packed structs

[dcharkes]

2.13.0-211.6.beta has support for packed structs (dart-lang/sdk#38158).

Can we detect with clang a struct is packed? and add the right annotation to it?

[mannprerak2]

We can get the alignment of a struct using this function

long long clang_Type_getAlignOf(CXType T);

and use it for annotation@Packed(<align-value>).

Do we need any other config for this?

EDIT: Will this be platform-independent though? Can this value be different on different platforms?
Maybe we only apply @Packed annotation if align value is exactly 1.

[dcharkes]

Will this be platform-independent though? Can this value be different on different platforms?

I don't think always emitting @PACKED aways is platform independent.

Ideally, we would only emit @Packed(1, 2, 4, 8, or 16) if packing is used on the struct. For example https://github.com/dart-lang/sdk/blob/master/runtime/bin/ffi_test/ffi_test_functions_generated.cc#L470, but not the structs before that one in the file.

If we can't detect if packing is used from clang, then we might want to use a configuration option to specify packing instead.

[mannprerak2]

We can only use the align value returned by clang_Type_getAlignOf if the struct was packed, otherwise the generated code is not platform-independant.

There are 3 ways to specify how a struct is packed in C -

1. Using __attribute__((__packed__)) or__attribute__((__packed__, aligned(X))) on a struct -> This can be easily detected with libclang.
2. Using #pragma pack(...) -> Unfortunately libclang doesn't expose this (See Bindgen can't translate #pragma pack(n) into #[repr(packed = "n")] rust-lang/rust-bindgen#537 (comment))
   1. Workaround -> We can check if any of the members of the struct has an align value greater than the align value of the parent struct, and if it is then we know for sure that the struct was packed.
3. There's also __attribute__((aligned(X))) which sets the minimum alignment for a struct, (i.e alignment could be more if a member has a value greater than X -> For this we can pack if the maximum align value of any child is less than the align value of parent struct.

I think this covers everything, so there is no need for adding a config to specify the packing of a struct explicitly.

[dcharkes]

P.S. A good stresstest would be to compare the generated bindings with the ones that @timsneath has been generating from the win32 meta-data repository in https://github.com/timsneath/win32. (I'm sure the metadata repository has more information than the C header files, so ffigen might not be as precise, but it would be cool to see if we end up with the same generated bindings.)

[mannprerak2]

We will have to also provide users a way to specify the packing via the config since -

1. It's possible to pack structs via some command-line argument when compiling a shared library.
2. To work around some missing edge cases, plus it's better to have the flexibility.

The config can be something like this -

structs:
  pack:
    'ShortStruct': 2 # value can be [0, 1, 2, 4, 8 ,16]. 0 means no packing is applied.
    '.*': 1
    'NotPackedStruct': 0 

This allows users to override the generated values, so this config will only be needed if the generated Packed annotation is incorrect.

[dcharkes]

Maybe use none instead of 0.

And yes an option for .* (or other regex). Great thinking! 🚀

[timsneath]

In my Win32 code, structs.g.dart represents all the structs I'm able to generate automatically today; structs.dart represents all the structs I still have to generate manually (mostly because of the lack of union support).

[mannprerak2]

Thanks @timsneath I'll take a look.