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

ORiordan, Martin via llvm-dev llvm-dev at lists.llvm.org
Wed Oct 18 02:28:41 PDT 2017

Hi Chandler,

I don’t track the head revisions, instead I do a “Big Bang” update with each 6-month formal release of the LLVM suite, and I am now at v5.0.

Is the implementation of the new pass manager in LLVM v5.0 suitable for me to usefully experiment with it so as to provide feedback, or should I wait and try it out with v6.0?



From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Chandler Carruth via llvm-dev
Sent: Wednesday, October 18, 2017 7:51 AM
To: llvm-dev <llvm-dev at lists.llvm.org>
Subject: [llvm-dev] RFC: Switching to the new pass manager by default

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.


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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