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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 efficient 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):

https://github.com/dotnet/csharplang/issues/191

This is an alternate design proposal to [compiler intrinsics] (https://github.com/dotnet/csharplang/blob/master/proposals/intrinsics.md)

Detailed Design

Function pointers

The language will allow for the declaration of function pointers using the func* syntax. The full syntax is described in detail in the next section but it is meant to resemble the syntax used by delegate declarations.

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

    void Example(DAction d, func* void(int) f) {
        d(42);
        f(42);
    }
}

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

The ECMA-335 definition of method pointers includes the calling convention as part of the type signature (section 7.1). The default calling convention will be managed . Alternate forms can be specified by adding the appropriate modifier after the func* syntax: cdecl, fastcall, stdcall, thiscall or winapi. Example:

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

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

Conversions between func* types is done based on their signature including the calling convention.

unsafe class Example {
    void Conversions() {
        func* int(int, int) p1 = ...;
        func* managed int(int, int) p2 = ...;
        func* cdecl int(int, int) p3 = ...;

        p1 = p2; // okay Func1 and Func3 have compatible signatures
        Console.WriteLine(p2 == p1); // True
        p2 = p3; // error: calling conventions are incompatible
    }
}

A func* 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 func* parameter or return type can only be called from an unsafe context.
  • Cannot be converted to object.
  • Cannot be used as a generic argument.
  • Can implicitly convert func* to void*.
  • Can explicitly convert from void* to func*.

Restrictions:

  • Custom attributes cannot be applied to a func* or any of its elements.
  • A func* parameter cannot be marked as params
  • A func* type has all of the restrictions of a normal pointer type.

Function pointer syntax

The full function pointer syntax is represented by the following grammar:

funcptr_type = 
    'func' '*' [calling_convention] type method_arglist |
    '(' funcptr_type ')' ;

calling_convention = 
    'managed' |
    'unmanaged' |
    'cdecl' |
    'winapi' | 
    'fastcall' | 
    'stdcall' | 
    'thiscall' ;

The unmanaged calling convention represents the default calling convention for native code on the current platform.

When there is a nested function pointer, a function pointer which has or returns a function pointer, parens can be optionally used to disambiguate the signature. Though they are not required and the resulting types are equivalent.

delegate int Func1(string s);
delegate Func1 Func2(Func1 f);

// Function pointer equivalent without parens or calling convention
func* int(string);
func* func* int(string) int(func* int(string));

// Function pointer equivalent without parens and with calling convention
func* managed int(string);
func* managed func* managed int(string) int(func* managed int(string));

// Function pointer equivalent with parens and without calling convention
func* int(string);
func* (func* int(string)) int((func* int(string));

// Function pointer equivalent of with parens and calling convention
func* int(string)
func* managed (func* managed int(string)) int((func* managed int(string));

When the calling convention is omitted from the syntax then managed will be used as the calling convention. That means all of the forms of Func1 and Func2 defined above are equivalent signatures.

The calling convention cannot be omitted when the return type of the function pointer has the same name as a calling convention. In that case, the parser would process the return type as a calling convention instead of a type. To resolve this the developer must specify both the calling convention and the return type.

class cdecl { }

// Function pointer which has a cdecl calling convention, a cdecl return type and takes a single 
// parameter of type cdecl;
func* cdecl cdecl(cdecl);

Allow address-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 a func* which has the equivalent signature of the target method and a managed calling convention:

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

    void Use() {
        func* void() ptr1 = &Util.Log;

        // Error: type "func* void()" not compatible with "func int()";
        func* int() ptr2 = &Util.Log; 

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

The conversion of an address-of method group to func* 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 func* 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) { };

    void Use() {
        func* void() a1 = &Log; // Log()
        func* void(int) 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 func* is more specific than void*

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

Open Issues

NativeCallback Attribute

This is an attribute used by the CLR to avoid the managed to native prologue when invoking. Methods marked by this attribute are only callable from native code, not managed (can’t call methods, create a delegate, etc …). The attribute is not special to mscorlib; the runtime will treat any attribute with this name with the same semantics.

It's possible for the runtime and language to work together to fully support this. The language could choose to treat address-of static members with a NativeCallback attribute as a func* with the specified calling convention.

unsafe class NativeCallbackExample {
    [NativeCallback(CallingConvention.CDecl)]
    static extern bool CloseHandle(IntPtr p);

    void Use() {
        func* bool(IntPtr) p1 = &CloseHandle; // Error: Invalid calling convention

        func* cdecl bool(IntPtr) p2 = &CloseHandle; // Okay
    }
}

Additionally the language would likely also want to:

  • Flag any managed calls to a method tagged with NativeCallback as an error. Given the function can't be invoked from managed code the compiler should prevent developers from attempting such an invocation.
  • Prevent method group conversions to delegate when the method is tagged with NativeCallback.

This is not necessary to support NativeCallback though. The compiler can support the NativeCallback attribute as is using the existing syntax. The runtime would simply need to cast to void* before casting to the correct func* signature. That would be no worse than the support today.

void* v = &CloseHandle;
func* cdecl bool(IntPtr) f1 = (func* cdecl bool(IntPtr))v;

Considerations

Allow instance methods

The proposal could be extended to support instance methods by taking advantage of the EXPLICITTHIS CLI calling convention (named instance in C# code). This form of CLI function pointers puts the this parameter as an explicit first parameter of the function pointer syntax.

unsafe class Instance {
    void Use() {
        func* instance string(Instance) f = &ToString;
        f(this);
    }
}

This is sound but adds some complication to the proposal. Particularly because function pointers which differed by the calling convention instance and managed would be incompatible even though both cases are used to invoke managed methods with the same C# signature. Also in every case considered where this would be valuable to have there was a simple work around: use a static local function.

unsafe class Instance {
    void Use() {
        static string toString(Instance i) = i.ToString();
        func* string(Instance) f = &toString;
        f(this);
    }
}

Don't require unsafe at declaration

Instead of requiring unsafe at every use of a func*, only require it at the point where a method group is converted to a func*. 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 func* values the same way that they do for normal pointer types today. Consider:

unsafe struct Action {
    func* void() _ptr;

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

Using delegates

Instead of using a new syntax element, func*, simply use existing 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.

Named function pointers

The function pointer syntax can be cumbersome, particularly in complex cases like nested function pointers. Rather than have developers type out the signature every time the language could allow for named declarations of function pointers as is done with delegate.

func* void Action();

unsafe class NamedExample {
    void M(Action a) {
        a();
    }
}

Part of the problem here is the underlying CLI primitive doesn't have names hence this would be purely a C# invention and require a bit of metadata work to enable. That is doable but is a significant about of work. It essentially requires C# to have a companion to the type def table purely for these names.

Also when the arguments for named function pointers was examined we found they could apply equally well to a number of other scenarios. For example it would be just as convenient to declare named tuples to reduce the need to type out the full signature in all cases.

(int x, int y) Point;

class NamedTupleExample {
    void M(Point p) {
        Console.WriteLine(p.x);
    }
}

After discussion we decided to not allow named declaration of func* types. If we find there is significant need for this based on customer usage feedback then we will investigate a naming solution that works for function pointers, tuples, generics, etc ... This is likely to be similar in form to other suggestions like full typedef support in the language.

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

static delegates

This refers to the proposal to allow for the declaration of delegate types which can only refer to static members. The advantage being that such delegate instances can be allocation free and better in performance sensitive scenarios.

If the function pointer feature is implemented the static delegate proposal will likely be closed out. The proposed advantage of that feature is the allocation free nature. However recent investigations have found that is not possible to achieve due to assembly unloading. There must be a strong handle from the static delegate to the method it refers to in order to keep the assembly from being unloaded out from under it.

To maintain every static delegate instance would be required to allocate a new handle which runs counter to the goals of the proposal. There were some designs where the allocation could be amortized to a single allocation per call-site but that was a bit complex and didn't seem worth the trade off.

That means developers essentially have to decide between the following trade offs:

  1. Safety in the face of assembly unloading: this requires allocations and hence delegate is already a sufficient option.
  2. No safety in face of assembly unloading: use a func*. This can be wrapped in a struct to allow usage outside an unsafe context in the rest of the code.
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