[swift-dev] COFF indirect addressing and protocol conformance tables ABI issues

[Saleem Abdulrasool]

On Tue, Jul 5, 2016 at 6:28 PM, John McCall <rjmccall at apple.com> wrote:

> On Jul 5, 2016, at 5:56 PM, Saleem Abdulrasool <compnerd at compnerd.org>
> wrote:
> On Tue, Jul 5, 2016 at 10:30 AM, John McCall <rjmccall at apple.com> wrote:
>
>> > On Jul 3, 2016, at 2:40 PM, Saleem Abdulrasool <compnerd at compnerd.org>
>> wrote:
>> > Hello again,
>> >
>> > I come bearing more problems :-).
>> >
>> > I seem to have found another point in the ABI which prevents an easy
>> port of swift to Windows without an emulation layer underneath.  This time
>> it deals with the protocol conformance table.  The table is constructed
>> with a direct reference to protocol.  However, because the protocol lies in
>> an external module, this ends up being a problem as it must be indirectly
>> addressed.
>> >
>> > There is a workaround in place for generating a GOT equivalent entry,
>> however, that still doesnt indirect through the pointer, which needs to be
>> done to address the executable model on Windows.
>>
>> Is it not possible to emit the GOT equivalent entry as a reference to the
>> appropriate entry in the import lookup table?
>
>
> I had tried that, but it would still just be a constant reference to the
> value rather than the synthetic.  Perhaps I am misunderstanding something?
>
>
> I don't know what you mean.  I assume this is the reference to the
> protocol structure from the global protocol-conformances list.  This
> reference is a relative offset, either directly to the protocol structure
> or indirectly to a global containing a pointer to an external protocol
> structure.  PE's import lookup table is an array of pointers to external
> objects.
>

Exactly, in the case that identified the issue, it is _TMps12OutputStream


> I could easily believe that COFF doesn't allow us to express a relative
> offset to an entry in the import lookup table.
>

I don't think that there is any way to represent that in IR, and I don't
know if COFF allows us to encode such a thing.  I will see if I can
construct an object file/library and see if we can get away with that.


>  However, it sounds like you think this needs to be *doubly* indirected,
> i.e. the conformance list needs to be a relative offset to a variable
> containing a pointer to a variable containing a pointer to the import
> lookup table.  I'm not sure why that would be.
>
John.
>



-- 
Saleem Abdulrasool
compnerd (at) compnerd (dot) org
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