Support Function Pointers As Callees

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Introduction

Function pointers are a fundamental concept in programming, allowing developers to pass functions as arguments to other functions. In the context of Rust's compiler, function pointers are used by some standard library functions, such as max, which calls max_by with a function pointer of type FnOnce. However, the current implementation of the compiler does not fully support function pointers as callees, leading to limitations in how these functions can be used. In this article, we will explore the requirements for supporting function pointers as callees and the necessary changes to the compiler.

Current Implementation

The current implementation of the compiler uses the call_once function to execute function pointers. However, this function only supports function pointers that are present in the functions array in the Stable MIR JSON. This means that if a function pointer is not present in this array, it cannot be executed using the call_once function.

Requirements for Supporting Function Pointers as Callees

To support function pointers as callees, the following requirements must be met:

  • The callee name must be present in the functions array in the Stable MIR JSON. This requires a change in the stable-mir-json crate.
  • The Call terminator must support determining the func from an argument. Currently, it only supports operandConstant.

Changes to the stable-mir-json Crate

The first requirement is to add the callee name to the functions array in the Stable MIR JSON. This can be achieved by modifying the stable-mir-json crate to include the callee name in the functions array.

Here is an example of how the functions array might be modified:

{
  "functions": [
    {
      "name": "callee_name",
      "type": "fn",
      "args": []
    }
  ]
}

In this example, the callee_name function is added to the functions array with a type of fn and no arguments.

Changes to the Call Terminator

The second requirement is to modify the Call terminator to support determining the func from an argument. Currently, it only supports operandConstant. To support function pointers as callees, the Call terminator must be able to determine the func from an argument, such as a function pointer.

Here is an example of how the Call terminator might be modified:

struct Call {
    func: Option<Operand>,
    args: Vec<Operand>,
}

impl Call {
    fn new(func: Option<Operand>, args: Vec<Operand>) -> Self {
        Call { func, args }
    }
}

In this example, the Call terminator is modified to take an optional func operand and a vector of args operands. The func operand can be a function pointer, and the args operands can be the arguments passed to the function.

Example Use Case

To demonstrate the use of function pointers as callees, let's consider an example. Suppose we have a function max_by that takes a function pointer as an argument and returns the maximum value based on that function. We can use the call_once function to execute the function pointer and get the maximum value.

Here is an example of how this might be implemented:

fn max_by<F>(f: F) -> i32
where
    F: FnOnce(i32, i32) -> i32,
{
    let max = call_once(f, 1, 2);
    max
}

fn main() {
    let max = max_by(|x, y| x + y);
    println!("Maximum value: {}", max);
}

In this example, the max_by function takes a function pointer f as an argument and returns the maximum value based on that function. The call_once function is used to execute the function pointer and get the maximum value.

Conclusion

In conclusion, supporting function pointers as callees requires changes to the stable-mir-json crate and the Call terminator. The callee name must be present in the functions array in the Stable MIR JSON, and the Call terminator must support determining the func from an argument. By making these changes, we can enable the use of function pointers as callees in Rust's compiler, providing more flexibility and expressiveness in programming.

Future Work

There are several areas for future work in supporting function pointers as callees. One area is to improve the performance of the call_once function, which is currently a bottleneck in the compiler. Another area is to add support for more complex function pointers, such as those with multiple arguments or return types.

References

Introduction

In our previous article, we explored the requirements for supporting function pointers as callees in Rust's compiler. We discussed the need to modify the stable-mir-json crate and the Call terminator to support function pointers as callees. In this article, we will answer some frequently asked questions about supporting function pointers as callees.

Q: What is a function pointer?

A: A function pointer is a variable that holds the memory address of a function. It allows you to pass functions as arguments to other functions or return functions from functions.

Q: Why do we need to support function pointers as callees?

A: Supporting function pointers as callees allows you to write more flexible and expressive code. It enables you to pass functions as arguments to other functions or return functions from functions, which can be useful in a variety of scenarios.

Q: What changes are required to support function pointers as callees?

A: To support function pointers as callees, we need to make two changes:

  1. Add the callee name to the functions array in the Stable MIR JSON.
  2. Modify the Call terminator to support determining the func from an argument.

Q: How do I add the callee name to the functions array in the Stable MIR JSON?

A: To add the callee name to the functions array in the Stable MIR JSON, you need to modify the stable-mir-json crate. You can do this by adding the callee name to the functions array in the JSON file.

Here is an example of how you might modify the functions array:

{
  "functions": [
    {
      "name": "callee_name",
      "type": "fn",
      "args": []
    }
  ]
}

In this example, the callee_name function is added to the functions array with a type of fn and no arguments.

Q: How do I modify the Call terminator to support determining the func from an argument?

A: To modify the Call terminator to support determining the func from an argument, you need to change the Call terminator to take an optional func operand and a vector of args operands.

Here is an example of how you might modify the Call terminator:

struct Call {
    func: Option<Operand>,
    args: Vec<Operand>,
}

impl Call {
    fn new(func: Option<Operand>, args: Vec<Operand>) -> Self {
        Call { func, args }
    }
}

In this example, the Call terminator is modified to take an optional func operand and a vector of args operands.

Q: What are some use cases for supporting function pointers as callees?

A: There are several use cases for supporting function pointers as callees. Some examples include:

  • Passing functions as arguments to other functions
  • Returning functions from functions
  • Using function pointers to implement callbacks

Q: How do I implement a function pointer as a callee?

A: To implement a function pointer as a callee, you need to follow these steps:

  1. Define the function pointer type
  2. Create a function pointer instance
  3. Pass the function pointer instance to the callee function

Here is an example of how you might implement a function pointer as a callee:

fn max_by<F>(f: F) -> i32
where
    F: FnOnce(i32, i32) -> i32,
{
    let max = call_once(f, 1, 2);
    max
}

fn main() {
    let max = max_by(|x, y| x + y);
    println!("Maximum value: {}", max);
}

In this example, the max_by function takes a function pointer f as an argument and returns the maximum value based on that function. The call_once function is used to execute the function pointer and get the maximum value.

Conclusion

In conclusion, supporting function pointers as callees requires changes to the stable-mir-json crate and the Call terminator. By making these changes, we can enable the use of function pointers as callees in Rust's compiler, providing more flexibility and expressiveness in programming. We hope this Q&A article has been helpful in answering your questions about supporting function pointers as callees.