[llvm] [CycleInfo] Store blocks using Euler tour representation (PR #208614)

[Alexis Engelke via llvm-commits]

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@@ -333,20 +343,73 @@ void GenericCycleInfo<ContextT>::addBlockToCycle(BlockT *Block, CycleT *Cycle) {
   if (Number >= BlockMap.size())
     BlockMap.resize(GraphTraits<FunctionT *>::getMaxNumber(Block->getParent()));
 
-  // FixMe: Appending NewBlock is fine as a set of blocks in a cycle. When
-  // printing, cycle NewBlock is at the end of list but it should be in the
-  // middle to represent actual traversal of a cycle.
-  Cycle->appendBlock(Block);
+  // Insert Block at Pos (end of Cycle's slice) and shift every later cycle's
+  // range right, keyed on IdxBegin so a nested cycle sharing end == Pos is
+  // left alone; Cycle and its ancestors then grow to contain it, below.
+  // FIXME: appended at the slice end, not the traversal-order middle.
+  unsigned Pos = Cycle->IdxEnd;
+  BlockLayout.insert(BlockLayout.begin() + Pos, Block);
+  for (CycleT *TLC : toplevel_cycles())
+    for (auto It = df_begin(TLC), E = df_end(TLC); It != E;) {
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aengelke wrote:

NB: df_iterator is rather expensive. But addBlockToCycle is used rarely, so this is ok.

https://github.com/llvm/llvm-project/pull/208614