[llvm] [VPlan] Generalize `VPAllSuccessorsIterator` to support predecessors (PR #178724)

[Florian Hahn via llvm-commits]

================
@@ -25,90 +25,132 @@ namespace llvm {
 // GraphTraits specializations for VPlan Hierarchical Control-Flow Graphs     //
 //===----------------------------------------------------------------------===//
 
-/// Iterator to traverse all successors of a VPBlockBase node. This includes the
-/// entry node of VPRegionBlocks. Exit blocks of a region implicitly have their
-/// parent region's successors. This ensures all blocks in a region are visited
-/// before any blocks in a successor region when doing a reverse post-order
-// traversal of the graph. Region blocks themselves traverse only their entries
-// directly and not their successors. Those will be traversed when a region's
-// exiting block is traversed
-template <typename BlockPtrTy>
-class VPAllSuccessorsIterator
-    : public iterator_facade_base<VPAllSuccessorsIterator<BlockPtrTy>,
-                                  std::bidirectional_iterator_tag,
-                                  VPBlockBase> {
+/// Iterator to traverse all successors/predecessors of a VPBlockBase node, such
+/// that:
+///
+///     A
+///     |
+///  +-----+ <- Region R
+///  |  b  |
+///  |     |
+///  | ... |
+///  |     |
+///  |  e  |
+///  +-----+
+///     |
+///     B
+///
+///  children(A) == {R} ; Forward == true
+///  children(R) == {b} ; Forward == true
+///  children(e) == {B} ; Forward == true
+///
+///  children(B) == {R} ; Forward == false
+///  children(R) == {e} ; Forward == false
+///  children(b) == {A} ; Forward == false
+///
+/// This ensures that all blocks of the region are visited before continuing
+/// traversal outside the region when doing a reverse post-order traversal of
+/// the VPlan.
+///
+template <typename BlockPtrTy, bool Forward = true>
+class VPImmediateHierarchicalChildrenIterator
+    : public iterator_facade_base<
+          VPImmediateHierarchicalChildrenIterator<BlockPtrTy, Forward>,
+          std::bidirectional_iterator_tag, VPBlockBase> {
   BlockPtrTy Block;
-  /// Index of the current successor. For VPBasicBlock nodes, this simply is the
-  /// index for the successor array. For VPRegionBlock, SuccessorIdx == 0 is
-  /// used for the region's entry block, and SuccessorIdx - 1 are the indices
-  /// for the successor array.
-  size_t SuccessorIdx;
-
-  static BlockPtrTy getBlockWithSuccs(BlockPtrTy Current) {
-    while (Current && Current->getNumSuccessors() == 0)
+  /// Index of the current successor/predecessor.
+  ///
+  /// For VPBasicBlock nodes, this simply is the index for the
+  /// successors/predecessors array. For VPRegionBlock, EdgeIdx == 0 is used for
+  /// the region's entry/exiting block, and EdgeIdx - 1 are the indices for the
+  /// successors/predecessors array.
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fhahn wrote:

hard to tell from the diff if anything changes. if not, can retain original formatting?

https://github.com/llvm/llvm-project/pull/178724