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

[Rafael Espíndola]

> Okay so I understand thst the old LLVM specific archive format is now gone.

correct.

> However it seems you are allowing LLVM bitcode files ( I assume that's what
> you mean by "IL" - does that stand for "intermediate language"? It's not in
> [1] ) to be placed inside the more standard archive format understood by GNU
> ar and nm.

Correct, by having gnu ar use the plugin.

> Therefore LLVM is still supporting runtime libraries that consist of llvm
> bitcode files (even if the format is now different)

runtime? It is still possible to build .a files if that is what you men.

> For example, I can build a runtime library (e.g. a simple C library) as a
> collection of bitcode modules and place them in an archive using the latest
> llvm-ar ( I realise that currently the archive's symbol table will be
> missing symbols from the bitcode files).

Yes, it will produce an archive, but without the symbol table.

>> 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.
>
> Okay. Could you please clarify? Do you mean?

Once llvm-ar is producing .a files with symbol table for IR files, you
should be able to read those symbol tables to find which member
defines a symbol you are looking for.

> - Future changes to lib/Object will let me read the symbols in an archive so
> I can implement my own ( primitive bitcode only ) linking?
>
> Or
>
> - Future changes to lib/Object will  implement llvm module linking for me?

The first. Since lld is in its own repository, the logic for fetching
member and iterating will be there, but all the supporting logic will
be in llvm proper.

> I suspect you meant the first. If so, is the intention that if someone needs
> to link an in memory LLVM bitcode module to an archive of bitcode modules (
> produced by the new llvm-ar or at with llvmLTO wrapper to ar) then they
> should use the API of lld? Does that API exist now? I took a look at the lld
> source code and I couldn't find any methods that returned llvm::Module so I
> assumed it wasn't possible.

No, we still have to implement support for bitcode files in lld.

Cheers,
Rafael