invoke-polymorphic

The invoke-polymorphic instruction was introduced alongside invoke-custom in DEX version 038, specifically designed to call “signature polymorphic” methods like MethodHandle.invoke and MethodHandle.invokeExact. This functionality enables more flexible method invocation in bytecode, allowing methods to be called with a variable number and type of arguments, without the need for wrapping them in arrays or converting primitive types.

What Does “Signature Polymorphism” Mean?

In the case of invoke-polymorphic, “polymorphism” refers to methods capable of handling different types and numbers of arguments, known as “signature polymorphic” methods. Unlike standard methods, which require fixed parameter types and counts, signature polymorphic methods can accept any combination of argument types directly in bytecode. This contrasts with Java’s usual Object... (varargs) parameter style, where arguments are typically passed as an array of Objects, making invoke-polymorphic highly efficient for diverse method signatures.

Currently, the only signature polymorphic methods in Java are:

• MethodHandle.invoke(Object...)
• MethodHandle.invokeExact(Object...)

These methods are designed to accept different signatures, a capability defined in the JVM Specification, §2.9.3.

Syntax of invoke-polymorphic

invoke-polymorphic {vC, vD, ...}, Ljava/lang/invoke/MethodHandle;->methodName([Ljava/lang/Object;)Ljava/lang/Object;, (Signature)ReturnType

In this structure:

• Registers ({vC, vD, ...}): These contain the actual arguments for the MethodHandle invocation.
• Method Reference (Ljava/lang/invoke/MethodHandle;->methodName(...)): Refers to invoke or invokeExact method, specifying the signature and return type dynamically.
• Signature and Return Type: Directly specified in the instruction, allowing it to match the expected argument and return types of each invocation.

Practical Example of invoke-polymorphic

Consider this Java method that uses MethodHandle.invoke and MethodHandle.invokeExact:

void foo(MethodHandle handle) throws Throwable {
    handle.invoke(10, 20);
    handle.invokeExact("foo", "bar");
}

This code translates into Dalvik bytecode as follows:

const/16 v0, 0xa
const/16 v1, 0x14
invoke-polymorphic {p1, v0, v1}, Ljava/lang/invoke/MethodHandle;->invoke([Ljava/lang/Object;)Ljava/lang/Object;, (II)V

const-string v0, "foo"
const-string v1, "bar"
invoke-polymorphic {p1, v0, v1}, Ljava/lang/invoke/MethodHandle;->invokeExact([Ljava/lang/Object;)Ljava/lang/Object;, (Ljava/lang/String;Ljava/lang/String;)V

In this bytecode:

1. The invoke-polymorphic instruction is used to call MethodHandle.invoke and MethodHandle.invokeExact directly, with arguments passed in registers ({v0, v1}) instead of a single array.
2. The (II)V and (Ljava/lang/String;Ljava/lang/String;)V type signatures are explicitly included, allowing the MethodHandle to handle primitive and object types interchangeably.

Why Use invoke-polymorphic?

• Efficiency: Unlike varargs methods, invoke-polymorphic calls can handle primitive types directly without boxing, and arguments don’t need to be bundled into arrays.
• Dynamic Invocation: Provides a flexible, low-overhead way to call methods dynamically in DEX, particularly beneficial for applications requiring runtime method dispatch.

Differences Between invoke-custom and invoke-polymorphic

• Purpose: invoke-custom provides a general mechanism for dynamic method calls, while invoke-polymorphic is specifically designed for signature polymorphic methods in the MethodHandle class.
• Signature Flexibility: invoke-polymorphic allows different signatures for each call without needing wrappers, unlike invoke-custom, which relies on the call_site_item structure to define a method signature statically.

References

• Dalvik bytecode format | Android Open Source Project
• JVM Specification §2.9.3
• Stack Answer