[llvm-dev] Linker Option support for ELF

[James Henderson via llvm-dev]

By my understanding, the proposal is that the user input looks something
like:

#pragma linker_directive("lib", "m")

which is passed essentially as is to the linker, i.e. the directive payload
is a pair of strings, the first being "lib", the second "m". It is then up
to the linker to decide if it supports "lib" directives, and translate that
into the corresponding command, or do something else if it doesn't support
it, e.g. emit an error or ignore it. Neither the backend nor the frontend
need to do any conversion, and it makes it much clearer that these aren't
like any other command-line option (due to the limited set that can be
passed through, and the possible differences in behaviour, such as order on
the command-line).

As an aside, this approach would aid portability to linkers that aren't
command-line compatible with one another. If, for example, the switch is
"-lib" on linker A, and "-l" on linker B, the user doesn't have to have
different source code for each different linker their object is to be
consumed by, nor does the compiler have to know about the different
possible linkers.

James

On 9 January 2018 at 05:15, Saleem Abdulrasool <compnerd at compnerd.org>
wrote:

>
>
> > On Jan 7, 2018, at 5:02 PM, Cary Coutant <ccoutant at gmail.com> wrote:
> >
> >> I think we all agree that blindly allowing the linker to honor the
> options
> >> would be scary.  I agree that we should whitelist the options, and am
> of the
> >> opinion that we should force validation on the linker side (use of any
> >> option which the linker doesn't support in this form can be fatal).
> >> Starting small is the best way, with `-l` and `-L` as a starting
> point.  I
> >> want to retain the ability to add additional options which may not be
> >> available in all linkers.  However, whitelisting obviously requires
> working
> >> with the linker as would adding such options, so that could be handled
> at
> >> that time.
> >
> > This is actually why I'd prefer a new "language" over just
> > whitelisting options. With "lib", "file", and "path", as I suggested,
> > there's no question whether an option like "-no-pie" is supported, and
> > no temptation to even try. The new language should be tailored for
> > process-to-process communication, rather than user-to-shell
> > communication.
>
> I suppose I am slightly confused about what you are proposing here now.
> From the user side, it would be something to that effect.  Im suggesting
> that we take lib and transform it to a different representation in the
> frontend.  Taking an explicit example:
>
> The user input
> `#pragma comment(lib, “m”)`
>
> would get transformed to `-lm` in the object file encoding.  However, this
> would still not permit you from injecting any arbitrary options, but the
> backend doesn’t change for any new option, only the frontend and the linker.
>
> >> I’m thinking about future enhancements.  MachO does actually provide
> >> something like `-L` -`l` in a single go via `-framework`.  But, no such
> >> option exists for ELF since it doesn’t have the concept of framework
> bundles
> >> (but the layout itself is interesting), and I just want to try to keep
> the
> >> door open for such features.
> >
> > This is why I also included "path" in my suggestion. I imagine
> > something very much like -framework, where include files and library
> > search paths are handled together.
> >
> > -cary
>
>
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