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

Thu May 20 09:08:36 PDT 2021

So for us we don't use the passes from PassBuilder - we've got our own cut
down set to improve compile time by 2.5x over the default pipeline, while
still maintaining vectorization and all the other fun things (tangentially
- I was going to talk about this at EuroLLVM then COVID killed that!).

At the moment what I've hacked in locally is:

struct InlinerPass final : llvm::PassInfoMixin<InlinerPass>
{
    explicit InlinerPass(llvm::InlineParams&& params) : params(params),
pass()
    {
    }

    llvm::PreservedAnalyses run(llvm::LazyCallGraph::SCC&,
llvm::CGSCCAnalysisManager&, llvm::LazyCallGraph&,
llvm::CGSCCUpdateResult&);

private:
    const llvm::InlineParams params;
    llvm::InlinerPass pass;
};

llvm::PreservedAnalyses InlinerPass::run(llvm::LazyCallGraph::SCC& scc,
llvm::CGSCCAnalysisManager& analysisManager, llvm::LazyCallGraph&
callGraph, llvm::CGSCCUpdateResult& updater)
{
    llvm::Module& module = *scc.begin()->getFunction().getParent();
    llvm::FunctionAnalysisManager& functionAnalysisManager =
analysisManager.getResult<llvm::FunctionAnalysisManagerCGSCCProxy>(scc,
callGraph).getManager();

    // Do something really awful - `OwnedAdvisor` is private in the
`llvm::InlinerPass`, but we need to set it to our own advisor. Cast the
class to a struct
    // that happens to have the same layout, and set the field that way.
This is totally undefined behaviour and bad, but LLVM hasn't given us the
tools with
    // the new pass manager to do this properly :'(.

reinterpret_cast<LLVMInlinerPassPrivateMemberGetterHackeroo*>(&pass)->OwnedAdvisor.reset(new
BurstInlineAdvisor(module, functionAnalysisManager, params));

    return pass.run(scc, analysisManager, callGraph, updater);
}

So we can use the default llvm::InlinerPass but with our own custom
InlineAdvisor. I *know* this is a hack though, and I think the correct
solution (but please correct me if I'm wrong!) would be to add a version of
InlineAdvisorAnalysis that takes our own InlineAdvisor?

With the old pass manager the Inliner just had a virtual getInlineCost
method that we could extend and override, so we didn't have this problem
there.

On Thu, May 20, 2021 at 4:45 PM Mircea Trofin <mtrofin at google.com> wrote:

>
>
> 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
>>
>

-- 
Neil Henning
Senior Software Engineer Compiler
unity.com
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