[swift-dev] Relative Pointers and Windows ARM

[Joe Groff]

> On May 18, 2016, at 6:01 PM, Saleem Abdulrasool <compnerd at compnerd.org> wrote:
> 
> On Wednesday, May 18, 2016, Joe Groff <jgroff at apple.com> wrote:
> 
> > 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
> 
> Yeah, it would make interoperability harder.  But, is there a loader for macho on Windows?

Sorry, if it wasn't clear, I meant that you could use mach-o (or ELF, or any object format really) for .o and .a files. You'd still link them into PE executables and DLLs.

-Joe