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

Mon Nov 15 13:00:38 PST 2021

`llc` should always run codegenprepare on IR before isel.

On Mon, Nov 15, 2021 at 11:49 AM Mingming Liu <mingmingl at google.com> wrote:

>
>
> On Mon, Nov 15, 2021 at 10:34 AM Arthur Eubanks <aeubanks at google.com>
> wrote:
>
>> `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.
>>
>
> These all make sense to me.
>
> (The following idea side-tracks from the original topic, but just
> brainstorming how to make the tools more friendly).
>
> If it (piping `opt` and `llc` misses `CodeGenPrepare` and causes
> surprises) becomes a common question, `llc` tool might be enhanced by
> emitting a warning/hint to CLI users that the IR probably needs
> `CodeGenPrepare` pass (if input IR has metadata to record which middle-end
> passes ran)
>
> This wouldn't change even if we used the NPM for the codegen pipeline.
>>
>
> I get the point that CodeGenPrepare could be supported in `opt` (w/ NPM)
> since `opt` does IR to IR transformations.
>
>>
>> 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
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
>
> --
> Thanks,
> Mingming
>
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