Type info not available to library code about emscripten

[kripken]

Implemented in 414f1dc.

Runtime.typeInfo.X contains the type info for type X. Enabled with RUNTIME_TYPE_INFO, currently defaulted to off.

[max99x]

Any way to have it as easily accessible at compile-time without overhead?

[kripken]

We can write {{{ exp }}} in library.js, so you can in theory use Types.types there. However, this is evaluated before the type info is present I think, so it won't work. But, what is the use case for this?

[max99x]

If I need to allocate a struct and set its members in a library function, I have to know its layout. Since I know the exact type I need, there's no reason to require specific compilation settings and include extra runtime overhead in terms of code size. An ugly version of this can be seen in the time.h implementation (struct tm is described manually).

[kripken]

Good point, I am working on a patch now.

[max99x]

Awesome!

[kripken]

Enabled type info in library in ab56efe. Rewrote struct_tm to use that (so it functions as a test for this).

[max99x]

Nevermind. There was a GCC-specific field I didn't add to the list. Might be a good idea to verify that the number of fields passed to Runtime.generateStructInfo matches the number of fields in the type.

Previous message for reference:

Thanks for the patch! However, I've found a miscalculation (most likely not a result of this patch). Calling Runtime.generateStructInfo on struct dirent, defined in <bits/dirent.h> produces:

{"__size__":268,
 "d_ino":0,
 "d_off":4,
 "d_reclen":8,
 "d_name":10}

However, d_name should have an offset of 11. This is true for both native size and the generated JS code.

[kripken]

What does the .ll look like?

[max99x]

As I've mentioned in my edit, it was a mistake on my side.

[kripken]

Ah, I missed the edit.

Adding that check sounds like a good idea, I'll do that.

[kripken]

Pushed followup following IRC discussion, 9bcf21d

Allows generateStructInfo to process nested structs, for example

Runtime.generateStructInfo(['x', { us: ['x', 'y', 'z'] }, 'y'], 'StructType')

where the 2nd field is a struct as well.

[lucidvizion]

If I need to allocate a struct and set its members in a library function, I have to know its layout.

What is the solution to this problem with FastComp? I see that Runtime.typeInfo is not available anymore.

[kripken]

You can create a method in C that returns the size and/or positions of elements, and compile that, then call it. This is essentially what the webidl binder and embind do, so might be useful to look at their code or output.