[Mlir-commits] [mlir] [mlir][Transform] Fix crash in SequenceOp::getEffects when body region is empty (PR #185063)

[Mehdi Amini]

https://github.com/joker-eph created https://github.com/llvm/llvm-project/pull/185063

When walking operations post-order and erasing blocks, the inner body block of a nested transform.sequence can be erased while the outer op is still alive. If printAsOperand is called on the outer block at that point, it triggers verification, which calls SequenceOp::getEffects -> getPotentialTopLevelEffects -> getBodyBlock() -> Region::front() on an empty region, causing an assertion failure in ilist_iterator ('\!NodePtr->isKnownSentinel()').

Fix by checking that the body region is non-empty before passing its front block to detail::getPotentialTopLevelEffects in the PossibleTopLevelTransformOpTrait.

Fixes #60213

Assisted-by: Claude Code

>From 8a9527ee39b86353a6f5cf870231913649e3f613 Mon Sep 17 00:00:00 2001
From: Mehdi Amini <joker.eph at gmail.com>
Date: Fri, 6 Mar 2026 05:58:14 -0800
Subject: [PATCH] [mlir][Transform] Fix crash in SequenceOp::getEffects when
 body region is empty

When walking operations post-order and erasing blocks, the inner body block of
a nested transform.sequence can be erased while the outer op is still alive.
If printAsOperand is called on the outer block at that point, it triggers
verification, which calls SequenceOp::getEffects -> getPotentialTopLevelEffects
-> getBodyBlock() -> Region::front() on an empty region, causing an assertion
failure in ilist_iterator ('\!NodePtr->isKnownSentinel()').

Fix by checking that the body region is non-empty before passing its front block
to detail::getPotentialTopLevelEffects in the PossibleTopLevelTransformOpTrait.

Fixes #60213

Assisted-by: Claude Code
---
 .../Transform/Interfaces/TransformInterfaces.h   |  5 ++++-
 mlir/test/IR/visitors.mlir                       | 16 ++++++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/mlir/include/mlir/Dialect/Transform/Interfaces/TransformInterfaces.h b/mlir/include/mlir/Dialect/Transform/Interfaces/TransformInterfaces.h
index 8b0517a84a675..4a07dc9e7457b 100644
--- a/mlir/include/mlir/Dialect/Transform/Interfaces/TransformInterfaces.h
+++ b/mlir/include/mlir/Dialect/Transform/Interfaces/TransformInterfaces.h
@@ -1169,9 +1169,12 @@ class PossibleTopLevelTransformOpTrait
   /// Populates `effects` with side effects implied by this trait.
   void getPotentialTopLevelEffects(
       SmallVectorImpl<MemoryEffects::EffectInstance> &effects) {
+    Region &region = this->getOperation()->getRegion(0);
+    if (region.empty())
+      return;
     detail::getPotentialTopLevelEffects(
         this->getOperation(), cast<OpTy>(this->getOperation()).getRoot(),
-        *getBodyBlock(), effects);
+        region.front(), effects);
   }
 
   /// Sets up the mapping between the entry block of the given region of this op
diff --git a/mlir/test/IR/visitors.mlir b/mlir/test/IR/visitors.mlir
index 24929f9fb6cf1..85c3d52802c33 100644
--- a/mlir/test/IR/visitors.mlir
+++ b/mlir/test/IR/visitors.mlir
@@ -422,3 +422,19 @@ func.func @test_no_skip_block_erasure_block_args(%arg0: i32, %arg1: i32) -> i32
 module {}
 // CHECK-LABEL: Block post-order erasures (no skip)
 // CHECK-NEXT:  Erasing block ^bb0 from region 0 from operation 'builtin.module'
+
+// -----
+
+// Regression test for https://github.com/llvm/llvm-project/issues/60213:
+// testNoSkipErasureCallbacks should not crash when getEffects is called on a
+// transform.sequence op whose body block has already been erased. Previously
+// printAsOperand triggered verification which called SequenceOp::getEffects ->
+// getBodyBlock() -> Region::front() on an empty region, causing an assertion.
+// CHECK-LABEL: Block post-order erasures (no skip)
+// CHECK:  Erasing block {{.*}} from region 0 from operation 'transform.sequence'
+transform.sequence failures(propagate) {
+^bb0(%arg0: !pdl.operation):
+  sequence %arg0 : !pdl.operation failures(propagate) {
+  ^bb0(%arg1: !pdl.operation):
+  }
+}