[swift-dev] Metadata Representation

Joe Groff jgroff at apple.com
Thu Sep 21 12:53:06 CDT 2017

> On Sep 21, 2017, at 9:32 AM, Saleem Abdulrasool via swift-dev <swift-dev at 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.


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