llvm dev - Apr 2018 - exploring possibilities for unifying ThinLTO and FullLTO frontend + initial optimization pipeline

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<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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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.
>
>
>
>
>
> _______________________________________________
> 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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I think for ld64, you can mix thinLTO and fullLTO files and ld64 is going to
compile them separately and combine the result. (Mehdi can confirm). I think
this is aligned with the fact that whether to use full or thin LTO is decided
during clang invocation, not linker invocation. I am not against any of the
model, but I think we need to do some research before making the effort to
switch the model.

On the other hand, I think it should work if you feed thinLTO object file into
FullLTO code generator (if not, it is probably easy to implement). The issue
there is thin and full LTO uses different optimization pipeline. Probably need
to do some benchmark to figure out the impact of that.

Mehdi is correct that recompute summaries is expensive. You will get either
memory or disk IO overhead that might make the compile time even slower than
fullLTO. If you want to pick one format to use in both, it has to be the thinLTO
format with the summary. But now, you need to deal with what happen if there is
a legacy library with fullLTO info and user specify thinLTO on the linker
command line.

Steven


> On Apr 10, 2018, at 5:25 PM, Mehdi AMINI via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> 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.
> 
>  
> 
>  
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>_______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<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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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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