[swift-dev] Metadata Representation

Saleem Abdulrasool compnerd at compnerd.org
Thu Sep 28 12:01:52 CDT 2017

On Mon, Sep 25, 2017 at 10:18 PM, John McCall <rjmccall at apple.com> wrote:

> On Sep 26, 2017, at 12:35 AM, Saleem Abdulrasool <compnerd at compnerd.org>
> wrote:
> On Mon, Sep 25, 2017 at 11:47 AM, John McCall <rjmccall at apple.com> wrote:
>> > On Sep 25, 2017, at 12:24 PM, Joe Groff <jgroff at apple.com> wrote:
>> >> On Sep 24, 2017, at 10:30 PM, John McCall <rjmccall at apple.com> wrote:
>> >>> On Sep 22, 2017, at 8:39 PM, Saleem Abdulrasool <
>> compnerd at compnerd.org> wrote:
>> >>>
>> >>> On Thu, Sep 21, 2017 at 10:28 PM, John McCall <rjmccall at apple.com>
>> wrote:
>> >>>
>> >>>> On Sep 21, 2017, at 10:10 PM, Saleem Abdulrasool <
>> compnerd at compnerd.org> wrote:
>> >>>>
>> >>>> On Thu, Sep 21, 2017 at 5:18 PM, John McCall <rjmccall at apple.com>
>> wrote:
>> >>>>> On Sep 21, 2017, at 1:26 PM, Saleem Abdulrasool via swift-dev <
>> swift-dev at swift.org> wrote:
>> >>>>> On Thu, Sep 21, 2017 at 12:04 PM, Joe Groff <jgroff at apple.com>
>> wrote:
>> >>>>>
>> >>>>>
>> >>>>>> On Sep 21, 2017, at 11:49 AM, Saleem Abdulrasool <
>> compnerd at compnerd.org> wrote:
>> >>>>>>
>> >>>>>> On Thu, Sep 21, 2017 at 10:53 AM, Joe Groff <jgroff at apple.com>
>> wrote:
>> >>>>>>
>> >>>>>>
>> >>>>>>> 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.
>> >>>>>>
>> >>>>>>
>> >>>>>> 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.
>> >>>>
>> >>>> What's wrong with IMAGE_REL_AMD64_REL32?  We'd have to adjust the
>> relative-pointer logic to store an offset from the end of the relative
>> pointer instead of the beginning, but it doesn't seem to have a section
>> requirement.
>> >>>>
>> >>>> Hmm, is it possible to use RIP relative addressing in data?  If so,
>> yes, that could work.
>> >>>
>> >>> There's no inherent reason, but I wouldn't put it past the linker to
>> fall over and die.  But it should at least be section-agnostic about the
>> target, since this is likely to be used for all sorts of PC-relative
>> addressing.
>> >>>
>> >>>
>> >>> At least MC doesnt seem to like it.  Something like this for example:
>> >>>
>> >>> ```
>> >>>  .data
>> >>> data:
>> >>>  .long 0
>> >>>
>> >>>  .section .rodata
>> >>> rodata:
>> >>>  .quad data(%rip)
>> >>> ```
>> >>>
>> >>> Bails out due to the unexpected modifier.  Now, theoretically, we
>> could support that modififer, but it does seem pretty odd.
>> >>>
>> >>> Now, as it so happens, both PE and PE+ have limitations on the file
>> size at 4GiB.  This means that we are guaranteed that the relative
>> difference is guaranteed to fit within 32-bits. This is where things get
>> really interesting!
>> >>>
>> >>> We cannot generate the relocation because we are emitting the values
>> at pointer width.  However, the value that we are emitting is a relative
>> offset, which we just determined to be limited to 32-bits in width.  The
>> thing is, the IMAGE_REL_AMD64_REL32 doesn't actually seem to care about the
>> cross-setionness as you pointed out.  So, rather than emitting a
>> pointer-width value (`.quad`), we could emit a pad (`.long 0`) and follow
>> that with the relative displacement (`.long <expr>`).  This would be
>> representable in the PE/COFF model.
>> >>>
>> >>> If I understand the layout correctly, the type metadata fields are
>> supposed to be pointer sized.  I assume that we would like to maintain that
>> across the formats.  It may be possible to alter the emission to change the
>> relative pointer emission to emit a pair of longs instead for PE/COFF with
>> a 64-bit pointer value.  Basically, we cannot truncate the relocation to a
>> IMAGE_REL_AMD64_REL32 but we could generate the appropriate relocation and
>> pad to the desired width.
>> >>>
>> >>> Are there any pitfalls that I should be aware of trying to adjust the
>> emission to do this?  The only downsides that I can see is that the
>> emission would need to be taret dependent (that is check the output object
>> format and the target pointer width).
>> >>>
>> >>> Thanks for the hint John!  It seems that was spot on :-).
>> >>
>> >> Honestly, I don't know that there's a great reason for this pointer to
>> be relative in the first place.  The struct metadata will already have an
>> absolute pointer to the value witness table which requires load-time
>> relocation, so  maybe we should just make this an absolute pointer, too,
>> unless we're seriously considering making that a relative pointer before
>> allocation.
>> >>
>> >> In practice this will just be a rebase, not a full relocation, so it
>> should be relatively cheap.
>> >
>> > At one point we discussed the possibility of also making the value
>> witness table pointer relative, which would allow concrete value type
>> metadata to be fully read-only, and since code invoking a value witness is
>> almost certainly going to have the base type metadata pointer live,
>> probably not an undue burden on code size.
>> Yes, that's true.  It would make the base of the load (metadata +
>> loaded-offset + immediate-offset), which I think would require an extra
>> instruction even on x86, but maybe that's not so bad.
>> On the other hand, yes, it would not be possible to refer to prebuilt
>> vwtables from the runtime, and it would need to be a 64-bit relative offset
>> in order to handle dynamic instantiation correctly, which as you say is
>> problematic on some platforms.
> Hmm, Im not sure I understand the desired approach.  Would we want to
> switch to a rebased pointer?
> That's what we're discussing.  Switching to an absolute pointer (i.e. a
> normal pointer, which would need to be rebased) has proven to be generally
> more portable because many linkers do not support 64-bit relative
> pointers.  Also, since this is adjacent to another absolute pointer, the
> benefits of a relative pointer seem pretty weak: it would eliminate a very
> small amount of work at load time and (probably) some binary-size overhead,
> but that's relatively minor compared to, say, whether the loader has to
> dirty any memory.  Now, maybe we can avoid it being adjacent to another
> absolute pointer by making the vwtable relative, and that would have some
> significant upsides, but it would also have some significant drawbacks, and
> it's not clear that anybody actually wants to put any time into that
> investigation before we reach ABI stability.
> I'm personally leaning towards saying that vwtables should just stay
> absolute, and thus that nominal-type-descriptor pointers should just become
> absolute to make things easier.  I'm not worried about the binary-size
> impact; it's just a rebase, and Mach-O encodes rebases pretty efficiently.
> It's a little unfortunate for ELF, which has wastefully large loader
> encodings, but we could address that specifically if we felt the urge (or
> maybe just do ELF infrastructure work on more efficient encodings).
> Would this be for all of the metadata or just the struct type?
> Only structs, enums, and classes have nominal type metadata, and classes
> use an absolute pointer.

I thought that I would take a stab at this since this is the penultimate
issue preventing the Windows x86_64 stdlib build.  Maybe I'm misreading
something, but the compiler seems to indicate that the class metadata has a
relative pointer?

template <class Impl>
class ClassMetadataBuilderBase : public ClassMetadataVisitor<Impl> {
  void addNominalTypeDescriptor() {
    auto descriptor = ClassNominalTypeDescriptorBuilder(IGM, Target).emit();

The addFarRelativeAddress would handle this the same way as the other cases

> Are there no other instances of the same pattern?
> At the very least, none of the other instances have the 64-bit problem.
> They're also just generally more likely to be internal to a section.
> John.
>> John.
>> > It's a fair question though whether we'll ever get around to that
>> analysis, and I think the nominal type descriptor reference is the only
>> place we statically emit a pointer-sized rather than 32-bit relative
>> offset, which has caused problems for ports to other platforms that only
>> support 32-bit relative offsets.
>> >
>> > -Joe
> --
> Saleem Abdulrasool
> compnerd (at) compnerd (dot) org

Saleem Abdulrasool
compnerd (at) compnerd (dot) org
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