F# compiler internal error when try to compile an inherited padded generic class with inline members

[zpodlovics]

Maybe it's a known (compiler) limitation but it seems that an inherited padded generic class with inline members will result compiler internal errors. Please note I already have a struct based field padding implementation - but I would like to also make this work with this object inheritance based padding mechanism. Unfortunately a single class and padding fields will not work, because the JIT free to reorder the fields within a clas but it will not reorder the fields across "inheritance borders".

The non-full example code looks like this:

[<NoEquality; NoComparison>]
[<Class>]
type PaddedReferenceRightPad<'T when 'T : not struct> =
    inherit PaddedReferenceBase<'T>
    val mutable Right: PaddedReferenceRightPadding
    new() = { inherit PaddedReferenceBase<'T>(); Right = PaddedReferenceRightPadding() }
    new(v: 'T) = { inherit PaddedReferenceBase<'T>(v); Right = PaddedReferenceRightPadding() }

[<NoEquality; NoComparison>]
[<Class>]
type PaddedReference<'T when 'T : not struct> =
    inherit PaddedReferenceRightPad<'T>
    new(value) = { inherit PaddedReferenceRightPad<'T>(value) }

    member inline x.Read() =
        GenericOperations.read x.Value

    member inline x.ReadByRef() =
        GenericOperations.readByRef &x.Value

The full example code are available here:
https://gist.github.com/zpodlovics/aa333619a39facdeba90

Compilation will result the following errors

/tmp/fsharpgenericpaddedinlinemember.fs(84,21): error FS0073: internal error: the mustinline value 'Read' was not inferred to have a known value
/tmp/fsharpgenericpaddedinlinemember.fs(87,21): error FS0073: internal error: the mustinline value 'ReadByRef' was not inferred to have a known value
/tmp/fsharpgenericpaddedinlinemember.fs(84,21): error FS1113: The value 'Read' was marked inline but its implementation makes use of an internal or private function which is not sufficiently accessible
/tmp/fsharpgenericpaddedinlinemember.fs(87,21): error FS1113: The value 'ReadByRef' was marked inline but its implementation makes use of an internal or private function which is not sufficiently accessible
/tmp/fsharpgenericpaddedinlinemember.fs(84,21): error FS0073: internal error: the mustinline value 'Read' was not inferred to have a known value
/tmp/fsharpgenericpaddedinlinemember.fs(87,21): error FS0073: internal error: the mustinline value 'ReadByRef' was not inferred to have a known value
/tmp/fsharpgenericpaddedinlinemember.fs(84,21): error FS1113: The value 'Read' was marked inline but its implementation makes use of an internal or private function which is not sufficiently accessible
/tmp/fsharpgenericpaddedinlinemember.fs(87,21): error FS1113: The value 'ReadByRef' was marked inline but its implementation makes use of an internal or private function which is not sufficiently accessible

Is there any workaround available to use this object inheritance based padding mechanism?

Thanks for your help,
Zoltan

[abelbraaksma]

Note, for one of those errors, the error:

error FS0073: internal error: the mustinline value 'Read' was not inferred to have a known value

you do not need to have a padded, or even generic class, it simply happens "always", as soon as you inherit a class and it has any inline method in the derived class. I reported that earlier here, and there's a StackOverflow question that shows this bug has been around since F# 2.0.

[mpetruska]

Started to investigate this bug, here's my current progress:

• When trying to compile the module below the internal error is reported and assertion failures are triggered in line 3058 of Optimizer.fs

let inline foo x = x

type Base<'T>() = class end

type Class<'T>() =
    inherit Base<'T>()

    member inline __.Bar x = foo x

• After removing the line inherit Base<'T>() the module is compiled successfully.
• Debugging the Optimizer revealed that this expression in the OptimizeLambdas function sets the Info field to UnknownValue, which will cause the IsPartialExprVal function to return true.
• There's a comment "can't inline any values with semi-recursive object references to self or base" at the problematic expression, which suggests the fix for this bug might not be trivial.

[mpetruska]

I think PR #763 contains the fix for this bug.

[abelbraaksma]

Have you also seen #763? It contains a fix for my later bug #740, which I think is the same issue (though I'm not 100% sure). Would be a pity if you'd be doing the work twice... ;)

[abelbraaksma]

Oops, sorry, should've refreshed my window... you found it already ;)

[mpetruska]

Hi, tested the changes in PR #763 on .Net 4.5, it seems that it solves this issue (both the original and the simplified code) compilation succeeds.

[abelbraaksma]

Glad to hear it fixes the issue! The PR is in need of test cases, I haven't gotten around to looking into it yet (I'm not that accustomed to the whole process flow yet), but if you have test cases and you want to / can add them for #763, please do.

[eriawan]

@KevinRansom ,

Just FYI, if this issue is fixed, (according to previous conversations) we should close this issue. I also think this issue doesn't need to be tracked anymore.

cc @dsyme