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

Mon Nov 15 14:27:12 PST 2021

`llc -O3` does not run the optimization pipeline on the IR, so IR-level
optimizations aren't being run unless you use `opt -O3`.
`opt -O3` optimizes the IR.
`llc -O3` (mostly) enables MIR optimizations and better isel. But if the
input IR isn't optimized then you lose most optimization opportunities.
So a typical `clang -O3` would be somewhat equivalent to running Clang's
output IR through `opt -O3` then `llc -O3`.

On Mon, Nov 15, 2021 at 1:43 PM Mingming Liu <mingmingl at google.com> wrote:

> I used "llc -print-after-all -O3 <file.ll>" on this IR gives an assembly (
> https://godbolt.org/z/K6cszrPPf), and codegenprepare indeed runs in `llc`
> (from `print-after-all` output)
>
> The source of my confusion is:
>
>    1. Running the same IR by `opt -O3 -codegenprepare` gives a more
>    optimized IR (https://godbolt.org/z/fdqTGsqG4)
>    2. Piping the IR of step 1 (https://godbolt.org/z/544GMqaco) to `llc
>    -O3` gives a better assembly (tail call generated).
>
> I'm missing something here..
>
> On Mon, Nov 15, 2021 at 1:00 PM Arthur Eubanks <aeubanks at google.com>
> wrote:
>
>> `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
>>>
>>
>
> --
> Thanks,
> Mingming
>
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