Fsharp: `[||]` should compile to System.Array.Empty<object>

[chinwobble]

Array clamps should compile to System.Array.Empty<T>() instead of new T[0];

This causes unnecessary object allocations.
In fact FsharpList already compiles to FSharpList.Empty

Provide the steps required to reproduce the problem:

1. Create a new project with .net core 2.1 and fsharp.core 4.7

Create a type like this

type TestType =
    member __.EmptyArray() = [||]

Expected behavior
Should compile to

System.Array.Empty<T>()

Actual behavior
inspect the generated C# and it looks like this

public b[] EmptyArray<b>()
        {
            return new b[0];
        }

Known workarounds

None

Related information

Provide any related information (optional):

• Win 10 x 64
• .NET core 2.1 Fsharp Core 4.7
• VS 2019 16.2

[rojepp]

@forki Array.Empty<T> caches the array which helps reduce allocations.
The empty array is immutable so this is perfectly safe.
https://github.com/dotnet/coreclr/blob/ef1e2ab328087c61a6878c1e84f4fc5d710aebce/src/mscorlib/src/System/Array.cs#L1062-L1069

[forki]

Thanks

Robert Jeppesen [e-mail] schrieb am Do., 19. Sep. 2019,
08:47:

@forki https://github.com/forki Array.Empty caches the array which
helps reduce allocations.
The empty array is immutable so this is perfectly safe.

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[smoothdeveloper]

Wondering if String.Empty versus "" is exhibiting the same issue?

[rojepp]

@smoothdeveloper They should be equivalent due to string interning. https://stackoverflow.com/questions/263191/in-c-should-i-use-string-empty-or-string-empty-or-to-intitialize-a-string?rq=1

[smoothdeveloper]

thanks @rojepp I guess I'll turn to you on allocation issues :)

[chinwobble]

@smoothdeveloper @rojepp @forki I checked FSharpList and it uses the optimised static member FSharpList.Empty

[forki]

Yes I know we do it for lists. Just wanted to make sure it's the same pattern

[auduchinok]

It's currently done this way due to we'd had to target net45 in compiler and FCS for a broader reach and EmptyArray`1 type was only introduced in net461. Since we've switched to a newer target framework in the compiler we can now use the singleton, it's just needs to be done by someone. :)

[chinwobble]

@auduchinok would be happy to contribute if you gave some pointers on where to look to improve this?

[cartermp]

This could be done for F# 5.0, since we're going to drop the net45 binary from FSharp.Core and move to be .NET Standard 2.0-only.

[forki]

Well it could already be done now if we ifdef it, right?

Phillip Carter [e-mail] schrieb am Sa., 21. Sep. 2019,
18:25:

This could be done for F# 5.0, since we're going to drop the net45 binary
from FSharp.Core and move to be .NET Standard 2.0-only.

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[cartermp]

Yes, though I'd prefer to avoid any and all ifdefs given how many of those horrible things we have

[forki]

They are not horrible. Just little creepy. It's all fine if we don't feed
them after midnight.

Phillip Carter [e-mail] schrieb am Sa., 21. Sep. 2019,
18:32:

Yes, though I'd prefer to avoid any and all ifdefs given how many of those
horrible things we have

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[chinwobble]

@forki bump

[forki]

@chinwobble I think you can try to send a pull request.

[forki]

maybe @dsyme can point you to the correct location in the source

[dsyme]

maybe @dsyme can point you to the correct location in the source

I guess in IlxGen.fs GenNewArray and/or GenNewArraySimple (the latter being used for small arrays)