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

Davide Italiano via llvm-dev llvm-dev at lists.llvm.org
Wed Sep 6 13:28:12 PDT 2017

On Wed, Sep 6, 2017 at 1:10 PM, Johan Engelen via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> On Tue, Sep 5, 2017 at 11:34 PM, Davide Italiano <dccitaliano at gmail.com>
> wrote:
>> 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.
> ThinLTOCodeGenerator::addModule is called by the linker, right? (I can't
> find any callers)
> When it is called, we cannot retrieve the offset of the module inside the
> archive, because the linker didn't tell us about it.

See here for the fix.
You pass the the archive name + offset to `lto::InputFile::create`,
assuming your linker uses the new LTO API (and I'm not sure whether
ld64 already switched).
The linker knows the archive name from which it's fetching the member,
as well as its offset (it asks libArchive about it, for lld).
I'm not sure how it works ld64, but of course, to get this mechanism
working, you need some linker modifications.
CC:ed some Mach-O people, they probably know more about this than I do.



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