diff --git a/include/mlir-hlo/Dialect/mhlo/IR/chlo_ops.h b/include/mlir-hlo/Dialect/mhlo/IR/chlo_ops.h
index c5483e9..14a22e9 100644
--- a/include/mlir-hlo/Dialect/mhlo/IR/chlo_ops.h
+++ b/include/mlir-hlo/Dialect/mhlo/IR/chlo_ops.h
@@ -17,6 +17,7 @@ limitations under the License.
 #define TENSORFLOW_COMPILER_MLIR_HLO_INCLUDE_MLIR_HLO_DIALECT_MHLO_IR_CHLO_OPS_H_
 
 #include "llvm/ADT/StringRef.h"
+#include "mlir-hlo/Dialect/mhlo/IR/infer_fusibility_op_interface.h"
 #include "mlir/IR/Dialect.h"
 #include "mlir/IR/DialectImplementation.h"
 #include "mlir/IR/MLIRContext.h"
diff --git a/include/mlir-hlo/Dialect/mhlo/IR/chlo_ops.td b/include/mlir-hlo/Dialect/mhlo/IR/chlo_ops.td
index 79d6fb2..d7cdd12 100644
--- a/include/mlir-hlo/Dialect/mhlo/IR/chlo_ops.td
+++ b/include/mlir-hlo/Dialect/mhlo/IR/chlo_ops.td
@@ -33,6 +33,7 @@ include "mlir/IR/OpBase.td"
 include "mlir/Interfaces/InferTypeOpInterface.td"
 include "mlir/Interfaces/SideEffectInterfaces.td"
 include "mlir-hlo/Dialect/mhlo/IR/hlo_ops_base.td"
+include "mlir-hlo/Dialect/mhlo/IR/infer_fusibility_op_interface.td"
 
 def HLOClient_Dialect : Dialect {
   let name = "chlo";
@@ -338,6 +339,31 @@ def HLOClient_BroadcastComplexOp : HLOClient_BroadcastBinaryElementwiseOp<
   let results = (outs HLO_ComplexTensor);
 }
 
+//===----------------------------------------------------------------------===//
+// Unary op
+//===----------------------------------------------------------------------===//
+
+class HLOClient_UnaryElementwiseOp<string mnemonic, list<OpTrait> traits,
+    Type TensorType>: HLOClient_Op<mnemonic,
+      !listconcat(traits, [InferFusibilityOpInterface])> {
+  let arguments = (ins TensorType:$operand);
+  let results = (outs TensorType);
+}
+
+def HLOClient_AcosOp: HLOClient_UnaryElementwiseOp<"acos",
+    [NoSideEffect, SameOperandsAndResultType], HLO_FpOrComplexTensor> {
+  let summary = "Acos operator";
+
+  let description = [{
+    Returns `Acos(operand)` element-wise.
+
+    $$
+    \acos(x) = 2 * \atan(\sqrt(1 - x^2) / (1 + x)) if x != -1
+             = pi                                  if x == -1
+    $$
+  }];
+}
+
 //===----------------------------------------------------------------------===//
 // Broadcasting compare op
 //===----------------------------------------------------------------------===//