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