[Mlir-commits] [mlir] [mlir][Builtin] Fix crash folding self-referential unrealized_conversion_cast in graph regions (PR #207185)

[Anutosh Bhat]

https://github.com/anutosh491 updated https://github.com/llvm/llvm-project/pull/207185

>From 92a760069ab67b5d4fdaab65f711aff53619573b Mon Sep 17 00:00:00 2001
From: anutosh491 <andersonbhat491 at gmail.com>
Date: Thu, 2 Jul 2026 18:50:24 +0530
Subject: [PATCH] [mlir][Builtin] Fix crash folding self-referential
 unrealized_conversion_cast

---
 mlir/lib/IR/BuiltinDialect.cpp              |  5 +++++
 mlir/test/Dialect/Builtin/canonicalize.mlir | 11 +++++++++++
 2 files changed, 16 insertions(+)

diff --git a/mlir/lib/IR/BuiltinDialect.cpp b/mlir/lib/IR/BuiltinDialect.cpp
index c88b328282275..00cad06122dd8 100644
--- a/mlir/lib/IR/BuiltinDialect.cpp
+++ b/mlir/lib/IR/BuiltinDialect.cpp
@@ -201,6 +201,11 @@ UnrealizedConversionCastOp::fold(FoldAdaptor adaptor,
   ResultRange results = getOutputs();
 
   if (operands.getType() == results.getType()) {
+    // Skip if any operand is this op's own result (self-referential cycle).
+    if (llvm::any_of(operands, [this](Value v) {
+          return v.getDefiningOp() == getOperation();
+        }))
+      return failure();
     foldResults.append(operands.begin(), operands.end());
     return success();
   }
diff --git a/mlir/test/Dialect/Builtin/canonicalize.mlir b/mlir/test/Dialect/Builtin/canonicalize.mlir
index 2e36b7ee371c3..537868e990d78 100644
--- a/mlir/test/Dialect/Builtin/canonicalize.mlir
+++ b/mlir/test/Dialect/Builtin/canonicalize.mlir
@@ -23,3 +23,14 @@ func.func @multiple_conversion_casts_failure(%arg0: i32, %arg1: i32, %arg2: i64)
   %outputs:2 = builtin.unrealized_conversion_cast %arg2, %inputs#1 : i64, i64 to i32, i32
   return %outputs#0, %outputs#1 : i32, i32
 }
+
+// Test a self-referential cast in a graph region must not crash.
+// CHECK-LABEL: func @self_referential_cast
+// CHECK-NOT: unrealized_conversion_cast
+func.func @self_referential_cast() {
+  test.graph_region {
+    %0 = builtin.unrealized_conversion_cast %0 : i32 to i32
+    "test.return"() : () -> ()
+  }
+  return
+}