Hitchikers Guide to Porting Phobos / D Runtime to other architectures

[Iain Buclaw]

On 8 January 2014 11:13, Mike <none at none.com> wrote:
> On Wednesday, 8 January 2014 at 01:15:45 UTC, Iain Buclaw wrote:
>>>
>>> Anyway, once you add the word "class" or "struct" you release an
>>> avalanche
>>> of snowballing required runtime stuff that you are likely not even using,
>>> directly or indirectly, in your program. And if using --gc-sections in
>>> your
>>> linker, it all just gets stripped out in the end. I was trying to
>>> articulate
>>> that here
>>> (http://forum.dlang.org/post/zewevdmburppufkjxdje@forum.dlang.org)
>>>
>>
>> If you use a struct, it generates typeinfo and initialiser symbols
>> that go on the comdat on the basis that you *may* access the typeinfo
>> at runtime.
>>
>> These are weak decls so the linker merges/removes duplicates.  There's
>> currently no solution in which the compiler decides whether to omit
>> sending the symbols to object file, but this can be worked upon.
>>
>
> Thanks Iain, things are starting to make sense.
>
> Can't the compiler just emit whatever the runtime provides for the TypeInfo
> stuff, right or wrong?  So, if the runtime does not provide a TypeInfo_x
> implementation, it doesn't emit one.  If the runtime does provide a
> TypeInfo_x, it emits exactly what the runtime provided, right or wrong.
>
> If the program provides a faulty TypeInfo_x implementation, the program
> behaves in a faulty manner, just as the programmer specified.
>
> If the program references a missing TypeInfo_x (or one of it's properties)
> implicitly/indirectly, the linker will emit and undefined reference error,
> won't it?
>
> Is this implementation possible?
>
>

Half and half. The TypeInfo support code is a separation layer between
runtime and compiler and the compiler is not fully aware of what is
implemented by runtime.

On the one hand, there's the internal TypeInfo you see in object.d,
these are required to be known at compile time for certain operations.
 If those aren't present then you'll get mismatch errors from the
compiler, and in the case of GDC, a handy little ICE to end the
compilation immediately.

On the other hand there's the TypeInfo support under rt.typeinfo (eg:
TypeInfo_AC).  For these, only the symbol is generated by the compiler
on the fly and referenced as 'extern'. So you get linker errors if
they aren't implemented.

This is why I suggested the 'all or nothing' approach.