[llvm-dev] [RFC] Deprecating the legacy pass manager for the optimization pipeline

Mon Aug 30 15:23:59 PDT 2021

What legacy PM stuff I'd like to remove soonish:

1) Support for the CMake flag DLLVM_ENABLE_NEW_PASS_MANAGER=off (which is
related to defaults for the next 3 items)
2) LTO support in various linkers
3) Clang support for the optimization pipeline
4) opt support for translating `-instcombine` into `-passes=instcombine`,
will require updating many tests
5) PassManagerBuilder for building an optimization pipeline with the legacy
PM (requires all of the above to be done first)

I can add these to the 13.x release notes.

If there are any existing users right now who continuously test against
LLVM trunk and would like some time to port legacy PM passes to the new PM
I'm happy to wait a couple of weeks/a month (and happy to answer any
questions), but otherwise we shouldn't worry about downstream users who
don't speak up. Porting most passes is generally pretty quick and easy, but
some passes can be tricky.
As mentioned earlier, having two ways to do something can result in
confusion upstream when debugging issues. And code cleanup is nice. Also, a
fair number of lit tests currently run against both pass managers and
removing some RUN lines against the legacy PM can speed up tests.

What won't be removed anytime soon:
1) General legacy PM infrastructure, such as llvm::legacy::PassManager,
since codegen still uses it
2) Most legacy passes, since some IR passes run as part of the codegen
pipeline (although no reason to keep around some legacy passes like
LTO-specific passes)
        Some backends, via TargetPassConfig::addIRPasses(), will add
various passes to the codegen IR pipeline.
3) Testing legacy passes via opt, since again some IR passes run as part of
the codegen pipeline
        However, I'd like to explicitly enumerate all the passes that we
allow opt to run under the legacy PM. For example, target-specific codegen
IR passes like "amdgpu-lower-ctor-dtor" should be run under the legacy PM
via `opt -amdgpu-lower-ctor-dtor`. But most passes like instcombine should
only be invokable via the `opt -passes=instcombine` syntax so that we don't
have to worry about people testing the wrong pass manager by using the
wrong syntax. Even if some passes like instcombine are added to the codegen
pipeline in a legacy pass manager, I don't think the confusion of `opt
-instcombine` vs `opt -passes=instcombine` is worth the test coverage we
may gain by being able to run instcombine with the legacy PM. Perhaps if we
find that some passes do regress only under the legacy PM and not the new
PM we can revisit this point.

On Fri, Aug 27, 2021 at 6:17 PM Fāng-ruì Sòng <maskray at google.com> wrote:

> On Fri, Aug 27, 2021 at 3:55 PM Mehdi AMINI via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > These revisions are deprecating (IIUC):
> > 1) Clang user flags to use the legacy pass manager.
> > 2) The CMake build flag that defines the *default* pass manager.
> >
> > These don't seem like the most impactful for downstream users, are they?
>
> The previous disabling mechanism
> -DENABLE_EXPERIMENTAL_NEW_PASS_MANAGER=off is ignored in main and
> release/13.x.
> If a user switches (-DLLVM_ENABLE_NEW_PASS_MANAGER=off), they should
> notice the warning.
>
> So I think we don't need something like
> -DLLVM_FORCE_USE_OLD_TOOLCHAIN=on which was done in 2019 to bump the
> toolchain requirement.
>
> > On the other hand, what seems to me to be critical to clarify is if you
> also intend to deprecate other things, like the use of the legacy pass
> manager in opt (or as a pass manager in a downstream tool/compiler)? Are we
> gonna also remove all the legacy adapters that make the passes work in the
> legacy pass manager as well?
> >
> > This looks more important to me because this affects directly how other
> compilers are built on top of LLVM and push them to migrate, while the
> deprecation revision you sent may just have no effect on them.
> >
> > Thanks,
> >
> > --
> > Mehdi
> >
> >
> > On Fri, Aug 27, 2021 at 1:44 PM Chris Tetreault via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >>
> >> I think that’s a sufficiently obnoxious warning. I still strongly
> prefer that no removals of functionality come until we branch for LLVM 14,
> but I think this will do for a notice of deprecation.
> >>
> >>
> >>
> >> Thanks,
> >>
> >>    Chris Tetreault
> >>
> >>
> >>
> >> From: Arthur Eubanks <aeubanks at google.com>
> >> Sent: Friday, August 27, 2021 12:42 PM
> >> To: Chris Tetreault <ctetreau at quicinc.com>
> >> Cc: llvm-dev <llvm-dev at lists.llvm.org>
> >> Subject: Re: [llvm-dev] [RFC] Deprecating the legacy pass manager for
> the optimization pipeline
> >>
> >>
> >>
> >> WARNING: This email originated from outside of Qualcomm. Please be wary
> of any links or attachments, and do not enable macros.
> >>
> >> Are https://reviews.llvm.org/D108789 and
> https://reviews.llvm.org/D108775 sufficient if we cherrypick them into 13?
> >>
> >>
> >>
> >> On Wed, Aug 25, 2021 at 11:22 AM Philip Reames <
> listmail at philipreames.com> wrote:
> >>
> >> I'd vote for immediate removal.  I don't have much sympathy for
> downstream consumers who haven't moved.  This effort has been underway for
> literal years.  Many - though not by any means all - downstream projects
> moved *before* upstream finally switched.  Let's put a nail in this coffin,
> and remove code aggressively.
> >>
> >> Supporting both has serious ongoing costs.  As a real example, I have
> twice spent time in the last two weeks tracking down some odd quirk of the
> unrolling implementation to find it supports some quirk of the legacy pass.
> That slows down development, compromises quality, and is generally a "bad
> thing".
> >>
> >> We might want to wait a couple of weeks/months to ensure stability, but
> we should only consider the needs to the upstream project itself when doing
> so.  Giving downstream projects time to react should be an explicit
> non-goal.
> >>
> >> Philip
> >>
> >> p.s. I don't expect this to be the actual decision reached, but since I
> only see opinions down-thread arguing for migration windows, I wanted to
> make a point of sharing the opposite opinion.  Fair warning, I probably
> won't reply to this thread further.  I don't have sufficient interest in
> the topic to make it worthwhile.
> >>
> >> On 8/24/21 10:44 AM, Arthur Eubanks via llvm-dev wrote:
> >>
> >> The new pass manager has been on by default since the 13 branch. Now
> that we've branched for 14, I'd like to start the process of deprecating
> and removing legacy pass manager support for the optimization pipeline.
> This includes clang, opt, and lld LTO support.
> >>
> >>
> >>
> >> Note that this doesn't apply to the codegen pipeline since there's no
> new pass manager support for that yet.
> >>
> >>
> >>
> >> Are there any objections?
> >>
> >>
> >>
> >> _______________________________________________
> >>
> >> LLVM Developers mailing list
> >>
> >> llvm-dev at lists.llvm.org
> >>
> >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >>
> >> _______________________________________________
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> >
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>
>
>
> --
> 宋方睿
>
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