[swift-dev] Relative Pointers and Windows ARM

[Joe Groff]

> On May 18, 2016, at 1:51 PM, Saleem Abdulrasool via swift-dev <swift-dev at swift.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.

That's unfortunate. One possibly-crazy solution would be to use a different object format that does support the necessary relocations, such as LLVM's win32-macho target. That would forgo interoperability with non-LLVM toolchains, of course. 

-Joe