[llvm-dev] exploring possibilities for unifying ThinLTO and FullLTO frontend + initial optimization pipeline

Wed Apr 11 06:55:18 PDT 2018

On Tue, Apr 10, 2018 at 11:52 PM, Mehdi AMINI <joker.eph at gmail.com> wrote:

>
>
> Le mar. 10 avr. 2018 à 23:18, <katya.romanova at sony.com> a écrit :
>
>> Hi Mehdi,
>>
>>
>>
>> Awesome! It’s a very clear design. The only question left is which
>> pipeline to choose for unified compile-phase optimization pipeline.
>>
>> -        ThinLTO compile-phase pipeline? It might very negatively affect
>> compile-time and the memory footprint for FullLTO link-phase. That was the
>> reason why so many optimization were moved from the link-phase to the
>> parallel compile-phase for FullLTO in the first place.
>>
>
> Just to clarify: "optimizations" were not "moved from the link-phase to
> the parallel compile-phase for FullLTO", they have never been in the link
> phase for FullLTO. It has always been this way.
>
> I think that the ThinLTO compile-phase pipeline will only affect FullLTO
> in the sense that we need to add more passes during the link phase, is this
> what you meant?
>
>
>> -        FullLTO compile-phase pipeline?  More optimization passes at
>> compile-phase will obviously increase compile time for ThinLTO, though I
>> suspect it will be tolerable. It is not very clear how this choice will
>> affect the overall runtime performance for ThinLTO. Assuming we keep
>> well-tuned link-phase/backend optimization pipeline “as is” for ThinLTO and
>> FullLTO, we will repeat some optimization passes for ThinLTO at
>> compile-phase and later at link-phase which potentially could improve the
>> performance… or it could make it worse, because we might perform an
>> optimization early at compile-time, potentially preventing more aggressive
>> optimization at link-phase when we see a larger scope. Any prediction on
>> what would happen to the ThinLTO runtime performance at run-time?
>>
>
> Note: repeating optimization is not supposed to improve performance, at
> least this isn't the goal of the pipeline.
> The pipeline for ThinLTO has been modeled on O3, good or bad we felt there
> was no reason to really deviate and any improvement to one could (should!)
> reflect on the other.
>
> The rational behind the ThinLTO pipeline is not only compile time: it
> split the O3 pipeline at the point where we stop the "function
> simplification" / inliner loop and before we get into
> unrolling/vectorization.
>

Right - see my reply on this from last night, at the very least the ThinLTO
importing thresholds will need retuning if we will perform optimizations
like unrolling/vectorization/etc that tend to increase code side.

> I remember even trying to stop the compile-phase without inlining but the
> generated IR was too big: the inliner CGSCC visit actually reduces the size
> of the IR considerably in some cases.
>

To add on, this affects not only the importing thresholds, but also the
cost of doing the thin link (which will have a graph with many more
nodes/edges).

>
>
>> -        New “unified” compile-phase pipeline?
>>
>>
>>
>> I guess, there is not a definitive answer and we have to experiment,
>> measure compile-time/run-time performance and potentially make some
>> adjustments to the pipeline and to the thresholds. We have a few
>> proprietary tests in Sony that we could use for the performance
>> measurements, but it will be nicer if there are some open source benchmarks
>> that we could use. What did you use in Google/Apple for ThinLTO/FullLTO
>> measurements? Have you used some proprietary benchmarks also? It’s
>> important to make sure we won’t have run-time/compile-time performance
>> degradation, but it will be nicer if anyone can run previously used
>> ThinLTO/FullLTO benchmarks oneself, while making changes to the
>> optimization pipeline and heuristics.
>>
>
> We benchmarked multiple variants of the pipeline two years ago. There were
> some regressions when adoption the ThinLTO pipeline in FullLTO (and some
> improvements), but when investigated we didn't find any real regressions
> that couldn't be solved by fixing the optimizer.
> I.e. these are cases where FullLTO gets it right "by luck" and not by
> principle, and fixing such cases helps the non-LTO O3 (for example this
> test case https://bugs.llvm.org/show_bug.cgi?id=27395 )
>

We have a number of internal benchmarks/applications used to evaluate
ThinLTO changes. We don't use Full LTO (with very limited exceptions).

>
>
>>
>>
>> >> # No flag: use the compile-phase preference, perform ThinLTO on a.o
>> and FullLTO on b.o/c.o, but allow ThinLTO import between the LTO group and
>> the ThinLTO >> objects
>>
>> >> $ clang a.o b.o c.o
>>
>>
>>
>> If I understood you correctly, while doing ThinLTO on a.o, we could
>> import from b.o and c.o (this is possible since the summaries are
>> available), while we won’t see a.o when doing FullLTO for b.o/c.o. (i.e.,
>> the previous non-permeable barrier between ThinLTO and FullLTO groups will
>> become permeable in one direction).
>>
>
> It could be permeable in both direction: b.o+c.o become "like a single
> ThinLTO object" after they get merged.
>

Yes in fact right now (at least in the new LTO API, but certainly similar
in the old one) the Full LTO partition gets fully
merged/optimized/codegened before running any ThinLTO. If we wanted a
mixed-mode LTO compilation, it would be better to do some of the Full LTO
in parallel with the ThinLTO. The Thin Link will need to be split out and
done before any Full LTO optimization/codegen. E.g., if we are importing
from FullLTO into ThinLTO, then some symbols will need to be promoted in
the Full LTO IR. And to import from ThinLTO into FullLTO, we will also need
to have the Thin Link results. Either the ThinLink + ThinLTO optimizations
like importing/promotion would need to be done before any Full LTO IR
merging + backend, or as Mehdi suggests, do the Full LTO IR merging, then
treat as a new ThinLTO IR object during the ThinLink and beyond. For
distributed builds, we would need to serialize out the Full LTO IR. I'm not
sure if it will be worth it from an optimization standpoint in that case.

Before we start modifying this significantly, BTW, it would be good to
revisit the idea of migrating the old LTO library to the new LTO API. Is
anyone thinking of investing in that? I know Mehdi started this a long time
back, but I'm sure doesn't have the bandwidth.

>
>
>> However, do you think by doing this, we will achieve a better performance
>> than doing ThinLTO backend for all of the files (a.o, b.o, c.o)?
>>
>
> Performance is always very much use-case dependent.
> One may know that a group of files performs better when they get merged
> together with FullLTO while the rest of the app does not?
>

That's what I am wondering. There are still some places where Full LTO
outperforms ThinLTO due to optimizations not ported to ThinLTO (e.g. some
global variable optimizations). But you'd need to figure out how to detect
these opportunities ahead of time.

Katya - did you have a particular use case in mind?

Thanks,
Teresa

>
> I don't know but this all needs to be carefully looked at from a
> user-interface point of view I think (will it be intuitive for the users?
> Will it fit in every (most) scenarios? etc.).
>
> Cheers,
>
> --
> Mehdi
>
>
>
>
>>
>>
>> Thank you!
>>
>> Katya.
>>
>>
>>
>>
>>
>> *From:* Mehdi AMINI <joker.eph at gmail.com>
>> *Sent:* Tuesday, April 10, 2018 5:25 PM
>> *To:* Romanova, Katya <katya.romanova at sony.com>
>> *Cc:* David Blaikie <dblaikie at gmail.com>; Teresa Johnson <
>> tejohnson at google.com>; llvm-dev <llvm-dev at lists.llvm.org>
>> *Subject:* Re: [llvm-dev] exploring possibilities for unifying ThinLTO
>> and FullLTO frontend + initial optimization pipeline
>>
>>
>>
>> Hi,
>>
>>
>>
>> It is non trivial to recompute summaries (which is why we have summaries
>> in the bitcode in the first place by the way), because bitcode is expensive
>> to load.
>>
>>
>>
>> I think shipping two different variant of the bitcode, one with and one
>> without summaries isn't providing much benefit while complicating the flow.
>> We could achieve what you're looking for by revisiting the flow a little.
>>
>>
>>
>> I would try to consider if we can:
>>
>>
>>
>> 1) always generate summaries.
>>
>> 2) Use the same compile-phase optimization pipeline for ThinLTO and LTO.
>>
>> 3) Decide at link time if you want to do FullLTO or ThinLTO.
>>
>>
>>
>> We haven't got this route 2 years ago because during the bringup we
>> didn't want to affect FullLTO in any way, but it may make sense now to have
>> `clang -flto=thin` and `clang -flto=full` be identical and change the
>> linker plugins to operate either in full-LTO mode or in ThinLTO mode but
>> not differentiate based on the availability of the summaries.
>>
>>
>>
>> A possible behavior could be:
>>
>>
>>
>> # The -flto flag in the compile phase does not change the produced
>> bitcode but for a flag that record the preference in the bitcode (FullLTO
>> vs ThinLTO)
>>
>> $ clang -c -flto=thin a.cpp
>>
>> $ clang -c -flto=full b.cpp
>>
>> $ clang -c -flto=full c.cpp
>>
>>
>>
>> # At link time the behavior depends on the -flto flag passed in.
>>
>>
>>
>> # No flag: use the compile-phase preference, perform ThinLTO on a.o and
>> FullLTO on b.o/c.o, but allow ThinLTO import between the LTO group and the
>> ThinLTO objects
>>
>> $ clang a.o b.o c.o
>>
>>
>>
>> # Forces full LTO, merges all the objects, no cross module importing will
>> happen.
>>
>> clang a.o b.o c.o -flto=full
>>
>>
>>
>> # Forces ThinLTO for all objects, FullLTO won't happen, no objects will
>> be merged.
>>
>> clang a.o b.o c.o -flto=thin
>>
>>
>>
>> Cheers,
>>
>>
>>
>> --
>>
>> Mehdi
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> Le mar. 10 avr. 2018 à 15:51, via llvm-dev <llvm-dev at lists.llvm.org> a
>> écrit :
>>
>> Hi David,
>>
>> Thank you so much for your reply!
>>
>>
>>
>> >> You're dealing with a situation where you are shipped BC files offline
>> and then do one, or multiple builds with these BC files?
>> Yes, that’s exactly the case.
>>
>>
>>
>> >> If the scenario was more like a naive build: Multiple BC files
>> generated on a single (multi-core/threaded) machine (but some Thin, some
>>
>> >> Full) & then fed to the linker, I would wonder if it'd be relatively
>> cheap for the LTO step to support this by computing summaries for
>>
>> >> FullLTO files on the fly (without a separate tool/writing the summary
>> to disk, etc).
>>
>>
>>
>> I think so. My understanding that for FullLTO files, it’s possible to
>> perform name anonymous globals pass and compute summaries on the fly, which
>> should allow to perform ThinLTO at link phase.
>>
>>
>>
>> Katya.
>>
>>
>>
>> *From:* David Blaikie <dblaikie at gmail.com>
>> *Sent:* Tuesday, April 10, 2018 7:38 AM
>> *To:* Romanova, Katya <katya.romanova at sony.com>; Teresa Johnson <
>> tejohnson at google.com>
>> *Cc:* llvm-dev at lists.llvm.org
>> *Subject:* Re: [llvm-dev] exploring possibilities for unifying ThinLTO
>> and FullLTO frontend + initial optimization pipeline
>>
>>
>>
>> Hi Katya,
>>
>> [+Teresa since this is about ThinLTO & she's the owner there]
>>
>> I'm not sure how other folks feel, but terminologically I'm not sure I
>> think of these as different formats (for example you mention the idea of
>> stripping the summaries from ThinLTO BC files to then feed them in as
>> FullLTO files - I would imagine it'd be reasonable to modify/fix/improve
>> the linker integration to have it (perhaps optionally) /ignore/ the
>> summaries, or use the summaries but in a non-siloed way (so that there's
>> not that optimization boundary between ThinLTO and FullLTO))
>>
>> You're dealing with a situation where you are shipped BC files offline
>> and then do one, or multiple builds with these BC files?
>>
>> If the scenario was more like a naive build: Multiple BC files generated
>> on a single (multi-core/threaded) machine (but some Thin, some Full) & then
>> fed to the linker, I would wonder if it'd be relatively cheap for the LTO
>> step to support this by computing summaries for FullLTO files on the fly
>> (without a separate tool/writing the summary to disk, etc). Though I
>> suppose that'd produce a pretty wildly different behavior in the link when
>> just a single ThinLTO BC file was added to an otherwise FullLTO build.
>>
>> Anyway - just some (admittedly fairly uninformed) thoughts. I'm sure
>> Teresa has more informed ideas about how this might all look.
>>
>> On Mon, Apr 9, 2018 at 12:20 PM via llvm-dev <llvm-dev at lists.llvm.org>
>> wrote:
>>
>> Hello,
>>
>> I am exploring the possibility of unifying the BC file generation phase
>> for ThinLTO and FullLTO. Our third party library providers prefer to give
>> us only one version of the BC archives, rather than test and ship both Thin
>> and Full LTO BC archives. We want to find a way to allow our users to pick
>> either Thin or Full LTO, while having only one “unified” version of the BC
>> archive.
>>
>> Note, I am not necessarily proposing to do this work in the upstream
>> compiler. If there is no interest from other companies, we might have to
>> keep this as a private patch for Sony.
>>
>> One of the ideas (not my preference) is to mix and match files in the
>> Thin and Full BC formats.  I'm not sure how well the "mix and match"
>> scenario works in general. I was wondering if Apple or Google are doing
>> this for production?
>>
>> I wrote a toy example, compiled one group of files with ThinLTO and the
>> rest with FullLTO, linked them with gold. I saw that irrespective of
>> whether the Thin or Full LTO option was used at the link step, files are
>> optimized within the Thin group and within the Full group separately, but
>> they don't know about the files in the other group (which makes sense).
>> Basically, the border between Thin and Full LTO bitcode files created an
>> artificial "barrier" which prevented cross-border optimization.
>>
>> Obviously, I am not too fond of this idea. Even if mixing and matching
>> ThinLTO and FullLTO bitcode files will work “as is”, I suspect we will see
>> a non-trivial runtime performance degradation because of the
>> "ThinLTO"/"FullLTO" border. Are you aware of any potential problems with
>> this solution, other than performance?
>>
>>
>>
>> Another, hopefully, better idea is to introduce a "unified" BC format,
>> which could either be FullLTO, ThinLTO, or neither (e.g., something in
>> between).
>>
>> If the user chooses FullLTO at the link step, but some of the files are
>> in the Thin BC format – the linker will call a special LTO API to convert
>> these files to the Full LTO BC format (i.e., stripping the module summary
>> section + potentially do some additional optimizations from the FullLTO
>> pass manager pipeline).
>>
>> If the user chooses ThinLTO at the link step, but some of the files are
>> in the Full BC format – the linker will call an LTO API to convert these
>> files to the Thin LTO bitcode format (by regenerating the module summary
>> section dynamically for the Full LTO bitcode files).
>>
>> I think the most reasonable idea for the unification of the Thin and Full
>> LTO compilation pipelines is to use Full LTO as the “unified” BC format. If
>> the user requests FullLTO – no additional work is needed, the linker will
>> perform FullLTO as usual. If the user request ThinLTO, the linker will call
>> an API to regenerate the module summary section for all the files in the
>> FullLTO format and perform ThinLTO as usual.
>>
>> In reality I suspect things will be much more complicated. The pipelines
>> for the Thin and Full LTO compilation phases are quite different. ThinLTO
>> can afford to do much more optimization in the linking phase (since it has
>> parallel backends & smaller IR compared to FullLTO), while for FullLTO we
>> are forced to move some optimizations from linking to the compilation phase.
>>
>> So, if we pick FullLTO as our unified format, we would increase the build
>> time for ThinLTO (we will be doing the FullLTO initial optimization
>> pipeline in the compile phase, which is more than what ThinLTO is currently
>> doing, but the pipeline of the optimizations in the backend will stay the
>> same). It’s not clear what will happen with the runtime performance: we
>> might improve it (because we repeat some of the optimizations several
>> times), or we might make it worse (because we might do an optimization in
>> the early compilation phase, potentially preventing more aggressive
>> optimization later). What are your expectations? Will this approach work in
>> general? If so, what do you think will happen with the runtime performance?
>>
>> I also noticed that the pass manager pipeline is different for
>> ThinLTO+Sample PGO (use profile case). This might create some additional
>> complications for unification of Thin and FullLTO BC generation phase too,
>> but it’s too small detail to worry about right now. I’m more interested in
>> choosing a right general direction for solving this problem now.
>>
>> Please share your thoughts!
>>
>> Thank you!
>>
>> Katya.
>>
>>
>>
>>
>>
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>>

-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |  408-460-2413
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