[LLVMdev] [LLVM] What has happened to LLVM bitcode archive support?

[Rafael Espíndola]

On 20 December 2013 05:39, Daniel Liew <daniel.liew at imperial.ac.uk> wrote:
> Hi Rafael and other LLVM devs,
>
> I'm currently upgrading a project that uses LLVM that links a bitcode
> archive (a C library) with a module. Originally we used
> Linker::LinkInFile() but that was removed by r172749. So I started
> looking for an alternative and I found
> Archive::findModulesDefiningSymbols() which looked very promising as
> it would allow me to very easily implement linking a bitcode archive
> with a module efficiently (i.e. only call Linker::LinkModules() on the
> relevant bitcode modules inside the bitcode archive).
>
> Before I started I thought I'd better check that this API was still
> going to be available in the upcoming 3.4 release and to my dismay
> ``llvm/Bitcode/Archive.h`` was moved by you in r184083 and finally
> removed by you in r186197.
>
> So...
>
> * Is LLVM completely moving away from the idea have bitcode archive
> runtime libraries? The LLVM Make build system seems to support
> building these but LLVM doesn't seem to have an API for linking
> runtime bitcode archives into a bitcode module any more (or have I
> missed something?). I have seen suggestions of using the LLVM gold
> plug-in but that's not an API I can use, is it?
>
> * I'm guessing I'm supposed to use the Object/Archive.h interface now?
> This doesn't seem provide a nice interface for getting the set of
> Modules I want to link with, so what am I supposed to do instead?

We are going this way, yes. The overview is

* In the old days llvm had its own archive index format for IL files
and special code to use it.
* llvm-ld was very incomplete, and not designed to grow into real
linker, so an alternative for LTO was needed.
* apple implemented libLTO and changed their linker to use it. A
wrapper (LLVMgold.so) was created for gold and bfd.
* Using this the gnu nm, ar and ld (and gold) were able to do LTO. In
particular, a IL files can be put in archives and show up in the
regular symbol table.
* llvm-ld was deprecated and then removed.
* with nothing able to read the old LLVM only symbol table, that was
also removed along with other archive related now dead code.

The more recent story is that llvm has grow support for object files,
so that llvm-ar is now a "real" (if immature) ar and lld is growing to
be a real linker. Currently llvm-ar is missing support for putting IL
files in the symbol table. The main reason is the desire to now get
this right this time, which required quiet a bit of refactoring. I
posted what I think is the last big patch for review (making the
Mangler not depend on target). With that in things should move fairly
quickly to have llvm-ar just work with IL files and with that
lib/Object should have the features you need.

> Any help would be appreciated,
>
> Thanks,
> Dan Liew.

Cheers,
Rafael