diff --git a/mlir/test/lib/TestDialect/TestOps.td b/mlir/test/lib/TestDialect/TestOps.td
--- a/mlir/test/lib/TestDialect/TestOps.td
+++ b/mlir/test/lib/TestDialect/TestOps.td
@@ -592,6 +592,21 @@
 def : Pattern<(OpNativeCodeCall3 $input),
               [(NativeCodeCall<"createOpI($_builder, $0)"> $input), (OpK)]>;
 
+// Test that the $_op hook works.
+def OpHook : TEST_Op<"op_hook"> {
+  let arguments = (ins
+    I32:$input,
+    I64Attr:$result_count
+  );
+  let results = (outs I32);
+}
+
+def CheckResultCount : Constraint<CPred<"$_op.getOperation()->getNumResults() == $0.getInt()">>;
+
+def : Pat<(OpHook:$op $input, $result_count),
+            (OpNativeCodeCall3 $input),
+            [(CheckResultCount $result_count)]>;
+
 // Test AllAttrConstraintsOf.
 def OpAllAttrConstraint1 : TEST_Op<"all_attr_constraint_of1"> {
   let arguments = (ins I64ArrayAttr:$attr);
diff --git a/mlir/test/mlir-tblgen/pattern.mlir b/mlir/test/mlir-tblgen/pattern.mlir
--- a/mlir/test/mlir-tblgen/pattern.mlir
+++ b/mlir/test/mlir-tblgen/pattern.mlir
@@ -76,6 +76,22 @@
   return %0 : i32
 }
 
+// CHECK-LABEL: verifyOpHookValid
+func @verifyOpHookValid(%arg0: i32) -> (i32) {
+  // CHECK: test.op_i
+  // CHECK: test.op_k
+  %0 = "test.op_hook"(%arg0) {result_count = 1} : (i32) -> (i32)
+  return %0 : i32
+}
+
+// CHECK-LABEL: verifyOpHookInvalid
+func @verifyOpHookInvalid(%arg0: i32) -> (i32) {
+  // The result count is 1, not 2, so lowering constraint should fail.
+  // CHECK: test.op_hook
+  %0 = "test.op_hook"(%arg0) {result_count = 2} : (i32) -> (i32)
+  return %0 : i32
+}
+
 // CHECK-LABEL: verifyAllAttrConstraintOf
 func @verifyAllAttrConstraintOf() -> (i32, i32, i32) {
   // CHECK: "test.all_attr_constraint_of2"
diff --git a/mlir/tools/mlir-tblgen/RewriterGen.cpp b/mlir/tools/mlir-tblgen/RewriterGen.cpp
--- a/mlir/tools/mlir-tblgen/RewriterGen.cpp
+++ b/mlir/tools/mlir-tblgen/RewriterGen.cpp
@@ -382,6 +382,8 @@
     auto &constraint = appliedConstraint.constraint;
     auto &entities = appliedConstraint.entities;
 
+    fmtCtx.withOp("op");
+
     auto condition = constraint.getConditionTemplate();
     auto cmd = "if (!({0})) return matchFailure();\n";