llvm dev - Aug 2017 - [RFC] Add IR level interprocedural outliner for code size.

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
>>
>
> _______________________________________________
> 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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...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
>>>>
>>>
>>> _______________________________________________
>>> 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
>>>
>>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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>
>
>
> Also as a side note, I think in the original MachineOutliner RFC thread
> there was some confusion as to whether it was possible to solve the code
> folding outlining problem exactly as a graph problem on SSA using standard
> value numbering algorithms in polynomial time.
>
> I can elaborate further, but
> 1. it is easy to see that you can map an arbitrary dag to an isomorphic
> data flow graph in an SSA IR e.g. in LLVM IR or pre-RA MIR
>
> 2. Given two dags, you can create their respective isomorphic data flow
> graphs (say, put them into two separate functions)
> 3. An exact graph based code folding outliner would be able to discover if
> the two dataflow graphs are isomorphic (that is basically what I mean by
> exact) and outline them.
> 4. Thus, graph isomorphism on dags can be solved with such an algorithm
> and thus the outlining problem is GI-hard and a polynomial time solution
> would be a big breakthrough in CS.
>
First, you'd have to reduce them in both directions to prove that ;)

All that's been shown is that you can reduce it to a hard problem.
You can also reduce it to 3-sat., but that doesn't mean anything unless you
can reduce 3-sat to it.

5. The actual problem the outliner is trying to solve is actually more
like
> finding subgraphs that are isomorphic, making the problem even harder
> (something like "given dags A and B does there exist a subgraph of A
that
> is isomorphic to a subgraph of B")
>
>
This assumes, strongly, that this reduction is the way to do it, and also
that SSA/etc imparts no structure in the reduction that enables you to
solve it faster (IE restricts the types of graphs, etc)

FWIW, the exact argument above holds for value numbering, and since the
days of kildall, it was believed not solvable in a complete fashion in less
than exponential time due to the need to do graph isomorphism on large
value graphs at join points.

Except, much like RA and other things, it turns out this is not right for
SSA, and in the past few years, it was proved you could do complete  value
numbering in polynomial time on SSA.

So with all due respect to quentin, i don't buy it yet.

Without more, i'd bet using similar techniques to solve value numbering in
polynomial time to SSA could be applied here.

This is because the complete polynomial value numbering techniques are in
fact, value graph isomorphism ....

So i'd assume the opposite (it can be done in polynomial time) without more.
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> On Aug 1, 2017, at 1:07 AM, Andrey Bokhanko via llvm-dev <llvm-dev at
lists.llvm.org> 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).
I want to point out that River has not provided full raw benchmark data, he has
provided summarized data. Also, as a community of engineers I think theoretical
arguments are dangerous to accept. The community has also not been presented
with sufficient information to understand some of the differences between the
numbers for the IR outliner and MIR outliner.

Since the two are using very similar algorithms for detecting outlining
candidates I would expect the numbers to be very similar, however in many cases
they are not. I think understanding those differences and comparing them will
help inform whether or not the IR outliner needs changes before being integrated
to LLVM. I'm not advocating that we shouldn't take the IR outliner (in
fact I think IR outlining is an interesting optimization). I'm advocating
that we should understand and explore how this pass will fit into LLVM today and
in the future.
> 
> 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!
I completely disagree. When considering accepting a new pass to LLVM it is
completely reasonable, appropriate, and not orthogonal to consider how that pass
would interact with other existing passes. Also, given that in the past
we've requested much more of contributors trying to make large contributions
(see the many SPIR-V discussions) I think it is totally fair to request River
consider Jessica's suggestion, and if the community felt that was the right
approach it would be fair to request him to implement it.

Please keep in mind, every new piece of functionality added to LLVM adds a
maintenance burden on the community. If we are going to accept the burden of
maintaining a new pass, it is reasonable for us, as a community, to request that
the pass be engineered in a way that will make it worth that maintenance.

Further, I am incredibly frustrated by the fact that people in this thread seem
intent on squashing relevant technical discussions or turning them into
combative arguments. There is nothing wrong with asking technical questions on
an RFC, and we should be encouraging cooperative discussions of present and
future implications of all proposals.

-Chris
> 
> 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 <mailto: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 <mailto: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:
> IR outline
> Inline
> 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 <mailto: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 <mailto:llvm-dev at lists.llvm.org>> wrote:
>> Hi,
>> 
>> On Wed, Jul 26, 2017 at 12:54 PM, Sean Silva <chisophugis at
gmail.com <mailto: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 <mailto: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 <mailto: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 <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 <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 <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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2017-08-01 11:03 GMT-07:00 Chris Bieneman <beanz at apple.com>:
>
> On Jul 31, 2017, at 10:38 PM, Mehdi AMINI <joker.eph at gmail.com>
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 think it is particularly relevant because based on the limited
> performance numbers we've seen it looks like the MIR and IR outliners
have
> different benefits. Figuring out a pipeline where one doesn't prevent
the
> other from performing good optimizations seems like a reasonable
> precondition to accepting these patches.
>
>
>
>
>> 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.
>
>
> I believe the technical merit has not been shown clearly enough. The only
> data we've seen has been cherry-picked and there are outstanding
technical
> questions about the approach.
>
> Discussing inclusion as part of any of the default pipeline is a different
> story.
>
>
> The patches that were sent out *do* include it in default pass pipelines.
>
Saying we shouldn't include this in any default pipeline for now seems
totally reasonable to me. My understanding was that the patch were sent so
that folks can reproduce the results, I think these were "WIP" not
patches
ready to commit. But again, discussing the implementation and other details
is perfectly appropriate. That isn't what I was trying to point at.



>
>
> 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.
>
>
> The reasoning for this was covered in the discussions and in Jessica's
> LLVM dev meeting talk. It may not have been widely discussed because it was
> widely agreed on.
>
I disagree with your assessment and I'm puzzle that you can claim that
while your previous email was along the line "the community should
discussed and decide about a new proposal".
It was widely agreed on by the people who started this project, that's far
from a reason to not discuss the pros/cons upstream.
Again, *shrug* on my side as long as it does not preclude other approach at
the IR level.


>
> 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.
>
>
> Nobody is precluding an IR implementation. We are merely holding the IR
> implementation to the same high standards of justification that we held the
> MIR one to. You may not recall this, but the MIR one took *months* to go
> from RFC to landing in-tree.
>
I'm opposing to holding the RFC itself solely based on the existence of
MIR. I don't disagree with reviewing carefully the IR outliner
implementation, and requiring incremental individual patches, etc, on the
opposite this should happen.



>
> 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.
>
>
> There is no pushback based solely on the presence of the MIR outliner.
>
Then it seems we're on perfect agreement, but that wasn't my first
impression.

-- 
Mehdi


> One source of inquiry about the merits of the IR outliner is its
> comparison to the MIR outliner, and whether or not the two can play well
> together. This seems like a reasonable line of inquiry to me.
>
>
>
>
>> 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.
>
>
> Sure, and several of us are trying to have a conversation with River about
> how the IR outliner will best fit into LLVM and what technical
> considerations have to be made. You arguing that we should just accept the
> patches as they are is counter productive to us being able to ensure that
> the IR outliner is at an appropriate quality and has sufficient technical
> merit.
>
> -Chris
>
>
> --
> 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
>>>
>>
>> _______________________________________________
>> 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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