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Bizarre F# Generated IL
f#compiler

Hi folks,

When I compile the following piece of F# code:

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let arr = Array.zeroCreate 10

let mutable i = 1

let mutable x = 1

arr.[ i ] <- x

the compiler generates this IL:

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.method public static void main@() cil managed

{

    .entrypoint

    .maxstack 5

    L_0000: nop 

    L_0001: ldc.i4.s 10

    L_0003: newarr int32

    L_0008: stsfld int32[] <StartupCode$MonadsLearn>.$Program::arr@205

    L_000d: ldc.i4.1 

    L_000e: stsfld int32 Program::i

    L_0013: ldc.i4.1 

    L_0014: stsfld int32 Program::x

    L_0019: call int32 Program::get_i()

    L_001e: call int32 Program::get_x()

    L_0023: newobj instance void [FSharp.Core]Microsoft.FSharp.Core.Tuple`2<int32, int32>::.ctor(!0, !1)

    L_0028: stsfld class [FSharp.Core]Microsoft.FSharp.Core.Tuple`2<int32, int32> <StartupCode$MonadsLearn>.$Program::tupledArg@1

    L_002d: call int32[] Program::get_arr()

    L_0032: call class [FSharp.Core]Microsoft.FSharp.Core.Tuple`2<int32, int32> Program::get_tupledArg@1()

    L_0037: call instance !0 [FSharp.Core]Microsoft.FSharp.Core.Tuple`2<int32, int32>::get_Item1()

    L_003c: call class [FSharp.Core]Microsoft.FSharp.Core.Tuple`2<int32, int32> Program::get_tupledArg@1()

    L_0041: call instance !1 [FSharp.Core]Microsoft.FSharp.Core.Tuple`2<int32, int32>::get_Item2()

    L_0046: stelem.any int32

    L_004b: ret 

}

which corresponds to this piece of C# code:

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public static void main@()

{

    arr@205 = new int[10];

    Program.i = 1;

    Program.x = 1;

    tupledArg@1 = new Tuple<int, int>(Program.i, Program.x);

    Program.arr[Program.tupledArg@1.Item1] = Program.tupledArg@1.Item2;

}

Does anyone know why the compiler builds the tuple object? This only happens if [b]i[/b] is mutable.

I'm using F# May 2009 CTP for VS2008 in Release mode.

Thanks!

.Reply
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I can't reproduce the problem with F# 1.9.6.16 in VS2008, neither in Release nor in Debug mode.

Do you still have the problem, when you put the code into a function? Could you provide a sample with code that does something useful?

By on 7/1/2009 2:55 PM ()Reply
Hide this comment

I can't reproduce the problem with F# 1.9.6.16 in VS2008, neither in Release nor in Debug mode.

Do you still have the problem, when you put the code into a function? Could you provide a sample with code that does something useful?

Of course! I uploaded a simple project [1]. With this one, the compiler does something more bizarre: it creates a tuple, but only uses the first item.

[1] [link:www.ezbakhe.es]

By on 7/1/2009 3:57 PM ()Reply
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Ok, now I was able to reproduce the problem. Apparently the code generator has an issue with mutable indices to arrays. You should report this bug to fsbugs (at) microsoft (dot) com

The fact that the compiler creates a temporary tuple is not as bizarre as you might think. I suppose the reason for this behaviour is that it internally represents the array setter as a function taking a tuple with 2 elements: the index and the value. Normally the tuple code gets "optimized away", but in this case the mutability of the index seems to prevent that.

A small repro sample:

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module _Test.Test

let loop n =
    let a = Array.zeroCreate n    
    let mutable i = -1
    for j = 1 to n do
        i <- i + 1        
        a.[ i ] <- j // creates an unnecessary temporary tuple
        //let i = i in a.[ i ] <- j // has no problem        
    a

[<EntryPoint>]
let main(args) =
    printfn "%A" (loop 10)
    0
By on 7/2/2009 3:23 AM ()Reply
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I've forwarded this thread to the fsbugs alias Stephan mentioned - thanks for reporting the issue.

Kind regards

don

By on 7/5/2009 12:44 PM ()Reply

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