[LLVMdev] How to do bitcode archive linking correctly?

[Daniel Liew]

Hi,

We're currently upgrading KLEE to work with LLVM >=3.3 and we've hit a problem.

It seems r172749 removed support for linking a bitcode archive into a
module. KLEE unfortunately depends on this to link in its runtime (
which amongst other things provides a C library [5] ).

A first attempt at linking in a bitcode archive ourselves can be seen in [1].

This approach does not work correctly, it's incredibly slow and I'm
not convinced it will resolve all undefined or weak symbols like
linking with an archive normally should. This leads me to ask

* How does Linker::LinkModules() [2] behave? Does it just blindly
concatenate modules ( that might explain the slow down ) or does it
actually check the symbol tables of the modules and only link in `Src`
 if `Dest` has unresolved or weak symbols that `Src` can provide?

* If Linker::LinkModules() does decide to merge `Src` into `Dest` does
it merge the entire contents of the `Src` module or just the symbols
that are needed by `Dest`?

* IIRC when linking archives a linker must must link constituent
modules multiple times until a fixed point is reach (the set of
undefined symbols does not change ). Does that apply here? I guess the
answer depends on the behaviour of Linker:: LinkModules(). If it does
that would make [1] incorrect as it only links in the archive's
modules once.

* Why is the approach so slow compared to using the old
Linker::linkInFile() API in prior LLVM versions?

What ever the answers are to the above, the approach in [1] isn't
right. So what is the right way to programmatically link in LLVM
bitcode archives into a bitcode module?

I noted that the r172749 commit says...

```
the "right" way to get this support is to use the
platform-specific linker-integrated LTO mechanisms, or the forthcoming
LLVM linker.
```

I'm not sure how this helps us

- For "platform-specific linker-integrated LTO mechanisms", is that
referring to [3]? If so that's not particularly helpful as I'd like an
API I can use to do the archive linking rather than having to invoke
an external program (i.e. my system's linker).
- For "forthcoming LLVM linker", is that referring to lld [4]?

[1] https://github.com/MartinNowack/klee/blob/51d4a2b34511a8d6b00b16a50f90b0bc1a793a69/lib/Module/ModuleUtil.cpp#L61
[2] http://llvm.org/docs/doxygen/html/classllvm_1_1Linker.html#a244da8c7e9789b1b675b9741bd692c63
[3] http://llvm.org/docs/GoldPlugin.html
[4] http://lld.llvm.org/
[5] https://github.com/ccadar/klee-uclibc/tree/klee_0_9_29

Thanks,
Dan Liew.