[NativeAOT] Ensure that frozen objects respect the minimum object size

[filipnavara]

Ref: #75421 (comment)

When System.Memory.Tests was compiled with NativeAOT it produced incorrect data in the frozen heap section for some empty objects:

First object:

(lldb) image lookup -va 0x10b1c5058
      Address: System.Memory.Tests[0x0000000100d7c058] (System.Memory.Tests.__DATA.__data + 268888)
      Summary: __FrozenObj_xunit_assert_Xunit_Sdk_AssertEqualityComparer_1___c<S_P_CoreLib_System_ReadOnlyMemory_1<Int32>>1
       Module: file = "/Users/teamcity/runtime/runtime/artifacts/bin/System.Memory.Tests/Release/net7.0/native/System.Memory.Tests", arch = "x86_64"
       Symbol: id = {0x00061771}, range = [0x000000010b1c5058-0x000000010b1c5068), mangled="__FrozenObj_xunit_assert_Xunit_Sdk_AssertEqualityComparer_1___c<S_P_CoreLib_System_ReadOnlyMemory_1<Int32>>1"

Next object:

      Address: System.Memory.Tests[0x0000000100d7c068] (System.Memory.Tests.__DATA.__data + 268904)
      Summary: __FrozenObj_S_P_CoreLib_System_Array_1_ArrayEnumerator<S_P_CoreLib_System_Nullable_1<Int32>>3
       Module: file = "/Users/teamcity/runtime/runtime/artifacts/bin/System.Memory.Tests/Release/net7.0/native/System.Memory.Tests", arch = "x86_64"
       Symbol: id = {0x00061772}, range = [0x000000010b1c5068-0x000000010b1c5088), mangled="__FrozenObj_S_P_CoreLib_System_Array_1_ArrayEnumerator<S_P_CoreLib_System_Nullable_1<Int32>>3"

The dumped vtable of the first object:

(lldb) x/8 0x10b3f3798
0x10b3f3798: 0x52000000 0x00000018 0x0b2b94f8 0x00000001
0x10b3f37a8: 0x00000003 0x63cb6bdc 0x0a6746e0 0x00000001

Second word is the size (0x00000018) but the difference between the address of the two objects in the data is only 0x10. This fixes the IL Compiler to properly align empty objects and prevents a GC crash as runtime.

[filipnavara]

It solves the problem but I am not sure about the preferred code organisation. I'd have preferred to use EETypeNode.BaseSize directly in ObjectInstance.WriteContent to emit the padding but there is an indirection through IEETypeNode so I couldn't just access the member. This patch feels a bit finicky.

Also, once the details are ironed out, is this candidate for release/7.0 fix?

[jkotas]

LGTM. Thank you!

[jkotas]

/azp run runtime-extra-platforms

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jkotas]

The failures are #75699

[jkotas]

/backport to release/7.0

[github-actions]

Started backporting to release/7.0: https://github.com/dotnet/runtime/actions/runs/3109813547

[jkotas]

@filipnavara Thank you!

[filipnavara]

Thanks for help with it!