Improve getStaticFieldContent for classes without cctors

[EgorBo]

When a class has no static constructor (cctor), its pEnclosingMT->IsClassInited() returns false. It shouldn't stop us from folding RVA fields. Example:

public class Program
{
    static void Main()
    {
        Test();
    }

    static ReadOnlySpan<int> RvaData => [1, 2, 3, 4];

    [MethodImpl(MethodImplOptions.NoInlining)]
    static int Test() => RvaData[2];
}

Codegen of Test() in Main (Tier1 or FullOpts):

; Method Program:Test():int (FullOpts)
       mov      rax, 0x2AEC82D3450
       mov      eax, dword ptr [rax]
       ret

New codegen of Test():

; Method Program:Test():int (FullOpts)
       mov      eax, 3
       ret

PS: I have an impression that pEnclosingMT->IsClassInited() used to return true in such cases 🤔 Or maybe entrypoint holders always used to have cctors? cc @jkotas
Perhaps it's some change from DavidWr's statics PR?

The same snippet works well on NAOT.

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

[jkotas]

Perhaps it's some change from DavidWr's statics PR?

Yes, it looks like a regression from @davidwrighton's change. IsClassInited used to return true for types without static constructor before that change.

Are there other callers of IsClassInited that will hit the same problem?

[EgorBo]

Perhaps it's some change from DavidWr's statics PR?

Yes, it looks like a regression from @davidwrighton's change. IsClassInited used to return true for types without static constructor before that change.

Are there other callers of IsClassInited that will hit the same problem?

Hard to say, it seems like the problematic case is <PrivateImplementationDetails> class (RVA fields holder, I guess)

[EgorBo]

@jkotas actually, the problem is on the JIT side since the same snippet works when I use byte for RVA.

It seems that when we expand RuntimeHelpers.CreateSpan<int>((RuntimeFieldHandle)token)); in JIT we don't call initClass for the class containing that field. I presume it used to work because CreateSpan wasn't expanded in Tier0 previously and its non-intrinsified path used to properly init it as is, so, presumably, it's #91403 that broke this expansion.

byte RVA works because Roslyn doesn't emit CreateSpan for it (as expected).

[EgorBo]

@MihuBot

[jkotas]

I am not following why we need to do this. The comment should explain why we need to do this.

This comment suggests that the non-expanded version of RuntimeHelpers.CreateSpan performs equivalent operation, but I do not see an equivalent operation in non-expanded version of RuntimeHelpers.CreateSpan. I am not sure whether this is the right fix.

[EgorBo]

Case 1: ReadOnlySpan<byte> RVA: sharplab

here JIT triggers class initialization for <PrivateImplementationDetails> when it imports get_Data() method and sees ldsflda for which it calls getFieldInfo and triggers class init in jit time here

Nothing like that happens for CreateSpan case - there is no ld[s]fld opcode anywhere, only ldtoken field

This comment suggests that the non-expanded version of RuntimeHelpers.CreateSpan performs equivalent operation,

Then I guess it should do the same somehow?

[EgorBo]

Maybe David's PR indeed broke this behavior, but it seems that if we put some real cctor to <PrivateImplementationDetails> - nobody will call it

[jkotas]

Then I guess it should do the same somehow?

RuntimeHelpers.CreateSpan and RuntimeHelpers.InitializeArray APIs never triggered static constructors. They are special APIs. Nothing says that they have to trigger static constructor. I do not see a problem with it.

[EgorBo]

Then I guess it should do the same somehow?

RuntimeHelpers.CreateSpan and RuntimeHelpers.InitializeArray APIs never triggered static constructors. They are special APIs. Nothing says that they have to trigger static constructor. I do not see a problem with it.

Ok, I will leave this problem to @davidwrighton then since I presume we came to conclusion that MethodTable::IsClassInited should return true for such classes