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

[Hal Finkel via llvm-dev]

On 10/25/2017 12:16 PM, Xinliang David Li via llvm-dev wrote:
>
>
> On Tue, Oct 17, 2017 at 11:50 PM, Chandler Carruth via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto: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
>     <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.

Why?

  -Hal

>
> 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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>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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