[Mlir-commits] [mlir] [MLIR][CF] Avoid collapsing blocks which participate in cycles (PR #160783)

[Matthias Springer]

================
@@ -122,6 +122,18 @@ static LogicalResult collapseBranch(Block *&successor,
   Block *successorDest = successorBranch.getDest();
   if (successorDest == successor)
     return failure();
+  // Don't try to collapse branches which participate in a cycle.
+  Block *currBlock = successorDest;
+  BranchOp nextBranch = dyn_cast<BranchOp>(currBlock->getTerminator());
+  while (nextBranch) {
----------------
matthias-springer wrote:

Here's an example that gets into an endless loop.
```
func.func @cycle_4_blocks(%c: i1) {
  cf.cond_br %c, ^bb6, ^bb7
  ^bb6:
  cf.br ^bb5 {F}
  ^bb5:
  cf.br ^bb1 {A}
  ^bb1:
    cf.br ^bb2 {B}
  ^bb2:
    cf.br ^bb3 {C}
  ^bb3:
    cf.br ^bb4 {D}
  ^bb4:
    cf.br ^bb1 {E}
  ^bb7:
    cf.br ^bb6
}
```


https://github.com/llvm/llvm-project/pull/160783