Possible violation of the `pthread_create` interface? about ponyc HOT 4 OPEN

Considered relevant during sync: https://stackoverflow.com/questions/9116951/what-is-the-meaning-of-restrict-in-pthread-create

from ponyc.

I think I was convinced over the duration of the sync call that this is indeed a spec violation (but I may still be under-informed on the issue, so I could be wrong). That is, I was convinced that some future version of clang could add an optimization that leverages the restrict qualifier in a way that breaks our usage of pthread_create.

The violation seems to be that our first arg and final arg to pthread_create are overlapping in memory, because the pony_thread_id_t struct reference we pass as the first arg is embedded within the scheduler_t reference which we pass as the final arg. Because they address an overlapping memory region, certain optimizations allowed by the restrict qualifier could cause a copy operation to clobber the view as seen by one or the other.

One way to avoid this would be to not embed the pony_thread_id_t - we could, for example, pool-allocate it instead. Or keep it in a separate struct array perhaps.

from ponyc.

I'm not convinced this is an issue. We should discuss together at some point @jemc. I agree it is a violation of the quoted posix spec but I'm not sure that matters, what matters is whether that spec is implemented by our target platforms.

from ponyc.

Joe and I discussed at sync today. We both agree this could be an issue.

I'm more worried that someone fixing this issue is likely to introduce bugs than clang and other compilers will start to follow this portion of the spec. I'm fine with someone fixing the issue herein so long as there is a test plan to make sure no bugs are introduced with the change to address the spec violation.

from ponyc.