When working on this change https://github.com/apple/swift/pull/32352 the windows tests failed unexpectedly. We tracked this down to the output containing these lines:
This valid one (that there should have been 2 of):
And this invalid one where the `__ll` was cut off from the beginning:
I found the missing `__ll` above in another line:
It seems like when using `-emit-ir` (and maybe other output methods) with `-num-threads` greater than 1, the output can become interleaved. I was never able to reproduce this on macOS, or my Windows VM, but it reproduced consistently on Windows CI.