[llvm-dev] status of CodeGen in new Pass Manager

[Arthur Eubanks via llvm-dev]

`opt` is concerned about the optimization pipeline and `llc` is concerned
about the codegen pipeline. codegenprepare is part of the codegen pipeline,
not the optimization pipeline. We happen to be able to use `opt` to run
codegenprepare on its own because of how legacy PM passes are structured
and `llc` is not well suited to run individual IR passes. This wouldn't
change even if we used the NPM for the codegen pipeline.

On Fri, Nov 12, 2021 at 10:15 PM Mingming Liu via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Thank you so much Arthur and Yuanfang! These pointers are very educational.
>
> Now I realize there are two questions
> 1) Use NPM for machine passes; this is the desired state RFC
> <https://lists.llvm.org/pipermail/llvm-dev/2020-July/143309.html> and
> D85168 <https://reviews.llvm.org/D85168> tries to push forward.
> 2) Whether CodeGenPrepare should be enabled by default (e.g., user of opt
> CLI specifies an IR with sufficient target information, but doesn't enable
> CodeGenPrepare explicitly).
>
> From
> https://llvm.org/docs/NewPassManager.html#status-of-the-new-and-legacy-pass-managers,
> the preferred option is to not run CodeGenPrepare in the default settings
> (although users can still run it via specifying *-passes=codegenprepare*).
>
> I could make sense of the pointers, and understood the rationales better
> now.
>
> I'm curious if there were proposals to turn on CodeGenPrepare by default
> (if IR has sufficient target information). (didn't find one with this
> search query
> <https://www.google.com/search?q=llvm+rfc+turning+on+codegenpreare+opt&newwindow=1&sxsrf=AOaemvIqK3A44HhoAdT538LwKCQ_tbhq1g%3A1636783711790&ei=X1aPYcPSL8rU-gSnoq-IDg&oq=llvm+rfc+turning+on+codegenpreare+opt&gs_lcp=Cgdnd3Mtd2l6EAMyBwgAEEcQsAMyBwgAEEcQsAMyBwgAEEcQsAMyBwgAEEcQsAMyBwgAEEcQsAMyBwgAEEcQsAMyBwgAEEcQsAMyBwgAEEcQsANKBAhBGABQAFgAYNYCaAFwAngAgAEAiAEAkgEAmAEAyAEIwAEB&sclient=gws-wiz&ved=0ahUKEwiD_tu91pT0AhVKqp4KHSfRC-EQ4dUDCA4&uact=5>
> )
> The good thing is that, when someone (e.g., like me when ramping up on the
> llvm infra) pipes the *opt CLI* and *llc CLI *together, the machine
> assembly is closer to the machine assembly of Clang (in cpp to assembly
> mode).
>
> On Fri, Nov 12, 2021 at 2:17 PM <Yuanfang.Chen at sony.com> wrote:
>
>> Hi Mingming,
>>
>> About the status of using the new pass manager for the codegen pipeline,
>> the RFC was here (
>> http://lists.llvm.org/pipermail/llvm-dev/2020-July/143309.html) but
>> there was no Bugzilla ticket for it, sorry! I've just created one
>> https://bugs.llvm.org/show_bug.cgi?id=52493 with updates for anyone who
>> might be interested. I haven't been able to follow up on it for a while but
>> a few in-flight patches are still relevant and in good shape (check
>> PR52493). I'll see if I could push them forward in the near future.
>>
>> About codegen-prepare, I don't have much to add other than Arthur's
>> answer, except that D85168 would enable the use case, although it has some
>> dependencies so it's not like that it could be landed soon.
>>
>> HTH,
>> - Yuanfang
>>
>> ________________________________________
>> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Mingming
>> Liu via llvm-dev <llvm-dev at lists.llvm.org>
>> Sent: Friday, November 12, 2021 10:26 AM
>> To: llvm-dev at lists.llvm.org
>> Subject: [llvm-dev] status of CodeGen in new Pass Manager
>>
>> Hi,
>>    This is a newbie question around CodeGen related passes and the
>> current status in new Pass Manager.
>>
>>    From
>> https://llvm.org/docs/NewPassManager.html#status-of-the-new-and-legacy-pass-managers
>> <
>> https://urldefense.com/v3/__https://llvm.org/docs/NewPassManager.html*status-of-the-new-and-legacy-pass-managers__;Iw!!JmoZiZGBv3RvKRSx!tI8u93htbfzW8OQkAVIdBlQTDHabCnLJtB2D5fD_OjBuK1ACPDpumEw6GK_dphuBDA$>,
>> there are ongoing efforts to make the codegen pipeline work in the new Pass
>> Manager (which is great!). Searching in the bug list (
>> https://bugs.llvm.org/buglist.cgi?component=opt&list_id=226453&product=tools&query_format=advanced&resolution=---&short_desc=codegen&short_desc_type=allwordssubstr
>> <
>> https://urldefense.com/v3/__https://bugs.llvm.org/buglist.cgi?component=opt&list_id=226453&product=tools&query_format=advanced&resolution=---&short_desc=codegen&short_desc_type=allwordssubstr__;!!JmoZiZGBv3RvKRSx!tI8u93htbfzW8OQkAVIdBlQTDHabCnLJtB2D5fD_OjBuK1ACPDpumEw6GK-25d1S-w$>)
>> gives no result.
>>
>>    I'm wondering if anyone has more information on the current status of
>> CodeGen in the new Pass Manager (a tracking bug or other pointers)?
>>
>>    The context is that, I'm using opt CLI (by default new PM is used),
>> and surprised that codegenprepare pass doesn't run, so dig down and having
>> more questions :-)
>>
>>    Any related information will be appreciated!
>>
>> --
>> Thanks,
>> Mingming
>>
>
>
> --
> Thanks,
> Mingming
> _______________________________________________
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