[llvm-dev] Linker Option support for ELF

Saleem Abdulrasool via llvm-dev llvm-dev at lists.llvm.org
Wed Jan 3 21:51:08 PST 2018

On Wed, Jan 3, 2018 at 8:08 PM, Sean Silva <chisophugis at gmail.com> wrote:

> On Wed, Jan 3, 2018 at 4:02 PM, Saleem Abdulrasool <compnerd at compnerd.org>
> wrote:
>> Hello all,
>> There was some interest from a number of a few people about adding
>> support for embedded linker options to ELF.  This would be an extension
>> that requires linker support to actually work, but has significant prior
>> art with PE/COFF as well as MachO both having support for this.
>> The desire here is to actually add support to LLVM to pass along the
>> necessary information into the object file.  In order to keep this focused
>> on that, this thread is specifically for the *backend*, we are not
>> discussing how to get the information to the backend here at all, but
>> assuming that the information is present in the same LLVM IR encoding
>> (llvm.linker-options module metadata string).
> Do we have agreement about this assumption? I think one main point of
> disagreement is how open-ended we want things, and llvm.linker.options
> implies, at least at first glance, a very open-ended approach. Can you
> describe how open-ended llvm.linker.options is, in fact/in practice? I.e.
> what subset of linker options do the COFF/MachO targets actually support?
This is something which is already well established.  I'm not proposing to
change that.  llvm.linker.options is something which is a string passed by
the frontend.  There is nothing that is interpreted by either side.  This
is not new metadata that I am introducing, it is the existing
infrastructure.  Now, if you like, a newer more restrictive mechanism could
be introduced, but that would be beyond the scope of this change IMO.  Both
of those do not have any restrictions AFAIK; and any control of what they
permit is from the *frontend* side.

>> In order to have compatibility with existing linkers, I am suggesting the
>> use of an ELF note.  These are implicitly dropped by the linker so we can
>> be certain that the options will not end up in the final binary even if the
>> extension is not supported.  The payload would be a 4-byte version
>> identifier (to allow future enhancements) and a null-terminated string of
>> options.
> One thing that is on my mind is that the fact that llvm.linker.options is
> metadata means it can be dropped. So, playing devil's advocate here, it is
> correct for ELF targets to just ignore it (as they currently do AFAIK). if
> your intended use case does not actually behave correctly with the
> llvm.linker.options dropped, then that suggests that something is fishy.

Yeah, the metadata can be dropped.  If the metadata is dropped, then
nothing gets embedded into the object file.  The changes being discussed
would be embedding additional metadata into the object file.  A separate
change would be needed to actually process that in the linker as well as
one to the frontend to actually emit the metadata.

> I guess the fact that llvm.linker.options is currently used for COFF/MachO
> suggests that it is not dropped in practice in the situations that matter,
> but it does provide some evidence that we may want to move away from
> llvm.linker.options. For example, we could let frontends parse legacy
> open-ended linker pragmas and emit a new IR format with constrained
> semantics.

Right, this is inline with what I was suggesting as a second mechanism for
this that could be designed.  But, again, that is beyond the scope of the
changes that I am proposing.

> (also, do you know if the COFF/MachO representation for the linker
> options  in the .o file can/cannot be dropped? AFAIK, SHT_NOTE can be
> ignored)

Well, the content is only in the object files.  The final linked binary
does not contain it (which is why Im abusing the SHT_NOTE).  Do you mean
does the linker ignore it?  Well, if the linker doesn't support the
feature, it would.  In PE/COFF, it is encoded as a special section
(.drectve).  In fact, GNU ld doesn't have as complete of an implementation
as lld/link and does ignore a bunch of options.  MachO has a special load
command (LC_LINKOPT) that encodes this.  But, in both cases, it requires
the linker to interpret it, and if it does not, then the same behavior
would be observed.

> I just find it very fishy to consider linker options as advisory.
> Presumably if a user is passing them, then they are required for
> correctness.
Sure, but that failure would generally be pretty obvious: linking would

> -- Sean Silva
>> This allows for the backend to be entirely oblivious to the data as the
>> other backends and allows for extensions in the future without having to
>> teach the backend anything about the new functionality (again, something
>> which both of the other file formats support).
>> As an example of how this can be useful, it would help with Swift support
>> on Linux where currently the linker options are pushed into a custom
>> section, and a secondary program is used to extract the options from this
>> section prior to the linker being invoked.  This adds a lot of complexity
>> to the driver as well as additional tools being invoked in the build chain
>> slowing down the build.
>> --
>> Saleem Abdulrasool
>> compnerd (at) compnerd (dot) org

Saleem Abdulrasool
compnerd (at) compnerd (dot) org
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