>>>
>>>
>>>>
>>>>>
>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> 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();
B.addFarRelativeAddress(descriptor);
}
...
}
The addFarRelativeAddress would handle this the same way as the other cases
AFAICT.
···
On Mon, Sep 25, 2017 at 10:18 PM, John McCall <rjmccall@apple.com> wrote:
On Sep 26, 2017, at 12:35 AM, Saleem Abdulrasool <compnerd@compnerd.org> > wrote:
On Mon, Sep 25, 2017 at 11:47 AM, John McCall <rjmccall@apple.com> wrote:> On Sep 25, 2017, at 12:24 PM, Joe Groff <jgroff@apple.com> wrote:
>> On Sep 24, 2017, at 10:30 PM, John McCall <rjmccall@apple.com> wrote:
>>> On Sep 22, 2017, at 8:39 PM, Saleem Abdulrasool < >> compnerd@compnerd.org> wrote:
>>> On Thu, Sep 21, 2017 at 10:28 PM, John McCall <rjmccall@apple.com> >> wrote:
>>>> On Sep 21, 2017, at 10:10 PM, Saleem Abdulrasool < >> compnerd@compnerd.org> wrote:
>>>> On Thu, Sep 21, 2017 at 5:18 PM, John McCall <rjmccall@apple.com> >> wrote:
>>>>> On Sep 21, 2017, at 1:26 PM, Saleem Abdulrasool via swift-dev < >> swift-dev@swift.org> wrote:
>>>>> On Thu, Sep 21, 2017 at 12:04 PM, Joe Groff <jgroff@apple.com> >> wrote:
>>>>>> On Sep 21, 2017, at 11:49 AM, Saleem Abdulrasool < >> compnerd@compnerd.org> wrote:
>>>>>> On Thu, Sep 21, 2017 at 10:53 AM, Joe Groff <jgroff@apple.com> >> wrote:
>>>>>>> On Sep 21, 2017, at 9:32 AM, Saleem Abdulrasool via swift-dev < >> swift-dev@swift.org> wrote:
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