<div dir="ltr">On Thu, May 19, 2016 at 12:29 PM, Joe Groff <span dir="ltr"><<a href="mailto:jgroff@apple.com" target="_blank">jgroff@apple.com</a>></span> wrote:<br><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><span class=""><br>
> On May 19, 2016, at 12:22 PM, Tom Birch <<a href="mailto:froody@gmail.com">froody@gmail.com</a>> wrote:<br>
><br>
> Would it be acceptable to make relative pointers optional, so we can pay the extra load-time cost on platforms where it's hard/undesirable to implement them?<br>
<br>
</span>That's also a reasonable answer, since sliding DLLs is already fairly costly. We'd need a bunch of extra tests to ensure both the relative and absolute forms work, though maybe with David Farler's work to abstract relative addresses it's already straightforward to have the RelativePointer<..> templates in the runtime work in a platform-independent way.</blockquote><div><br></div><div>Would be interesting to at least see if this is possible. Any hints on where to get started with testing/prototyping such an approach?</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><span class="HOEnZb"><font color="#888888"><br>
-Joe<br>
</font></span><div class="HOEnZb"><div class="h5"><br>
> cheers,<br>
> Tom<br>
><br>
> On Thu, May 19, 2016 at 9:51 AM Joe Groff via swift-dev <<a href="mailto:swift-dev@swift.org">swift-dev@swift.org</a>> wrote:<br>
><br>
> > On May 18, 2016, at 6:01 PM, Saleem Abdulrasool <<a href="mailto:compnerd@compnerd.org">compnerd@compnerd.org</a>> wrote:<br>
> ><br>
> > On Wednesday, May 18, 2016, Joe Groff <<a href="mailto:jgroff@apple.com">jgroff@apple.com</a>> wrote:<br>
> ><br>
> > > On May 18, 2016, at 1:51 PM, Saleem Abdulrasool via swift-dev <<a href="mailto:swift-dev@swift.org">swift-dev@swift.org</a>> wrote:<br>
> > ><br>
> > > Hi,<br>
> > ><br>
> > > It seems that there are assumptions about the ability to create relative address across sections which doesn't seem possible on Windows ARM.<br>
> > ><br>
> > > Consider the following swift code:<br>
> > ><br>
> > > final class _ContiguousArrayStorage<Element> { }<br>
> > ><br>
> > > 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:<br>
> > ><br>
> > > __TMPCs23_ContiguousArrayStorage:<br>
> > > ...<br>
> > > .long __TMnCs23_ContiguousArrayStorage-(__MPCs23_ContiguousArrayStorage+128)<br>
> > > ...<br>
> > ><br>
> > > This generates a IMAGE_REL_I386_REL32 relocation which is the 32-bit relative displacement of the target.<br>
> > ><br>
> > > 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:<br>
> > ><br>
> > > _TMPCs23_ContiguousArrayStorage:<br>
> > > ...<br>
> > > .long _TMnCs23_ContiguousArrayStorage-(_MPCs23_ContiguousArrayStorage+128)<br>
> > > ...<br>
> > ><br>
> > > 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.<br>
> > ><br>
> > > 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.<br>
> ><br>
> > 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<br>
> ><br>
> > Yeah, it would make interoperability harder. But, is there a loader for macho on Windows?<br>
><br>
> 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.<br>
><br>
> -Joe<br>
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<br>
</div></div></blockquote></div><br><br clear="all"><div><br></div>-- <br><div class="gmail_signature">Saleem Abdulrasool<br>compnerd (at) compnerd (dot) org</div>
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