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

Wed Jul 28 03:56:58 PDT 2021

Many thanks for commenting on the ThinLTO part and the suggestions how to approach this!
I also think that specialisation benefits the out of line copy.

Cheers,
Sjoerd.
________________________________
From: Teresa Johnson <tejohnson at google.com>
Sent: 27 July 2021 16:00
To: Sjoerd Meijer <Sjoerd.Meijer at arm.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>; Florian Hahn <florian_hahn at apple.com>; chuanqi.xcq <yedeng.yd at linux.alibaba.com>
Subject: Re: [llvm-dev] [ThinLTO] Import functions to enable function specialization in ThinLTO

I can answer questions from the ThinLTO standpoint. Missed the earlier emails on this, but I just skimmed the other llvm-dev email and the patch. I have a high level question and can give some thoughts on the approach from a ThinLTO standpoint.

One important thing to note is that we import functions with available_externally linkage, because they still live in their original translation unit. The intention is to make them available for inlining, and then they get dropped. But specialization presumably mainly benefits the out of line copy, I think? So I'm not sure that the approach of allowing the specialization candidates to get imported will ultimately do what you want. It probably would be better to allow the necessary info to be propagated via the index back to the original TU so that it can be specialized in its original location.

Agree with others that the solution should be prototyped so you can show some results in terms of not only the benefit, but also the cost. I.e. adding information about all of the function arguments will increase the size of the index - how significant is that increase? We want to ensure that the thin link portion of the ThinLTO build stays efficient as that is the only monolithic, serial portion of a ThinLTO build. That being said, I think it is a great idea to try to extend function specialization to ThinLTO if it is giving good benefits with regular LTO.

Teresa

On Tue, Jul 27, 2021 at 5:56 AM Sjoerd Meijer via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
The motivation for this work was getting this working ThinLTO, which I know very little about that, so that's why I recommended getting buy in from ThinLTO folks for the general direction.

This general direction is that we change the cost-model, import more functions as a result, so that we can do more transformations. The questions that need answering are: what is this going to cost in compile-times, and what is the performance benefit?

To have a more efficient discussion on this, I think it is best if you prototype your proposal. This makes things more concrete and also allows to get some first costs and benefits of the approach to test the whole idea. Without this, it all just remains a bit abstract.

Regarding https://reviews.llvm.org/D105524, yes it is NFC and harmless, but it is not important. I.e., it is trivial and I'm sure we can get it approved in no-time, but more important are the bigger questions that needs answers first. It's also better to avoid unnecessary churn if things turns out to be different.

My 2 cents.
Sjoerd.
________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces at lists.llvm.org>> on behalf of chuanqi.xcq via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Sent: 26 July 2021 07:30
To: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>; Florian Hahn <florian_hahn at apple.com<mailto:florian_hahn at apple.com>>
Subject: Re: [llvm-dev] [ThinLTO] Import functions to enable function specialization in ThinLTO

gentle ping~
------------------------------------------------------------------
From:llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Send Time:2021年7月15日(星期四) 17:23
To:llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>; Florian Hahn <florian_hahn at apple.com<mailto:florian_hahn at apple.com>>
Subject:Re: [llvm-dev] [ThinLTO] Import functions to enable function specialization in ThinLTO

Hi Florian,

Thanks for looking into this!

> I’d try to summarise the heuristics you propose to add, rather than focusing on the implementation details to start with.

Thanks. I agree with it. The direction are more important than details.

> 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.

If I understand right, you mean that I could collect these numbers after we summarised the heuristics we decide to use instead of collecting now.
Collecting the numbers may not make sense to me. Since I think we should do statistics after we decided the direction.

> Also, the scope/focus on function specialisation seems a bit narrow for substantially increasing the number of imported functions.

I think there are two apsects.
- It depends on how much we could get from function specialization.
- It depends the cost model to import more functions.

For the first aspects, I think function specialization is potential because ipa-cp plays an important role in gcc.
And the interprocedural value range propogation based on function specialization seems very potential to me.
That's the reason that I want to look into function specialization.

For the second aspects, I think we need more time to tuning it. Or in another words, we need to decide the heuristics first.
I know it's hard to **decide**  heuristics without measuring. But the work process  I imaged maybe:
```
Get a set of heuristics -> Tuning the cost model to decrease the overhead -> if succed, we got it -> else go back to the first step.
```

> Do you think the proposed heuristics would also benefit other optimisations, like regular IPSCCP?

Yes, I think regular IPSCCP could benefit from the proposed heuristics. Out of curiousty, I guess we could replace regular IPSCCP
once the function specialization pass is strong enough. What's your opinion?

Finally, let me clarify the intention of this mail. I don't ask to get a solution right now. I know it is hard for every one.
The intention is that if we could get a consensus that it is valuable that importing functions by heuristics for function specialization.
If yes, we could continue the review process for https://reviews.llvm.org/D105524, which extract the analysis part from function
specialization pass. I believe this patch should be NFC and harmless.

Of course, it could much much better that we could discuss the heuristics and the cost model.
Let me introduce the heuristics and the cost model I proposed simply.
The heuristics are:

  *   Specialize Function Cost. An unsigned number, which is an estimation for the cost to clone one function.
  *   Base Bonus for specializing specific argument. A map from ArgNo to the corresponding base bonus. Here the base bonus means the bonus we could calculate by visiting the function body itself only (in other words, doesn't need to see the call site).
  *   ArgUsage. A map from ArgNo to the extra bonus for each call site. The extra bonus means the bonus we could calculate for specific call site. For example, if we passed a function in the call, we could calculate the possibility that the function could get inlined at the specific call site only.

And the cost model are the same with function specialization pass right now and we could adjust it in the future.
The first one `Specialize Function Cost` is the number of instruction in a function multiplies a factor.
The second one `Base Bonus` is calculated by how many possible loads for this argument and its alias now.
I plan to add more situations for example:
```
int foo(bool cond) {
   if (cond) { // We could get more base bonus by the average instruction count in the branches.
   } else {
   }
}
```
Or
```
int foo(int cond) {
   if (cond > 10) { // We could get the condition if cond is constant, so we could get more bonus.
   } else {
   }
}
```
The third one `ArgUsage`, which describes the infomation in callsites and we could use it to calculate the extra bonus.
Now it only contains the instruction count if the corresponding parameter is a function. Then we could get extra bonus
from the potential inlining.

Thanks,
Chuanqi

------------------------------------------------------------------
From:Florian Hahn <florian_hahn at apple.com<mailto:florian_hahn at apple.com>>
Send Time:2021年7月14日(星期三) 19:39
To:chuanqi.xcq <yedeng.yd at linux.alibaba.com<mailto:yedeng.yd at linux.alibaba.com>>; llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject:Re: [llvm-dev] [ThinLTO] Import functions to enable function specialization in ThinLTO

On 14 Jul 2021, at 09:38, chuanqi.xcq via llvm-dev <llvm-dev at lists.llvm.org<mailto: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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--
Teresa Johnson |         Software Engineer |     tejohnson at google.com<mailto:tejohnson at google.com> |
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