<div dir="ltr">On Thu, May 19, 2016 at 9:07 AM, John McCall <span dir="ltr"><<a href="mailto:rjmccall@apple.com" target="_blank">rjmccall@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="">> On May 18, 2016, at 1:51 PM, Saleem Abdulrasool <<a href="mailto:compnerd@compnerd.org">compnerd@compnerd.org</a>> wrote:<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>
</span>You can build PIC on Windows ARM, right? How does Microsoft compile this:<br>
<br>
static int x;<br>
int *get_x_addr() { return &x; }</blockquote><div><br></div><div>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.</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>
John.</font></span></blockquote></div><br>-- <br><div class="gmail_signature">Saleem Abdulrasool<br>compnerd (at) compnerd (dot) org</div>
</div></div>