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

[llvmlistbot at llvm.org]

================
@@ -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.
+  BranchOp nextBranch = dyn_cast<BranchOp>(successorDest->getTerminator());
+  llvm::DenseSet<Block *> visited{successorDest};
+  while (nextBranch) {
+    Block *nextBranchDest = nextBranch.getDest();
+    if (nextBranchDest == successor)
+      return failure();
+    else if (visited.contains(nextBranchDest))
+      break;
----------------
benwu25 wrote:

BTW, we would need to update the `flang` test to check for this cycle instead of what it currently checks. Not a large change.
```
! CHECK-LABEL:  func.func @_QPsb
! CHECK:          omp.parallel
! CHECK:            cf.cond_br %{{[0-9]+}}, ^bb1, ^bb2
! CHECK-NEXT:     ^bb1:
! CHECK:            cf.br ^bb2
! CHECK-NEXT:     ^bb2:
! CHECK-NEXT:       cf.br ^bb3
! CHECK-NEXT:     ^bb3:
! CHECK-NEXT:       cf.br ^bb3
! CHECK-NEXT:     }
```

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