skinny delegates

[Jonathan Marler]

On Monday, 30 July 2018 at 21:02:56 UTC, Steven Schveighoffer 
wrote:
> Would it be a valid optimization to have D remove the 
> requirement for allocation when it can determine that the 
> entire data structure of the item in question is an rvalue, and 
> would fit into the data pointer part of the delegate?
>
> Here's what I'm looking at:
>
> auto foo(int x)
> {
>    return { return x + 10; };
> }
>
> In this case, D allocates a pointer on the heap to hold "x", 
> and then return a delegate which uses the pointer to read x, 
> and then return that plus 10.
>
> However, we could store x itself in the storage of the pointer 
> of the delegate. This removes an indirection, and also saves 
> the heap allocation.
>
> Think of it like "automatic functors".
>
> Does it make sense? Would it be feasible for the language to do 
> this? The type system already casts the delegate pointer to a 
> void *, so it can't make any assumptions, but this is a slight 
> break of the type system.
>
> The two requirements I can think of are:
> 1. The data in question must fit into a word
> 2. It must be guaranteed that the data is not going to be 
> mutated (either via the function or any other function). Maybe 
> it's best to require the state to be const/immutable.
>
> I've had several cases where I was tempted to not use delegates 
> because of the allocation cost, and simply return a specialized 
> struct, but it's so annoying to do this compared to making a 
> delegate. Plus something like this would be seamless with 
> normal delegates as well (in case you do need a real delegate).
>
> -Steve

I think the number of cases where you could optimize this is very 
small.  And the complexity of getting the compiler to analyze 
cases to determine when this is possible would be very large.

In addition, a developer can already do this explicitly if they 
want, i.e.

auto foo(int x)
{
     static struct DummyStructToMakeFunctionWithDelegateAbi
     {
         int passthru() const { return cast(int)&this; }
     }
     DummyStructToMakeFunctionWithDelegateAbi dummyStruct;
     auto dg = &dummyStruct.passthru;
     dg.ptr = cast(void*)(x + 10); // treat the void* pointer as 
an int value
     return dg;
}

void main(string[] args)
{
     auto dg = foo(32);
     import std.stdio;
     writefln("dg() = %s", dg());
}

It's definitely ugly but it works.  This will print the number 
"42" as expected.

This would be a case where DIP1011 extern(delegate) would come in 
handy :) i.e.

extern(delegate) int passthru(void* ptr) { return cast(int)ptr; }
int delegate() foo2(int x)
{
     return &(cast(void*)(x + 10)).passthru;
}