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

[Fāng-ruì Sòng via llvm-dev]

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?
>>
>>
>>
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