[swift-dev] Relative Pointers and Windows ARM

[John McCall]

> On May 19, 2016, at 5:39 PM, Saleem Abdulrasool <compnerd at compnerd.org> wrote:
> On Thu, May 19, 2016 at 9:07 AM, John McCall <rjmccall at apple.com <mailto:rjmccall at apple.com>> wrote:
> > On May 18, 2016, at 1:51 PM, Saleem Abdulrasool <compnerd at compnerd.org <mailto: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.

Okay, so an absolute address and metadata to do a fix-up, i.e. not PIC.  Score one for Joe.

I guess it's probably not reasonable to assume that IMAGE_REL_ARM_BRANCH24 will just work. :)  16M is not really a reasonable max image size anyway.

Well, if we wanted to be adventurous, we could ask Microsoft to add an IMAGE_REL_ARM_REL32 relocation in a future toolchain.  What we're asking of the ELF and Mach-O linkers isn't all that much less ridiculous... In the meantime, yeah, we should probably just switch to absolute addressing; it should be easy enough to generalize that in the metadata scheme.

John.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-dev/attachments/20160519/2d9e1346/attachment.html>