Re: [PATCH] lto-plugin: add support for feature detection

[Rui Ueyama <rui314@gmail.com>]

On Mon, May 16, 2022 at 2:38 PM Alexander Monakov <amonakov@ispras.ru> wrote:
>
> On Mon, 16 May 2022, Rui Ueyama wrote:
>
> > If it is a guaranteed behavior that GCC of all versions that support
> > only get_symbols_v2 don't leave a temporary file behind if it is
> > suddenly disrupted during get_symbols_v2 execution, then yes, mold can
> > restart itself when get_symbols_v2 is called for the first time.
> >
> > Is this what you want? I'm fine with that approach if it is guaranteed
> > to work by GCC developers.
>
> I cannot answer that, hopefully someone in Cc: will. This sub-thread started
> with Richard proposing an alternative solution for API level discovery [1]
> (invoking onload twice, first with only the _v3 entrypoint in the "transfer
> vector"), and then suggesting an onload_v2 variant that would allow to
> discover which entrypoints the plugin is going to use [2].
>
> [1] https://gcc.gnu.org/pipermail/gcc-patches/2022-May/594058.html
> [2] https://gcc.gnu.org/pipermail/gcc-patches/2022-May/594059.html
>
> ... at which point I butted in, because the whole _v3 thing was shrouded in
> mystery. Hopefully, now it makes more sense.
>
>
> From my side I want to add that thread-safety remains a major unsolved point.
> Compiler driver _always_ adds the plugin to linker command line, so I expect
> that if you add a mutex around your claim_file hook invocation, you'll find
> that it serializes the linker too much. Have you given some thought to that?
> Will you be needing a plugin API upgrade to discover thread-safe entrypoints,
> or do you have another solution in mind?

From my linker's point of view, the easiest way to handle the
situation is to implement a logic like this:

if (gcc_version >= 12.2)
  assume that claim_file_hook is thread safe
else
  assume that claim_file_hook is not thread safe

And that is also _very_ useful to identify the existence of
get_symbols_v3, because we can do the same thing with s/12.2/12.1/.