[llvm-dev] [RFC] Add IR level interprocedural outliner for code size.

River Riddle via llvm-dev llvm-dev at lists.llvm.org
Tue Aug 1 10:36:54 PDT 2017 

Hey Jessica,
  I was interested in the same relationship you are. Last summer i
implemented many variations of the algorithm that is outlined in the paper
you've provided, even including hot switch splitting.

When I implemented this last year there were several road blocks: Inliner
not always playing nicely, the inliner didn't preserve pgo data at that
point, aa was not very forgiving about the loss of information, etc. Now
that some of these problems have been improved/fixed it would be
interesting to review what can be gained from this type of transformation.

   It's also important to note that the two outlining transformations would
use completely different algorithms, as they are satisfying different
benefit functions.
 The algorithm used for enabling partial inlining is pgo driven, making it
very context, or occurrence, sensitive. You are looking at subsections of
the graph to identify cold sections that could increase performance if
outlined. There isn't a concept for more than once occurrence because the
benefit function is very sensitive to the pgo data of that one instance of
the subgraph. Given that you are only considering one instance you don't
have to worry about trying to solve graph isomorphism, which substantially
increases the amount of candidates you can consider. This type of outlining
also interacts much better with the inliner because all of the cost
modeling for both is contained within the context of a single call.

So effectively we have two different types of outliners, which I'm assuming
is part of the reason why Dan B brought up the semantics of what to call
these transformations in the original MachineOutliner RFC. Having
implemented both, I can attest to there not really being a lot of shared
code. Providing pgo data to the IR outliner will somewhat emulate this
behavior, but the amount of benefit to performance is not likely to be as
high. I'm sensing that this will hold even if/when we extend IR outlining
to handle regions as well.

We kind of have an outliner of this type in the form of the partial inliner
in LLVM. It currently only cares about enabling more inlining but that
could definitely be changed or extended depending on the performance
benefits that we could get.

Thanks,
  River Riddle

On Tue, Aug 1, 2017 at 9:15 AM, Jessica Paquette via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I think I agree with this sentiment (and the rather heated response to
> this suggestion says that I ought to as well!)
>
> The later pass would have to have a strong guarantee to “undo” bad
> decisions, which, given that we have to use heuristics to make code size
> decisions, is very difficult. I don’t know if I’d say *impossible*, but it
> would definitely force us to work in a more restrictive space. I was mostly
> just thinking of how the two separate technologies might work together in
> practice.
>
> For example, in “Function outlining and partial inlining” by Peng Zhao and
> J. N. Amaral, they found that you could get partial inlining for free by
> enabling inlining and outlining at the IR-level. What I think would be cool
> to explore is whether or not there would be a similar relationship between
> IR-level outlining and MIR-level outlining. Of course, this is just a
> suggestion!
>
> (Paper for anyone interested: https://www.researchgate.net/
> profile/Jose_Amaral3/publication/221306447_Function_Outlining_and_
> Partial_Inlining/links/00463522754960618b000000.pdf)
>
> - Jessica
>
>
> On Aug 1, 2017, at 1:17 AM, Andrey Bokhanko via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> ...and adding $0.02 to the "IR outline + inline + MIR outline" idea, my
> gut feeling (yes, only a "feeling" -- and one coming from my gut, not
> head!) is that inlining correcting wrong IR outlining decisions with MIR
> outlining correcting wrong inlining decisions is absolutely unrealistic and
> a heuristics nightmare at best.
>
> Inliner's heuristics are already complex enough and not 100% bulletproof;
> if we add IR outliner heuristics to the mix -- and then just a little bit
> of MIR outliner heuristics (which are more precise but, as demonstrated
> above, not 100% precise as well) on top... you can imagine.
>
> Yours,
> Andrey
>
>
> On Tue, Aug 1, 2017 at 10:07 AM, Andrey Bokhanko <andreybokhanko at gmail.com
> > wrote:
>
>> All,
>>
>> +1 to what Mehdi said.
>>
>> It's a fair concern to question whatever we need yet another Outlining
>> pass. I believe this concern has been cleared by River -- both with
>> theoretical arguments and practical data (benchmark numbers).
>>
>> Jessica's pipeline proposal is completely orthogonal. It's not fair to
>> request River to implement / fit into what she suggested. Sure, it's a
>> valid topic to discuss -- but yet completely orthogonal one. If anything,
>> accepting River's implementation would enable us to do experiments /
>> developments like pipeline changes of this ilk!
>>
>> Yours,
>> Andrey
>> ===
>> Compiler Architect
>> NXP
>>
>>
>> On Tue, Aug 1, 2017 at 7:38 AM, Mehdi AMINI via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>>
>>>
>>> 2017-07-28 21:58 GMT-07:00 Chris Bieneman via llvm-dev <
>>> llvm-dev at lists.llvm.org>:
>>>
>>>> Apologies for delayed joining of this discussion, but I had a few notes
>>>> from this thread that I really wanted to chime in about.
>>>>
>>>> River,
>>>>
>>>> I don't mean to put you on the spot, but I do want to start on a
>>>> semantic issue. In several places in the thread you used the words "we" and
>>>> "our" to imply that you're not alone in writing this (which is totally
>>>> fine), but your initial thread presented this as entirely your own work.
>>>> So, when you said things like "we feel there's an advantage to being at the
>>>> IR level", can you please clarify who is "we"?
>>>>
>>>> Given that there are a number of disagreements and opinions floating
>>>> around I think it benefits us all to speak clearly about who is taking what
>>>> stances.
>>>>
>>>> One particular disagreement that I think very much needs to be
>>>> revisited in this thread was Jessica's proposal of a pipeline of:
>>>>
>>>>    1. IR outline
>>>>    2. Inline
>>>>    3. MIR outline
>>>>
>>>> In your response to that proposal you dismissed it out of hand with
>>>> "feelings" but not data. Given that the proposal came from Jessica (a
>>>> community member with significant relevant experience in outlining), and it
>>>> was also recognized as interesting by Eric Christopher (a long-time member
>>>> of the community with wide reaching expertise), I think dismissing it may
>>>> have been a little premature.
>>>>
>>>
>>> It isn't clear to me how much the *exact* pipeline and ordering of
>>> passes is relevant to consider if "having an outliner at the IR level" is a
>>> good idea.
>>>
>>>
>>>
>>>> I also want to visit a few procedural notes.
>>>>
>>>> Mehdi commented on the thread that it wouldn't be fair to ask for a
>>>> comparative study because the MIR outliner didn't have one. While I don't
>>>> think anyone is asking for a comparative study, I want to point out that I
>>>> think it is completely fair.
>>>>
>>> If a new contributor approached the community with a new SROA pass and
>>>> wanted to land it in-tree it would be appropriate to ask for a comparative
>>>> analysis against the existing pass. How is this different?
>>>>
>>>
>>> It seems quite different to me because there is no outliner at the IR
>>> level and so they don't provide the same functionality. The "Why at the IR
>>> level" section of the original email combined with the performance numbers
>>> seems largely enough to me to explain why it isn't redundant to the
>>> Machine-level outliner.
>>> I'd consider this work for inclusion upstream purely on its technical
>>> merit at this point.
>>> Discussing inclusion as part of any of the default pipeline is a
>>> different story.
>>>
>>> Similarly last year, the IR-level PGO was also implemented even though
>>> we already had a PGO implementation, because 1) it provided a generic
>>> solutions for other frontend (just like here it could be said that it
>>> provides a generic solution for targets) and 2) it supported cases that
>>> FE-PGO didn't (especially around better counter-context using pre-inlining
>>> and such).
>>>
>>>
>>>
>>>>
>>>> Adding a new IR outliner is a different situation from when the MIR one
>>>> was added. When the MIR outliner was introduced there was no in-tree
>>>> analog.
>>>>
>>>
>>> We still usually discuss design extensively. Skipping the IR-level
>>> option didn't seem obvious to me, to say the least. And it wasn't really
>>> much discussed/considered extensively upstream.
>>> If the idea is that implementing a concept at the machine level may
>>> preclude a future implementation at the IR level, it means we should be *a
>>> lot* more picky before accepting such contribution.
>>> In this case, if I had anticipated any push-back on an IR-level
>>> implementation only based on the fact that we have now a Machine-level one,
>>> I'd likely have pushed back on the machine-level one.
>>>
>>>
>>>
>>>> When someone comes to the community with something that has no existing
>>>> in-tree analog it isn't fair to necessarily ask them to implement it
>>>> multiple different ways to prove their solution is the best.
>>>>
>>>
>>> It may or may not be fair, but there is a tradeoff in how much effort we
>>> would require them to convince the community that this is *the* right way
>>> to go, depending on what it implies for future approaches.
>>>
>>> --
>>> Mehdi
>>>
>>>
>>>> However, as a community, we do still exercise the right to reject
>>>> contributions we disagree with, and we frequently request changes to the
>>>> implementation (as is shown every time someone tries to add SPIR-V support).
>>>>
>>>> In the LLVM community we have a long history of approaching large
>>>> contributions (especially ones from new contributors) with scrutiny and
>>>> discussion. It would be a disservice to the project to forget that.
>>>>
>>>> River, as a last note. I see that you've started uploading patches to
>>>> Phabricator, and I know you're relatively new to the community. When
>>>> uploading patches it helps to include appropriate reviewers so that the
>>>> right people see the patches as they come in. To that end can you please
>>>> include Jessica as a reviewer? Given her relevant domain experience I think
>>>> her feedback on the patches will be very valuable.
>>>>
>>>> Thank you,
>>>> -Chris
>>>>
>>>> On Jul 26, 2017, at 1:52 PM, River Riddle via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>> Hey Sanjoy,
>>>>
>>>> On Wed, Jul 26, 2017 at 1:41 PM, Sanjoy Das via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> On Wed, Jul 26, 2017 at 12:54 PM, Sean Silva <chisophugis at gmail.com>
>>>>> wrote:
>>>>> > The way I interpret Quentin's statement is something like:
>>>>> >
>>>>> > - Inlining turns an interprocedural problem into an intraprocedural
>>>>> problem
>>>>> > - Outlining turns an intraprocedural problem into an interprocedural
>>>>> problem
>>>>> >
>>>>> > Insofar as our intraprocedural analyses and transformations are
>>>>> strictly
>>>>> > more powerful than interprocedural, then there is a precise sense in
>>>>> which
>>>>> > inlining exposes optimization opportunities while outlining does not.
>>>>>
>>>>> While I think our intra-proc optimizations are *generally* more
>>>>> powerful, I don't think they are *always* more powerful.  For
>>>>> instance, LICM (today) won't hoist full regions but it can hoist
>>>>> single function calls.  If we can extract out a region into a
>>>>> readnone+nounwind function call then LICM will hoist it to the
>>>>> preheader if the safety checks pass.
>>>>>
>>>>> > Actually, for his internship last summer River wrote a profile-guided
>>>>> > outliner / partial inliner (it didn't try to do deduplication; so it
>>>>> was
>>>>> > more like PartialInliner.cpp). IIRC he found that LLVM's
>>>>> interprocedural
>>>>> > analyses were so bad that there were pretty adverse effects from
>>>>> many of the
>>>>> > outlining decisions. E.g. if you outline from the left side of a
>>>>> diamond,
>>>>> > that side basically becomes a black box to most LLVM analyses and
>>>>> forces
>>>>> > downstream dataflow meet points to give an overly conservative
>>>>> result, even
>>>>> > though our standard intraprocedural analyses would have happily dug
>>>>> through
>>>>> > the left side of the diamond if the code had not been outlined.
>>>>> >
>>>>> > Also, River's patch (the one in this thread) does parameterized
>>>>> outlining.
>>>>> > For example, two sequences containing stores can be outlined even if
>>>>> the
>>>>> > corresponding stores have different pointers. The pointer to be
>>>>> loaded from
>>>>> > is passed as a parameter to the outlined function. In that sense, the
>>>>> > outlined function's behavior becomes a conservative approximation of
>>>>> both
>>>>> > which in principle loses precision.
>>>>>
>>>>> Can we outline only once we've already done all of these optimizations
>>>>> that outlining would block?
>>>>>
>>>>
>>>>   The outliner is able to run at any point in the interprocedural
>>>> pipeline. There are currently two locations: Early outlining(pre inliner)
>>>> and late outlining(practically the last pass to run). It is configured to
>>>> run either Early+Late, or just Late.
>>>>
>>>>
>>>>> > I like your EarlyCSE example and it is interesting that combined with
>>>>> > functionattrs it can make a "cheap" pass get a transformation that an
>>>>> > "expensive" pass would otherwise be needed. Are there any cases
>>>>> where we
>>>>> > only have the "cheap" pass and thus the outlining would be essential
>>>>> for our
>>>>> > optimization pipeline to get the optimization right?
>>>>> >
>>>>> > The case that comes to mind for me is cases where we have some
>>>>> cutoff of
>>>>> > search depth. Reducing a sequence to a single call (+ functionattr
>>>>> > inference) can essentially summarize the sequence and effectively
>>>>> increase
>>>>> > search depth, which might give more results. That seems like a bit
>>>>> of a weak
>>>>> > example though.
>>>>>
>>>>> I don't know if River's patch outlines entire control flow regions at
>>>>> a time, but if it does then we could use cheap basic block scanning
>>>>> analyses for things that would normally require CFG-level analysis.
>>>>>
>>>>
>>>>   The current patch currently just supports outlining from within a
>>>> single block. Although, I had a working prototype for Region based
>>>> outlining, I kept it from this patch for simplicity. So its entirely
>>>> possible to add that kind of functionality because I've already tried.
>>>> Thanks,
>>>>   River Riddle
>>>>
>>>>
>>>>>
>>>>> -- Sanjoy
>>>>>
>>>>> >
>>>>> > -- Sean Silva
>>>>> >
>>>>> > On Wed, Jul 26, 2017 at 12:07 PM, Sanjoy Das via llvm-dev
>>>>> > <llvm-dev at lists.llvm.org> wrote:
>>>>> >>
>>>>> >> Hi,
>>>>> >>
>>>>> >> On Wed, Jul 26, 2017 at 10:10 AM, Quentin Colombet via llvm-dev
>>>>> >> <llvm-dev at lists.llvm.org> wrote:
>>>>> >> > No, I mean in terms of enabling other optimizations in the
>>>>> pipeline like
>>>>> >> > vectorizer. Outliner does not expose any of that.
>>>>> >>
>>>>> >> I have not made a lot of effort to understand the full discussion
>>>>> here (so
>>>>> >> what
>>>>> >> I say below may be off-base), but I think there are some cases where
>>>>> >> outlining
>>>>> >> (especially working with function-attrs) can make optimization
>>>>> easier.
>>>>> >>
>>>>> >> It can help transforms that duplicate code (like loop unrolling and
>>>>> >> inlining) be
>>>>> >> more profitable -- I'm thinking of cases where unrolling/inlining
>>>>> would
>>>>> >> have to
>>>>> >> duplicate a lot of code, but after outlining would require
>>>>> duplicating
>>>>> >> only a
>>>>> >> few call instructions.
>>>>> >>
>>>>> >>
>>>>> >> It can help EarlyCSE do things that require GVN today:
>>>>> >>
>>>>> >> void foo() {
>>>>> >>   ... complex computation that computes func()
>>>>> >>   ... complex computation that computes func()
>>>>> >> }
>>>>> >>
>>>>> >> outlining=>
>>>>> >>
>>>>> >> int func() { ... }
>>>>> >>
>>>>> >> void foo() {
>>>>> >>   int x = func();
>>>>> >>   int y = func();
>>>>> >> }
>>>>> >>
>>>>> >> functionattrs=>
>>>>> >>
>>>>> >> int func() readonly { ... }
>>>>> >>
>>>>> >> void foo(int a, int b) {
>>>>> >>   int x = func();
>>>>> >>   int y = func();
>>>>> >> }
>>>>> >>
>>>>> >> earlycse=>
>>>>> >>
>>>>> >> int func(int t) readnone { ... }
>>>>> >>
>>>>> >> void foo(int a, int b) {
>>>>> >>   int x = func(a);
>>>>> >>   int y = x;
>>>>> >> }
>>>>> >>
>>>>> >> GVN will catch this, but EarlyCSE is (at least supposed to be!)
>>>>> cheaper.
>>>>> >>
>>>>> >>
>>>>> >> Once we have an analysis that can prove that certain functions
>>>>> can't trap,
>>>>> >> outlining can allow LICM etc. to speculate entire outlined regions
>>>>> out of
>>>>> >> loops.
>>>>> >>
>>>>> >>
>>>>> >> Generally, I think outlining exposes information that certain
>>>>> regions of
>>>>> >> the
>>>>> >> program are doing identical things.  We should expect to get some
>>>>> mileage
>>>>> >> out of
>>>>> >> this information.
>>>>> >>
>>>>> >> -- Sanjoy
>>>>> >> _______________________________________________
>>>>> >> LLVM Developers mailing list
>>>>> >> llvm-dev at lists.llvm.org
>>>>> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>> >
>>>>> >
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>
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>>>>
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>>>
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>>
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