[llvm-dev] Linker Option support for ELF

[Saleem Abdulrasool via llvm-dev]

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).

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.

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
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