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

[Xinliang David Li via llvm-dev]

On Tue, Oct 17, 2017 at 11:50 PM, Chandler Carruth via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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?
>


Missing support of 'print-after-all' or 'print-after-passX'  is another
major usability loss.

Regarding the default switch, maybe do it in two steps:

1) Switch the default for PGO build first
2) Switch the default for all modes.

David




>
> 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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