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function-pointers.md

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Function Pointers

Summary

This proposal provides language constructs that expose IL opcodes that cannot currently be accessed efficiently, or at all, in C# today: ldftn and calli. These IL opcodes can be important in high performance code and developers need an effecient way to access them.

Motivation

The motivations and background for this feature are described in the following issue (as is a potential implementation of the feature):

dotnet#191

This is an alternate design propsoal to compiler intrinsics

Detailed Design

funcptr

The language will allow for the declaration of function pointers using the funcptr contextual keyword. The declaration and usage of function pointers closely resemble that of delegate:

unsafe class Example {
    delegate void DAction(int a);
    funcptr void FAction(int a);

    void Example(DAction d, FAction f) {
        d(42);
        f(42);
    }
}

These types are represented using the function pointer type as outlined in ECMA-335. This means invocation of a funcptr will use calli where invocation of a delegate will use callvirt on the Invoke method. Syntactically though invocation is identical for both constructs.

The calli instruction requires the calling convention be specified as a part of the invocation. The default for funcptr will be managed. Alternate forms can be specified by adding the appropriate modifier after the funcptr keyword: cdecl, fastcall, stdcall, thiscall or winapi. Example:

// This method will be invoked using the cdecl calling convention
funcptr cdecl int F1(int value);

// This method will be invoked using the stdcall calling convention
funcptr stdcall int F2(int value);

Conversions between funcptr types is done based on their signature, not name. When two funcptr declarations have the same signature they have an identity conversion no matter what the name is.

unsafe class Example {
    funcptr int Func1(int left, int right);
    funcptr int Func2(int x);
    funcptr int Func3(int x, int y);
    funcptr cdecl int Func4(int x, int y);

    void Conversions() {
        Func1 p1 = ...;
        Func2 p2 = ...;

        Func3 p3 = p1; // okay Func1 and Func3 have compatible signatures
        Console.WriteLine(p3 == p1); // True
        Func3 p4 = p2; // error: Func3 and Func2 have incompatible signatures (parameter)
        Func4 p5 = p1; // error: Func4 and Func1 have incompatible signatures (calling convention)
    }
}

In addition to declaring a named funcptr type, as you declare a delegate, it is possible to use an unnamed funcptr type without first declaring it. This type can be used anywhere a type declaration would occur:

unsafe struct Example {
    funcptr int (int) Field;
    unsafe void UnnamedExample(funcptr int(int) ptr) {
        int x = ptr(42);
        Field = ptr;
        ...
    }
}

A funcptr type is a pointer type which means it has all of the capabilities and restrictions of a standard pointer type:

  • Only valid in an unsafe context.
  • Methods which contain a funcptr parameter or return type can only be called from an unsafe context.
  • Cannot be converted to object.
  • Cannot be used an a generic argument.
  • Can implicitly convert funcptr to void*.
  • Can explicitly convert from void* to funcptr.

Restrictions:

  • Cannot overload when the only difference in parameter types is the name of the function pointer.
  • Custom attributes cannot be applied to a funcptr or any of its elements.
  • A funcptr parameter cannot be marked as params
  • A funcptr type has all of the restrictions of a normal pointer type.

Allow addresss-of to target methods

Method groups will now be allowed as arguments to an address-of expression. The type of such an expression will be an unnamed funcptr which has the equivalent signature of the target method and a managed calling convention:

unsafe class Util { 
    public static void Log() { } 

    funcptr void Action();
    funcptr int Func();
    void Use() {
        funcptr void() ptr1 = &Util.Log; 
        Action ptr2 = &Util.Log;

        // Error: type "funcptr void()" not compatible with "funcptr int()";
        Func ptr3 = &Util.Log; 

        // Okay. Conversion to void* is always allowed.
        void* v = &Util.Log;
   }
}

The conversion of an address-of method group to funcptr has roughly the same process as method group to delegate
conversion. There are two additional restrictions to the existing process:

  • Only members of the method group that are marked as static will be considered.
  • Only a funcptr with a managed calling convention can be the target of such a conversion.

This means developers can depend on overload resolution rules to work in conjunction with the address-of operator:

unsafe class Util { 
    public static void Log() { } 
    public static void Log(string p1) { } 
    public static void Log(int i) { };

    funcptr void Action1();
    funcptr void Action2();

    void Use() {
        Action1 a1 = &Log; // Log()
        Action2 a2 = &Log; // Log(int i)

        // Error: ambiguous conversion from method group Log to "void*"
        void* v = &Log; 
    }

The address-of operator will be implemented using the ldftn instruction.

Restrictions of this feature:

  • Only applies to methods marked as static.
  • Local functions cannot be used in &. The implementation details of these methods are deliberately not specified by the language. This includes whether they are static vs. instance or exactly what signature they are emitted with.

Better function member

The better function member specification will be changed to include the following line:

A funcptr is more specific than void*

This means that it is possible to overload on void* and a funcptr and still sensibly use the address-of operator.

Open Issuess

  • Round tripping function pointer names, as well as parameter names, through metadata will require additional work. The function pointer type itself is natively supported by CLI but that does not include any names. This is not anticipated to be a big issue, just needs design work.
  • The address-of operator is limited to static methods in this proposal. It can be made to work with instance methods but the behavior can be confusing to developers. The this type becomes an explicit first parameter on the funcptr type. This means the behavior and usage would differ significantly from delegate. This extra confusion was the main reason it was not included in the design.

Considerations

Don't require unsafe at declaration

Instead of requiring unsafe at every use of a funcptr, only require it at the point where a method group is converted to a funcptr. This is where the core safety issues come into play (knowing that the containing assembly cannot be unloaded while the value is alive). Requiring unsafe on the other locations can be seen as excessive.

This is how the design was originally intended. But the resulting language rules felt very awkward. It's impossible to hide the fact that this is a pointer value and it kept peeking through even without the unsafe keyword. For example the conversion to object can't be allowed, it can't be a member of a class, etc ... The C# design is to require unsafe for all pointer uses and hence this design follows that.

Developers will still be capable of preventing a safe wrapper on top of funcptr values the same way that they do for normal pointer types today. Consider:

unsafe struct Action {
    funcptr void() _ptr;

    Action(funcptr void() ptr) => _ptr = ptr;
    public void Invoke() => _ptr();
}

Using delegates

Instead of using a new syntax element, funcptr, simply use exisiting delegate types with a * following the type:

Func<object, object, bool>* ptr = &object.ReferenceEquals;

Handling calling convention can be done by annotating the delegate types with an attribute that specifies a CallingConvention value. The lack of an attribute would signify the managed calling convention.

Encoding this in IL is problematic. The underlying value needs to be represented as a pointer yet it also must:

  1. Have a unique type to allow for overloads with different function pointer types.
  2. Be equivalent for OHI purposes across assembly boundaries.

The last point is particularly problematic. This mean that every assembly which uses Func<int>* must encode an equivalent type in metadata even though Func<int>* is defined in an assembly though don't control. Additionally any other type which is defined with the name System.Func<T> in an assembly that is not mscorlib must be different than the version defined in mscorlib.

One option that was explored was emitting such a pointer as mod_req(Func<int>) void*. This doesn't work though as a mod_req cannot bind to a TypeSpec and hence cannot target generic instantiations.

No names altogether

Given that a funcptr can be used without names why even allow names at all? The underlying CLI primitive doesn't have names hence the use of names is purely a C# invention. That ends up being a leaky abstraction in some cases (like not allowing overloads when funcptr differ by only names).

At the same time, funcptr look and feel so much like delegate types, not allowing them to be named would be seen as an enormous gap by customers. The leaky abstraction is wort the trade offs here.

Requiring names always

Given that names are allowed for funcptr why not just require them always? Given that funcptr is an existing CLI type there are uses of it in the ecosystem today. None of those uses will have the metadata serialization format chosen by the C# compiler. This means the feature would be using CLI function pointers but not interopting with any existing usage.

Future Considerations

static local functions

This refers to the proposal to allow the static modifier on local functions. Such a function would be guaranteed to be emitted as static and with the exact signature specified in source code. Such a function should be a valid argument to & as it contains none of the problems local functions have today