[llvm-dev] [ThinLTO] static library failure with object files with the same name

[Davide Italiano via llvm-dev]

On Tue, Sep 5, 2017 at 2:09 PM, Teresa Johnson <tejohnson at google.com> wrote:
>
> Hi Johan,
>
> Right, per the bug this is fixed in lld (and was already handled in gold-plugin), but I guess not in ld64. Note that lld and gold-plugin use the new LTO API, while ld64 (and probably other linkers) are still using the legacy libLTO (which is what ThinLTOCodeGenerator.cpp is part of). Fixing it in the location you propose could work for all legacy libLTO users. But I don't think that adding just the size will (always) be enough to disambiguate (couldn't the 2 same named members have the same size?) - although lld is doing the same thing so this may be as good as what is done there. For gold-plugin we add the byte offset into the archive where the member starts, which will be unique.
> +davide for thoughts since he fixed it on the lld side.
>

Yes, Teresa is right, this is the correct fix.
I'm not sure what subset of GNU archives Mach-O supports, but the only
way of being safe is using offset in the archive + archive name.
Fun fact, you apparently can have a single GNU archive with two
different members with the same name. Using the offset is the only way
 to disambiguate.
I think we should really consider documenting this assumption
somewhere as many people have been bitten in the past and tracking
down is not trivial as it shows up as assertion failures in the mover
or duplicate/undefined symbols reported as linker errors, as the
ThinLTO logic will pick the an archive randomly.