[llvm-dev] (no subject)

[Michael Spencer via llvm-dev]

On Wed, Sep 23, 2020 at 1:45 PM Eric Astor <epastor at google.com> wrote:

> Hi all,
>
> While working on alias support for the LLVM-ML project, I ran into a
> feature implemented back in 2010: default-null weak externals in COFF, a
> GNU extension.
> https://reviews.llvm.org/rG17990d56907b
> I'd like to disable this feature when targeting MSVC compatibility. Does
> anyone have more context on this, and why it'd be a terrible idea?
>
> For context: This seems to be designed to let LLVM implement a GNU
> extension in COFF libraries. However, it leads to very different behavior
> than we see for cl.exe (and ml.exe) on Windows; for already-defined
> aliasees, it injects an alternate placeholder
> ".weak.<alias>.default.<uniquifier>" symbol which resolves back to the
> current location. I admit, I'm not quite sure how this helps. If anyone can
> explain the purpose, I'd really appreciate it!
>
> In Windows PE/COFF files, aliases typically just resolve to their target
> symbol. For an example, see https://reviews.llvm.org/D87403#inline-811289.
>
> Thanks,
> - Eric
>


Sadly I don't recall why I made that change, probably should have included
that in the commit message. I do know that at the time I was focused on
link.exe support, not mingw. I just looked through my commit history from
that time and it doesn't really explain much.

I believe that as long as the "compare weak symbol against null" does the
right thing any changes you make here are fine.

- Michael Spencer
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200923/ac9c444f/attachment.html>