[llvm-dev] How to use a custom InlineAdvisor with the new pass manager

[Mircea Trofin via llvm-dev]

On Thu, May 20, 2021 at 8:14 AM Neil Henning <neil.henning at unity3d.com>
wrote:

> I can't edit InlineAdvisorAnalysis::Result::tryCreate to add my own
> InlineAdvisor there is the issue.
>
Not sure I follow: ReplayInlineAdvisor is constructed
in InlinerPass::getAdvisor. That aside, on the comment about editing: I
think I'm still missing some aspects of your scenario: wouldn't your
advisor be part of llvm?

>
> I guess the easiest thing for our use-case might be to add another
> constructor to InlineAdvisorAnalysis that takes an InlineAdvisor, and this
> would set the Advisor field. That way I could just add to our
> AnalysisManager our own InlineAdvisorAnalysis, and we'd get the expected
> behaviour.
>
Oh - your scenario involves an advisor implemented outside the llvm tree,
is that the case? Can you share more details, e.g. how would it be loaded;
do you use the optimization pipelines and analysis managers in
PassBuilder.cpp, or you'd set up your own? By better understanding the
scenario, we can come up with a design that can probably help others, too.

Thanks!


>
> Seems like a small enough thing to do in a PR - is that an acceptable
> change you think?
>
> On Thu, May 20, 2021 at 3:16 PM Mircea Trofin <mtrofin at google.com> wrote:
>
>> Ah, I see. There's a precedent for using custom InlineAdvisors, see for
>> instance (in Inliner.cpp) how the ReplayInlineAdvisor is handled. I suppose
>> you could do the same?
>>
>> On Thu, May 20, 2021 at 12:04 AM Neil Henning <neil.henning at unity3d.com>
>> wrote:
>>
>>> So what I need is for the default LLVM inliner to be able to use my
>>> InlineAdvisor in some fashion - without modifying tip LLVM *locally *to
>>> do so.
>>>
>>> So what I think I will have to do is land one of the proposals I stated
>>> originally (or a better idea from any of you fine folk) into LLVM *before
>>> *the LLVM 13 cutoff, so that when we pick up the LLVM 13 release in
>>> future we'll have the APIs available to set our own InlinerAdvisor.
>>>
>>> So to be clear - I'm totally ok to do a patch to LLVM to fix this, I
>>> just can't patch LLVM myself *locally *post-release because we are
>>> provided with a pre-built LLVM for some platforms we support.
>>>
>>> Hopefully that makes it a bit clearer?
>>>
>>> On Wed, May 19, 2021 at 4:21 PM Mircea Trofin <mtrofin at google.com>
>>> wrote:
>>>
>>>>
>>>> On Wed, May 19, 2021 at 5:27 AM Neil Henning via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> Hey list,
>>>>>
>>>>> I'm currently porting our HPC# Burst compiler over from the legacy
>>>>> pass manager to the new pass manager. While nearly everything went fine,
>>>>> I've hit one major hiccup that I can't seem to workaround - how can we have
>>>>> a custom `InlineAdvisor` for Burst without modifying tip LLVM.
>>>>>
>>>>
>>>> I'm trying to understand this better - you mean you'd want to load the
>>>> InlineAdvisor from a dynamic library, or something like that?
>>>>
>>>>
>>>>>
>>>>> At present I've managed to completely bodge this locally by getting
>>>>> access to the `OwnedAdvisor` member of `InlinerPass` through very UB means
>>>>> (make a class of the same layout, casteroo, assign the field). Now this
>>>>> works in that I don't have codegen regressions anymore, but obviously this
>>>>> isn't the solution I want to ship!
>>>>>
>>>>> I was wondering if the list would object to us either:
>>>>>
>>>>>    1. Making the `OwnedAdvisor` field of `InlinerPass` protected, so
>>>>>    I could derive from `InlinerPass` and set the advisor.
>>>>>    2. I could make the `getAdvisor` virtual, and assign it that way.
>>>>>    3. Probably the 'best' fix would be to make
>>>>>    `InlineAdvisorAnalysis` somehow able to take a user-provided
>>>>>    `InlineAdvisor` - although I'd rather not use the static option
>>>>>    `UseInlineAdvisor` to set this. I don't really know how this solution would
>>>>>    look if I'm honest.
>>>>>
>>>>> I'm trying to understand what amount of changes to tip of tree are OK
>>>> for your scenario. Option 1 means modifying a .h; maybe option 2 needs a
>>>> recompile though (because virtual). So would option 3 (at this point, we
>>>> can talk about purpose-building support for your scenario, basically - if
>>>> rebuilding the compiler binaries is on the table)
>>>>
>>>> Thoughts from anyone? This is a blocker for us in the LLVM 13 timeframe
>>>>> when we hope to enable the new pass manager as the default.
>>>>>
>>>>> Cheers,
>>>>> -Neil.
>>>>>
>>>>> --
>>>>> Neil Henning
>>>>> Senior Software Engineer Compiler
>>>>> unity.com
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>
>>>
>>> --
>>> Neil Henning
>>> Senior Software Engineer Compiler
>>> unity.com
>>>
>>
>
> --
> Neil Henning
> Senior Software Engineer Compiler
> unity.com
>
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