Disallow insertion of a root op in a block

src/jdk.incubator.code/share/classes/jdk/incubator/code/Block.java

@@ -827,6 +827,10 @@ public Op.Result op(Op op) {
          */
         public Op.Result op(Op op, OpTransformer transformer) {
             check();
+            if (op instanceof Op.Root) {
+               throw new IllegalStateException("Operation %s is root and can't be inserted in a block".formatted(op.getClass().getSimpleName()));
+            }
+
             final Op.Result oprToTransform = op.result();
 
             Op transformedOp = op;

src/jdk.incubator.code/share/classes/jdk/incubator/code/Op.java

@@ -171,6 +171,12 @@ public interface BlockTerminating extends Terminating {
         List<Block.Reference> successors();
     }
 
+    /**
+     * An operation characteristic indicating the operation is a root operation
+     */
+    public interface Root {
+    }
+
     /**
      * A value that is the result of an operation.
      */

src/jdk.incubator.code/share/classes/jdk/incubator/code/op/CoreOp.java

@@ -117,7 +117,7 @@ protected CoreOp(ExternalizedOp def) {
      */
     @OpFactory.OpDeclaration(FuncOp.NAME)
     public static final class FuncOp extends CoreOp
-            implements Op.Invokable, Op.Isolated, Op.Lowerable {
+            implements Op.Invokable, Op.Isolated, Op.Lowerable, Op.Root {
 
         public static class Builder {
             final Body.Builder ancestorBody;

test/langtools/tools/javac/reflect/RootOpInsertionTest.java

@@ -0,0 +1,31 @@
+import jdk.incubator.code.Block;
+import jdk.incubator.code.Body;
+import jdk.incubator.code.CodeReflection;
+import jdk.incubator.code.Op;
+import jdk.incubator.code.op.CoreOp;
+import jdk.incubator.code.type.FunctionType;
+import org.testng.Assert;
+import org.testng.annotations.Test;
+
+import java.lang.reflect.Method;
+
+/*
+ * @test
+ * @modules jdk.incubator.code
+ * @run testng RootOpInsertionTest
+ */
+public class RootOpInsertionTest {
+
+    @CodeReflection
+    static void f() {}
+
+    @Test
+    void test() throws NoSuchMethodException {
+        Method mf = this.getClass().getDeclaredMethod("f");
+        CoreOp.FuncOp funcOp = Op.ofMethod(mf).orElseThrow();
+
+        Body.Builder bodyBuilder = Body.Builder.of(null, FunctionType.VOID);
+        Block.Builder entryBlock = bodyBuilder.entryBlock();
+        Assert.assertThrows(IllegalStateException.class, () -> entryBlock.op(funcOp));
+    }
+}