Unnecessary allocation when calling functions stored in arrays.

[TobyShaw]

We noticed an unnecessary allocation which occurs when calling a (unit -> unit) function obtained from an array. The allocation appears to go away when the action of getting from the array is abstracted into a helper function. The allocation does not occur when invoking a System.Action.

Repro steps

Compile the following code on version 4.1 of the F# compiler. (Would be worth testing on a later version, we can try this later).

let f (arr : (unit -> unit) array) =
	for i = 0 to arr.Length - 1 do
		arr.[i] ()

Observe the IL using ILSpy or run a profiler on this function:

.method public static 
	void f (
		class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<class [FSharp.Core]Microsoft.FSharp.Core.Unit, class [FSharp.Core]Microsoft.FSharp.Core.Unit>[] arr
	) cil managed 
{
	// Method begins at RVA 0x2050
	// Code size 29 (0x1d)
	.maxstack 6
	.locals init (
		[0] int32
	)

	IL_0000: ldc.i4.0
	IL_0001: stloc.0
	IL_0002: br.s IL_0016
	// loop start (head: IL_0016)
		IL_0004: newobj instance void X.Y/f@9::.ctor()
		IL_0009: ldarg.0
		IL_000a: ldloc.0
		IL_000b: ldnull
		IL_000c: call !!1 class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<class [FSharp.Core]Microsoft.FSharp.Core.Unit, class [FSharp.Core]Microsoft.FSharp.Core.Unit>[], int32>::InvokeFast<class [FSharp.Core]Microsoft.FSharp.Core.Unit, class [FSharp.Core]Microsoft.FSharp.Core.Unit>(class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<!0, class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<!1, class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<!!0, !!1>>>, !0, !1, !!0)
		IL_0011: pop
		IL_0012: ldloc.0
		IL_0013: ldc.i4.1
		IL_0014: add
		IL_0015: stloc.0

		IL_0016: ldloc.0
		IL_0017: ldarg.0
		IL_0018: ldlen
		IL_0019: conv.i4
		IL_001a: blt.s IL_0004
	// end loop

	IL_001c: ret
} // end of method Y::f

The point of interest is IL_0004, which is an unnecesary newobj call.

By refactoring the code very slightly, the allocation is avoided:

let g (arr : (unit -> unit) array) =
    let get i = arr.[i]
    for i = 0 to arr.Length - 1 do
        get i ()

Which produces the following IL:

.method assembly static 
	class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<class [FSharp.Core]Microsoft.FSharp.Core.Unit, class [FSharp.Core]Microsoft.FSharp.Core.Unit> get@12 (
		class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<class [FSharp.Core]Microsoft.FSharp.Core.Unit, class [FSharp.Core]Microsoft.FSharp.Core.Unit>[] arr,
		int32 i
	) cil managed 
{
	// Method begins at RVA 0x207c
	// Code size 8 (0x8)
	.maxstack 8

	IL_0000: ldarg.0
	IL_0001: ldarg.1
	IL_0002: ldelem class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<class [FSharp.Core]Microsoft.FSharp.Core.Unit, class [FSharp.Core]Microsoft.FSharp.Core.Unit>
	IL_0007: ret
} // end of method Y::get@12

.method public static 
	void g (
		class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<class [FSharp.Core]Microsoft.FSharp.Core.Unit, class [FSharp.Core]Microsoft.FSharp.Core.Unit>[] arr
	) cil managed 
{
	// Method begins at RVA 0x2088
	// Code size 29 (0x1d)
	.maxstack 4
	.locals init (
		[0] int32
	)

	IL_0000: ldc.i4.0
	IL_0001: stloc.0
	IL_0002: br.s IL_0016
	// loop start (head: IL_0016)
		IL_0004: ldarg.0
		IL_0005: ldloc.0
		IL_0006: call class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<class [FSharp.Core]Microsoft.FSharp.Core.Unit, class [FSharp.Core]Microsoft.FSharp.Core.Unit> X.Y::get@12(class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<class [FSharp.Core]Microsoft.FSharp.Core.Unit, class [FSharp.Core]Microsoft.FSharp.Core.Unit>[], int32)
		IL_000b: ldnull
		IL_000c: callvirt instance !1 class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<class [FSharp.Core]Microsoft.FSharp.Core.Unit, class [FSharp.Core]Microsoft.FSharp.Core.Unit>::Invoke(!0)
		IL_0011: pop
		IL_0012: ldloc.0
		IL_0013: ldc.i4.1
		IL_0014: add
		IL_0015: stloc.0

		IL_0016: ldloc.0
		IL_0017: ldarg.0
		IL_0018: ldlen
		IL_0019: conv.i4
		IL_001a: blt.s IL_0004
	// end loop

	IL_001c: ret
} // end of method Y::g

Expected behavior

The function f does not allocate at all.

Actual behavior

The function f allocates.

Known workarounds

Using g

Related information

• Windows 10
• .NET Framework 4.6.1
• F# Compiler version 10.1.0

[TobyShaw]

Looks like a duplicate of #3660, might close this

[dsyme]

Yup a duplicate, thanks