[swift-dev] Relative Pointers and Windows ARM

[Saleem Abdulrasool]

On Thu, May 19, 2016 at 9:07 AM, John McCall <rjmccall at apple.com> wrote:

> > On May 18, 2016, at 1:51 PM, Saleem Abdulrasool <compnerd at compnerd.org>
> wrote:
> > Hi,
> >
> > It seems that there are assumptions about the ability to create relative
> address across sections which doesn't seem possible on Windows ARM.
> >
> > Consider the following swift code:
> >
> > final class _ContiguousArrayStorage<Element> { }
> >
> > When compiled for Windows x86 (via swiftc -c -target i686-windows
> -parse-as-library -parse-stdlib -module-name Swift -o Swift.obj
> reduced.swift) it will generate the metadata pattern as:
> >
> >     __TMPCs23_ContiguousArrayStorage:
> >       ...
> >       .long
> __TMnCs23_ContiguousArrayStorage-(__MPCs23_ContiguousArrayStorage+128)
> >       ...
> >
> > This generates a IMAGE_REL_I386_REL32 relocation which is the 32-bit
> relative displacement of the target.
> >
> > On Windows ARM (swiftc -c -target i686-windows -parse-pas-library
> -parse-stdlib -module-name Swift -o Swift.obj reduced.swift) it will
> generate similar assembly:
> >
> >     _TMPCs23_ContiguousArrayStorage:
> >       ...
> >       .long
> _TMnCs23_ContiguousArrayStorage-(_MPCs23_ContiguousArrayStorage+128)
> >       ...
> >
> > However, this generates an IMAGE_REL_ARM_ADDR32 relocation which is the
> 32-bit VA of the target.  If the symbol are in the same section, it is
> possible to get a relative value.  However, I don't really see a way to
> generate a relative offset across sections.  There is no relocation in the
> COFF ARM specification which provides the 32-bit relative displacement of
> the target.  There are 20, 23, and 24 bit relative displacements designed
> specifically for branch instructions, but none that would operate on
> generic data.
> >
> > Is there a good way to address this ABI issue?  Or perhaps do we need
> something more invasive to support such targets?  Now, I might be
> completely overlooking something simple that I didn't consider, so pointing
> that out would be greatly appreciated as well.
>
> You can build PIC on Windows ARM, right?  How does Microsoft compile this:
>
>   static int x;
>   int *get_x_addr() { return &x; }


It will generate what they call a based relocation, relying on the DLL
sliding to adjust for the load at an address other than the preferred base
address.


>
> John.


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