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

[via llvm-dev]

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.

-        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?

-        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.

>> # 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). 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)?

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<mailto: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<mailto:dblaikie at gmail.com>>
Sent: Tuesday, April 10, 2018 7:38 AM
To: Romanova, Katya <katya.romanova at sony.com<mailto:katya.romanova at sony.com>>; Teresa Johnson <tejohnson at google.com<mailto:tejohnson at google.com>>
Cc: llvm-dev at lists.llvm.org<mailto: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<mailto: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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