[llvm-dev] Switching to the New Pass Manager by Default

[Leonard Chan via llvm-dev]

I think so far most individual projects have their own statistics on build
time/performance impact on switching to the new PM, but I agree that there
should at least be one place that people can reference on the impact.

On Fri, Aug 2, 2019 at 1:14 PM Philip Reames <listmail at philipreames.com>
wrote:

> Thanks for all the hard work that went into getting it here.
>
> +1
>
> Can you spell out what performance validation has been done?  Informally,
> I know there's been quite a bit, but getting a summary in one place for
> later reference would be super helpful.
>
> Philip
> On 8/2/19 10:25 AM, Leonard Chan via llvm-dev wrote:
>
> Hello all,
>
> As of now, all LLVM and Clang tests have been updated/addressed to run
> under the new/experimental pass manager (at least the ones that failed when
> using the new PM).
>
> For those who aren't aware of what the new pass manager (PM) is, the tl;dr
> is that this will serve as a replacement for the legacy PM, and promises
> faster build times by restructuring how passes are run over IR units. For
> some time, when someone wants to create a new pass, they may need to
> implement it for both pass managers since the legacy one is what is enabled
> by default. Passes that were initially made under the new PM have also
> slowly been ported over time.
>
> Now that LLVM 9.0.0 has branched, we have about 6 months before the next
> release. We think it would be a good idea to take this opportunity and make
> the new PM the default one which gives us enough time to work out any kinks
> that might come out of this switch before LLVM 10.0.0 branch point. We
> suspect that there may be other LLVM projects that will be affected by
> this, probably from unported passes.
>
> Does anyone have any opinions on this?
>
> Off the top of my head, the next immediate work would be to update the
> docs with instructions on how to write or port a new PM pass, and address
> any breakages for other LLVM projects.
>
> Thanks,
> Leonard
>
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