[libc-commits] [libc] [libc] Add baremetal minor archive support. (PR #203015)

[Simi Pallipurath via libc-commits]

simpal01 wrote:

@vhscampos and @michaelrj-google Thank you both for the comments.

Based on comments,

1. **Dependency closure Issue**

- LLVM libc entrypoints should not depend on other entrypoints. The known allowed exceptions are libc.src.errno.errno and libc.src.setjmp.longjmp.
- For the string case I tested, only strdup depends on errno.

So looks like dependency-closure issue appears very limited.

3. **Unintended dependency packaging**

In Our layout, each minor variant links something like:

variant libllvmstring.a  +  base libc.a

The base libc.a will already contain errno. If the variant libstring.a also contains libc_errno.cpp.obj, then errno exists in both layers.

Then the link order can make the minor layering archive visible before the base archive so linker only extracts one copy from one archive.
-lllvmstring -lc

 For layered multilibs, the cleaner model is:
```

  base libc.a:
    errno
    common libc objects
    default string implementation, if needed

  variant libstring.a:
    only specialized string objects
    no errno
```

I would not argue that errno duplication will always break. I would argue that it is a layering violation and an unnecessary dependency packaging.  Victor and I agreed that it’s okay to leave it as it is for now until we identify a suitable solution.

4. Yeah my thought was also  to pass a a cmake flag  from downsteam project to change the exported archive's name

These are my thoughts. I still need to complete the full testing with LIBC_CONFIG_PATH and update this patch based on the results.

https://github.com/llvm/llvm-project/pull/203015