[llvm-dev] RFC: Switching to the new pass manager by default

[Chandler Carruth via llvm-dev]

Greetings everyone!

The new pass manager is getting extremely close to the point where I'm not
aware of any significant outstanding work needed, and I'd like to see what
else would be needed to enable it by default. Here are the current
functionality I'm aware of outstanding:

1) Does not do non-trivial loop unswitching. Majority of this is in
https://reviews.llvm.org/D34200 but will need one or two small follow-ups.

2) Currently, sanitizers don't work correctly with it. Thanks to the work
of others, the missing infrastructure has been added and I'll send a patch
to wire this up this week.

3) Missing support for 'optnone'. I've been working on this, but the
existing testing wasn't as thorough as I wanted, so it is going slowly.
I've got about 1/4 of this implemented and should have patches this week or
next.

4) Missing opt-bisect (or similar) facility. This looks pretty trivial to
add, but I've not even started. If anyone is interested in it, go for it.
We might even be able to do something simpler using the generic debug
counters and get equivalent functionality.

... that's it?

Optimization quality / run-time performance:
- We've been using it at Google extensively and are very happy with the
optimization quality. Benchmarks look *very* good here.
- More data from other users would be important.
- You can try it out with `-fexperimental-new-pass-manager` to Clang

Compile-time performance:
- Sometimes *much* better due to cached analyses.
- Sometimes worse, typically due to more / different inlining in turn
running main pipeline (GVN + InstCombine) more times or over more code.
- Overall somewhat a wash, but the increased compile times typically due to
the optimizer "trying" harder, so not too concerning on our end.
- Again, more feedback from other users good:
`-fexperimental-new-pass-manager` to Clang

Once the four missing things land, I'll also happily work on collecting
some of the basics on the test-suite and CTMark. But I suspect more "in the
wild" data would really be useful here given the significance of the change.

Thoughts? What else (beyond the four items above and feedback on run-time
and compile-time) would folks like to see?

Once this happens, I'll also be preparing some batch, mechanical updates to
the test suite to primarily use the new pass manager. Also there is lots of
documentation updates that will be needed here.

-Chandler

PS: I'll be sending a note to cfe-dev as a "heads up" about this discussion
as in some ways, the default flip is mostly a Clang default flip. But
hopefully our doc updates will trigger this being "perceived" as the
default for other frontends, and I'll try to reach out to other major
frontends as well (Swift and Rust are on my radar, and I've already started
talking with Philip Reames about their Falcon JIT).
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