[llvm-dev] new pass manager version of opt vs. legacy pass manager version

[Min-Yih Hsu via llvm-dev]

On Thu, Apr 22, 2021 at 8:34 AM Snider, Todd via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello All,
>
>
>
> My development group has been maintaining a downstream version of the
> monorepo that stays in sync with the upstream “main” branch, but we are
> still using the legacy pass manager in our local copy of the monorepo.
>
>
>
> We’ve recently encountered a few instances of lit tests that are failing
> when run with the legacy pass manager version of opt, but pass when run
> with the new pass manager version of opt.
>

I don't think you're talking about upstream tests right? Did you rewrite
some of the new-PM-only tests into old PM syntax?


>
>
> The situation raises a couple of questions:
>
>    - Is the legacy pass manager behavior being adequately tested by the
>    buildbots?
>    - What expectation should there be that legacy pass manager behavior
>    will be maintained in light of changes made to code that affects both the
>    new pass manager version and the legacy pass manager version of opt?
>
>
>

AFAIK many Passes share the same code base between old and new PM (except
few like AlwaysInliner who has completely different behaviors between old /
new PM), so any changes to their underlying implementation should reflect
in both PMs. The biggest differences between old / new PM is probably their
Pass ordering in the pipeline.


> I suspect that my group is not the only ones trying to stay in sync with
> the upstream LLVM main branch and keep using the legacy pass manager, and I
> anticipate the only long-term remedy for our situation is to move to using
> the new pass manager as soon as we can.
>
>
>

Well, don't forget we're still using old PM to do the code generation
(wink) so old PM _by itself_ won't go away any time soon. But I think
_optimization_ Passes written in legacy syntax might be removed sooner.

-Min


> Thoughts?
>
>
>
> Regards.
>
>
>
> Todd Snider
>
> Texas Instruments Incorporated
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>


-- 
Min-Yih Hsu
Ph.D Student in ICS Department, University of California, Irvine (UCI).
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