksTraversal, true> {
LoopBlocksTraversal &LBT;
po_iterator_storage(LoopBlocksTraversal &lbs) : LBT(lbs) {}
// These functions are defined below.
bool insertEdge(BasicBlock *From, BasicBlock *To);
void finishPostorder(BasicBlock *BB);
/// Traverse the blocks in a loop using a depth-first search.
class LoopBlocksTraversal {
/// Graph traversal iterator.
typedef po_iterator<BasicBlock*, LoopBlocksTraversal, true> POTIterator;
LoopBlocksDFS &DFS;
LoopInfo *LI;
LoopBlocksTraversal(LoopBlocksDFS &Storage, LoopInfo *LInfo) :
DFS(Storage), LI(LInfo) {}
/// Postorder traversal over the graph. This only needs to be done once.
/// po_iterator "automatically" calls back to visitPreorder and
/// finishPostorder to record the DFS result.
POTIterator begin() {
assert(DFS.PostBlocks.empty() && "Need clear DFS result before traversing");
assert(DFS.L->getNumBlocks() && "po_iterator cannot handle an empty graph");
return po_ext_begin(DFS.L->getHeader(), *this);
POTIterator end() {
// po_ext_end interface requires a basic block, but ignores its value.
return po_ext_end(DFS.L->getHeader(), *this);
/// Called by po_iterator upon reaching a block via a CFG edge. If this block
/// is contained in the loop and has not been visited, then mark it preorder
/// visited and return true.
/// TODO: If anyone is interested, we could record preorder numbers here.
bool visitPreorder(BasicBlock *BB) {
if (!DFS.L->contains(LI->getLoopFor(BB)))
return false;
return DFS.PostNumbers.insert(std::make_pair(BB, 0)).second;
/// Called by po_iterator each time it advances, indicating a block's
/// postorder.
void finishPostorder(BasicBlock *BB) {
assert(DFS.PostNumbers.count(BB) && "Loop DFS skipped preorder");
DFS.PostNumbers[BB] = DFS.PostBlocks.size();
inline bool po_iterator_storage<LoopBlocksTraversal, true>::
insertEdge(BasicBlock *From, BasicBlock *To) {
return LBT.visitPreorder(To);
inline void po_iterator_storage<LoopBlocksTraversal, true>::
finishPostorder(BasicBlock *BB) {
} // End namespace llvm