<html><head><meta http-equiv="Content-Type" content="text/html; charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Sep 21, 2017, at 11:49 AM, Saleem Abdulrasool <<a href="mailto:compnerd@compnerd.org" class="">compnerd@compnerd.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">On Thu, Sep 21, 2017 at 10:53 AM, Joe Groff<span class="Apple-converted-space"> </span><span dir="ltr" class=""><<a href="mailto:jgroff@apple.com" target="_blank" class="">jgroff@apple.com</a>></span><span class="Apple-converted-space"> </span>wrote:<br class=""><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-style: solid; border-left-color: rgb(204, 204, 204); padding-left: 1ex;"><div style="word-wrap: break-word; line-break: after-white-space;" class=""><span class=""><br class=""><div class=""><br class=""><blockquote type="cite" class=""><div class="">On Sep 21, 2017, at 9:32 AM, Saleem Abdulrasool via swift-dev <<a href="mailto:swift-dev@swift.org" target="_blank" class="">swift-dev@swift.org</a>> wrote:</div><br class="m_-1276541751748296627Apple-interchange-newline"><div class=""><div dir="ltr" class="">Hello,<div class=""><br class=""></div><div class="">The current layout for the swift metadata for structure types, as emitted, seems to be unrepresentable in PE/COFF (at least for x86_64). There is a partial listing of the generated code following the message for reference.</div><div class=""><br class=""></div><div class="">When building the standard library, LLVM encounters a relocation which cannot be represented. Tracking down the relocation led to the type metadata for SwiftNSOperatingSystemVersion.<wbr class=""> <span class="Apple-converted-space"> </span>The metadata here is <span style="font-family: monospace, monospace;" class="">_T0SC30_<wbr class="">SwiftNSOperatingSystemVersionV<wbr class="">N</span><font face="arial, helvetica, sans-serif" class="">. At +32-bytes we find the Kind (1). So, this is a struct metadata type. Thus at Offset 1 (+40 bytes) we have the nominal type descriptor reference. This is the relocation which we fail to represent correctly. If I'm not mistaken, it seems that the field is supposed to be a relative offset to the nominal type descriptor. However, currently, the nominal type descriptor is emitted in a different section (.rodata) as opposed to the type descriptor (.data). This cross-section relocation cannot be represented in the file format.</font></div><div class=""><font face="arial, helvetica, sans-serif" class=""><br class=""></font></div><div class=""><font face="arial, helvetica, sans-serif" class="">My understanding is that the type metadata will be adjusted during the load for the field offsets. Furthermore, my guess is that the relative offset is used to encode the location to avoid a relocation for the load address base. In the case of windows, the based relocations are a given, and I'm not sure if there is a better approach to be taken. There are a couple of solutions which immediately spring to mind: moving the nominal type descriptor into the (RW) data segment and the other is to adjust the ABI to use an absolute relocation which would be rebased. Given that the type metadata may be adjusted means that we cannot emit it into the RO data segment. Is there another solution that I am overlooking which may be simpler or better?</font></div></div></div></blockquote><br class=""></div></span><div class="">IIRC, this came up when someone was trying to port Swift to Windows on ARM as well, and they were able to conditionalize the code so that we used absolute pointers on Windows/ARM, and we may have to do the same on Windows in general. It may be somewhat more complicated on Win64 since we generally assume that relative references can be 32-bit, whereas an absolute reference will be 64-bit, so some formats may have to change layout to make this work too. I believe Windows' executable loader still ultimately maps the final PE image contiguously, so alternatively, you could conceivably build a Swift toolchain that used ELF or Mach-O or some other format with better support for PIC as the intermediate object format and still linked a final PE executable. Using relative references should still be a win on Windows both because of the size benefit of being 32-bit and the fact that they don't need to be slid when running under ASLR or when a DLL needs to be rebased.</div><span class="HOEnZb"><font color="#888888" class=""><div class=""><br class=""></div></font></span></div></blockquote><div class=""><br class=""></div><div class="">Yeah, I tracked down the relativePointer thing. There is a nice subtle little warning that it is not fully portable :-). Would you happen to have a pointer to where the adjustment for the absolute pointers on WoA is?</div><div class=""><br class=""></div><div class="">You are correct that the image should be contiugously mapped on Windows. The idea of MachO as an intermediatary is rather intriguing. Thinking longer term, maybe we want to use that as a global solution? It would also provide a nicer autolinking mechanism for ELF which is the one target which currently is missing this functionality. However, if Im not mistaken, this would require a MachO linker (and the only current viable MachO linker would be ld64). The MachO binary would then need to be converted into ELF or COFF. This seems like it could take a while to implement though, but would not really break ABI, so pushing that off to later may be wise.</div></div></div></div></div></blockquote><div><br class=""></div><div>Intriguingly, LLVM does support `*-*-win32-macho` as a target triple already, though I don't know what Mach-O to PE linker (if any) that's intended to be used with. We implemented relative references using current-position-relative offsets for Darwin and Linux both because that still allows for a fairly convenient pointer-like C++ API for working with relative offsets, and because the established toolchains on those platforms already have to support PIC so had most of the relocations we needed to make them work already; is there another base we could use for relative offsets on Windows that would fit in the set of relocations supported by standard COFF linkers?</div><div><br class=""></div><div>-Joe</div><br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><div class="gmail_extra"><div class="gmail_quote"><div class=""><br class=""></div><div class="">I really hope that we can get the Windows build to the point where we can actually have that be built regularly, as it seems that there is still insufficient test coverage.</div><div class=""><br class=""></div><div class=""> </div><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-style: solid; border-left-color: rgb(204, 204, 204); padding-left: 1ex;"><div style="word-wrap: break-word; line-break: after-white-space;" class=""><span class="HOEnZb"><font color="#888888" class=""><div class=""></div><div class="">-Joe</div></font></span></div></blockquote></div><br clear="all" class=""><div class=""><br class=""></div>--<span class="Apple-converted-space"> </span><br class=""><div class="gmail_signature" data-smartmail="gmail_signature">Saleem Abdulrasool<br class="">compnerd (at) compnerd (dot) org</div></div></div></div></blockquote></div><br class=""></body></html>