[llvm] [ExtendLifetimes] Implement llvm.fake.use to extend variable lifetimes (PR #86149)

[Orlando Cazalet-Hyams via llvm-commits]

https://github.com/OCHyams commented:

I've looked over the code and test changes. The contents and style LGTM overall (some minor inline comments). I'm not familiar with the legalizer or X86FloatingPoint.cpp, so if someone could take a look at those parts specifically that would be great (or @arsenm have you looked at those bits already?).

Once that has been done, and once GlobalISel support has been added, I think that's everything covered by review.

I also work at Sony so I'm hesitant to Accept this once those bits have been done (I know that I am "allowed" to, but for a larger feature it feels right if others are involved too).

For added confidence, it should also be noted that the functionality has been working well for us downstream for over 5 years (possibly 7 years, but I haven't done much archeology on it) - we work at tot and we've been maintaining it since, so we know it works with today's LLVM. The feature was first mentioned upstream all the way back in 2015, here https://lists.llvm.org/pipermail/llvm-dev/2015-September/090610.html. That is all to say, as far as "new" features go this one has gone through a lot of real-world testing.

The feature this patch supports (-fextend-lifetimes, and improving the -Og pipeline to bring substantial debugging benefits) was discussed at EuroLLVM 2024 - many people seemed to support this idea and I didn't see anyone one raised any objection to it.

https://github.com/llvm/llvm-project/pull/86149