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

Thu Aug 19 10:05:18 PDT 2021

Sorry for the slow reply, I've been out of office. Some replies inline
below. Teresa

On Wed, Aug 11, 2021 at 9:00 PM chuanqi.xcq <yedeng.yd at linux.alibaba.com>
wrote:

> Sorry for the late reply. Thanks everyone for looking into this!
>
> > To Sjoerd Meijer:
> >> 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?
>
> For the cost and beneift, I marked them in `
> https://reviews.llvm.org/D105966` <https://reviews.llvm.org/D105966>.
> Simply, for compile-time, there is no significant change except 502.gcc_r
> increased 30%. For code-size, the average increasement is 6.7%. And the
> maximum is 19%. For the performance benefit, the ThinLTO for function
> specialization version gains all the benefit from FullLTO version.
> Obiviously, the number are not so satisfying and this is the reason I
> marked that patch as WIP. The things I want to say is:
> - The increasement for compile-time and code-size mainly comes the newly
> imported functions been specialized. Instead of calculating and storing the
> extra heuristics.
> - The main purpose of this mail is to make sure we are on the right
> direction. In other words, ThinLTO for function specialization is pursued.
>
> > To Sjoerd Meijer:
> >> 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.
>
> Yeah, I thought I made the proposal in D105524, D105966 and D107136.
> Since the codes for function specialization changes relatively fast. It may
> not be applied directly. I would try to rebase it recently.
> Another way to measure it is to `git reset --hard
> d4840175c95f6edcba21baae411589468`.
>
> > To Sjoerd Meijer:
> >> 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
>
> The problem is that function specialization gets changed fast. So that
> this patch couldn't be approved directly. It may be an extra cost. If we
> decide the ThinLTO for function specialization is a long-term direction, we
> should check it in first to avoid extra cost.
>
> > To Teresa Johnson:
> >> 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.
>
> For imported functions with available_externally linkage, the strategy
> now is that if we decide to specialize it, we would copy the importanted
> function and change the linkage for cloned to local so that it wouldn't
> affect other translation unit. It looks fine to me.  Correct me if there is
> any problems. Since linking is really magic.
>

Ah ok, I missed that aspect of the approach. Yes, making them internal
copies should work, in that they will get kept and linking will work fine.
However, this could definitely exacerbate the code size increases from
specialization. Here is a possible case:

Assume a.cc:A() calls b.cc:B() (i.e. A and B are all in different TUs
initially). Let's say B could be specialized when called via A. And
further, assume A is small and hot, and is called from many many other TUs.
Normally, we would be likely to import A aggressively since it is likely to
be inlined (as it is small and hot). But if B is very large we may not want
to normally import and inline it (and as such function specialization is
very desirable). With this approach, B could be imported and specialized as
a local copy many many times (wherever A is imported and inlined). It is
possible that linker ICF could merge these copies, but that's not enabled
by default and I'm not sure if it would always be able to do that. This
could seem like an extreme example, but we have applications with 20K+ TUs
internally.

> > To Teresa Johnson:
> >> 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.
>
> It looks hard to do. I think the key point is that we make decision to
> specialize functions in the process of ThinLTO compiling, which is pararrel
> to other compilation unit. It looks like we need to do synchronization for
> ThinLTO, which sounds like to violate the design principles for ThinLTO.
>

Actually, ThinLTO does utilize cross-module synchronization. The
synchronization is implemented via the index provided to the ThinLTO
backends in which whole program optimization decisions are recorded.

I could envision providing via the index a way to record the arg number and
specialized value on the FunctionSummary for the function to be
specialized, along with a globally unique suffix to append (decided during
the thin link), for each specialization, and then on each caller
FunctionSummary, which callsite should call a specialized version
(indicated by the globally unique suffix to apply to the the called
function name). I actually need to do something similar for context
sensitive heap optimization (WIP), so presumably they could rely on a
similar summary mechanism to coordinate the cross module renaming.

This approach will also require some additional info in the index for the
callsite arguments that are constant (i.e. pass the constant instead of a
marker that it is a constant) so we can make these decisions during the
thin link.

It's definitely a bit more work, but probably something we'd want to do
longer term, rather than creating many specialized local copies, so I'd
just make sure that the work done for specialization via importing is
extensible to this approach.

> > To Teresa Johnson:
> > 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.
>
> It's a great point to measure the size increased for index. I forget to
> measure it before. Then here are the numbers for SPEC2017 int. The method I
> used to measure the size for index is to compile the sizefor the `.o`
> objects compiled by `-flto=thin`. Correct me if the measurement is not
> precise.
> The results shows that for SPEC2017 intrate, the overall increased size is
> about  2.8%. More precisely,
>
> | benchamrk       | size change for all the .o files |
> |-----------------|--------|
> | 500.perlbench_r | +1.9%  |
> | 502.gcc_r       | +3.8%  |
> | 505.mcf_r       | +3%    |
> | 520.omnetpp_r | +12% |
> | 525.x264_r | +2.9% |
> | 531.deepsjeng_r | +1.4% |
> | 541.leela_r | +2% |
>

The omnetpp increase is a little large, and it is more likely
representative of large C++ applications. Do you know how much of this is
due to the ArgUsage info vs the FuncSpecCostInfo? Not sure if there is a
good way to shrink these though (I've only skimmed your patch).

Thanks,
Teresa

>
> The size changes less than 1% are omitted.
>
> > To Teresa Johnson:
> >> 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.
>
> Thanks for your approval for the direction very much!
>
> Thanks,
> Chuanqi
>
>
> ------------------------------------------------------------------
> From:Sjoerd Meijer <Sjoerd.Meijer at arm.com>
> Send Time:2021年7月28日(星期三) 18:57
> To:Teresa Johnson <tejohnson at google.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
>
> 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> 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> on behalf of
> chuanqi.xcq via llvm-dev <llvm-dev at lists.llvm.org>
> *Sent:* 26 July 2021 07:30
> *To:* llvm-dev <llvm-dev at lists.llvm.org>; Florian Hahn <
> 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>
> Send Time:2021年7月15日(星期四) 17:23
> To:llvm-dev <llvm-dev at lists.llvm.org>; Florian Hahn <
> 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>
> Send Time:2021年7月14日(星期三) 19:39
> To:chuanqi.xcq <yedeng.yd at linux.alibaba.com>; llvm-dev <
> 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> 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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> llvm-dev at lists.llvm.org
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>
>
> --
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>
>
>

-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |
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