On Thu, Sep 21, 2017 at 12:04 PM, Joe Groff <jgr...@apple.com> wrote:
> > > On Sep 21, 2017, at 11:49 AM, Saleem Abdulrasool <compn...@compnerd.org> > wrote: > > On Thu, Sep 21, 2017 at 10:53 AM, Joe Groff <jgr...@apple.com> wrote: > >> >> >> On Sep 21, 2017, at 9:32 AM, Saleem Abdulrasool via swift-dev < >> swift-dev@swift.org> wrote: >> >> Hello, >> >> 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. >> >> When building the standard library, LLVM encounters a relocation which >> cannot be represented. Tracking down the relocation led to the type >> metadata for SwiftNSOperatingSystemVersion. The metadata here is >> _T0SC30_SwiftNSOperatingSystemVersionVN. 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. >> >> 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? >> >> >> 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. >> >> > 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? > > 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. > > > 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? > Yes, the `-windows-macho` target is used for UEFI :-). The MachO binary is translated later to PE/COFF as required by the UEFI specification. There are only two relocation types which can be used for relative displacements: __ImageBase relative (IMAGE_REL_*_ADDR32NB) and section relative (IMAGE_REL_*_SECREL) which are relative to the beginning of the section. The latter is why I mentioned that moving them into the same section could be a solution as that would allow the relative distance to be encoded. Unfortunately, the section relative relocation is relative to the section within which the symbol is. > -Joe > > > 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. > > > >> -Joe >> > > > -- > Saleem Abdulrasool > compnerd (at) compnerd (dot) org > > -- Saleem Abdulrasool compnerd (at) compnerd (dot) org
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