llvm dev - Jul 2021 - [ThinLTO] Import functions to enable function specialization in ThinLTO

Hi all,

Before I sent a mail to ask the opinion to how to import functions to enable
function specialization in ThinLTO.
Maybe the question is too empty or the function specialization pass didn't
get checked in at that time. So I didn't get responses.

Then now, the function specialization passes got checked in and I also made it a
version in the downstream.
So I tried to update my patches. The first patch is to extract the analysis part
of function specialization as an analysis pass.
(https://reviews.llvm.org/D105524).

And @sjoerd.meijer suggests that it may be better that I ping this thread with
more infomation to make us confident we are
in the right direction.

Then I updated this one: https://reviews.llvm.org/D105966, which contains the
detailed description and numbers.
Response in this thread or in that review page is fine to me either.

BTW, could any one suggest some people who are familiar with ThinLTO?

Thanks,
Chuanqi
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> On 14 Jul 2021, at 09:38, chuanqi.xcq via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi all,
> 
> Before I sent a mail to ask the opinion to how to import functions to
enable function specialization in ThinLTO.
> Maybe the question is too empty or the function specialization pass
didn't get checked in at that time. So I didn't get responses.
> 
> Then now, the function specialization passes got checked in and I also made
it a version in the downstream.
> So I tried to update my patches. The first patch is to extract the analysis
part of function specialization as an analysis pass.
> (https://reviews.llvm.org/D105524).
> 
> And @sjoerd.meijer suggests that it may be better that I ping this thread
with more infomation to make us confident we are
> in the right direction.
> 
> Then I updated this one: https://reviews.llvm.org/D105966, which contains
the detailed description and numbers.
> Response in this thread or in that review page is fine to me either.
> 

I’d try to summarise the heuristics you propose to add, rather than focusing on
the implementation details to start with. I think it might be helpful to
motivate a set of heuristics first. You should also be able to collect and
provide date on the effectiveness of those heuristics, like the number of
additional functions imported, compared to the number of additionally
specialised  functions.

Also, the scope/focus on function specialisation seems a bit narrow for
substantially increasing the number of imported functions. Do you think the
proposed heuristics would also benefit other optimisations, like regular IPSCCP?

Cheers,
Florian

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