llvm dev - Mar 2016 - [RFC] Target-specific parametrization of function inliner

IMO, the appropriate thing for TTI to inform the inliner about is how
costly the actual act of a "call" is likely to be. I would hope that
this
would only be used on targets where there is some really dramatic overhead
of actually doing a function call such that the code size cost incurred by
inlining is completely dwarfed by the improvements. GPUs are one of the few
platforms that exhibit this kind of behavior, although I don't think
they're truly unique, just a common example.

This isn't quite the same thing as the cost of the call instruction, which
has much more to do with the size. Instead, it has to do with the expected
consequences of actually leaving a call edge in the program.

To me, this pretty accurately reflects the TTI hook we have for customizing
loop unrolling where the cost of having a cyclic CFG is modeled to help
indicate that on some targets (also GPUs) it is worth a very large amount
of code size growth to simplify the control flow in a particular way.

Does that make sense to you Hal? Based on that, it would really just be a
scaling factor of the inline heuristics. Unsure of how to more
scientifically express this construct.

-Chandler

On Thu, Mar 10, 2016 at 3:42 PM Hal Finkel via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> ----- Original Message -----
> > From: "Artem Belevich via llvm-dev" <llvm-dev at
lists.llvm.org>
> > To: "llvm-dev" <llvm-dev at lists.llvm.org>
> > Sent: Tuesday, March 1, 2016 6:31:06 PM
> > Subject: [llvm-dev] [RFC] Target-specific parametrization of function
> inliner
> >
> > Hi,
> >
> >
> > I propose to make function inliner parameters adjustable for specific
> > target.
> >
> > Currently function inlining pass appears to be target-agnostic with
> > various constants for calculating call cost hardcoded. While it
> > works reasonably well for general purpose CPUs, some quirkier
> > targets like NVPTX would benefit from target-specific tuning.
> >
> >
> > Currently it appears that there are two things that need to be done:
> >
> >
> > * add Inliner preferences to TargetTransformInfo in a way similar to
> > how we customize loop unrolling. Use it to provide inliner with
> > target-specific thresholds and other parameters.
> > * augment Inliner pass to use existing TargetTransformInfo API to
> > figure out cost of particular call on a given target.
> > TargetTransforInfo already has getCallCost(), though it does not
> > look like anything uses it.
> >
> >
> > Comments? Concerns? Suggestions?
> >
>
> Hi Art,
>
> I've long thought that we should have a more principled way of doing
> inline profitability. There is obviously some cost to executing a function
> body, some call site overhead, and some cost reduction associated with any
> post-inlining simplifications. If inlining reduces the overall call site
> cost by more than some factor, say 1% (this should probably depend on the
> optimization level), then we should inline. With profiling information, we
> might even use global speedup instead of local speedup.
>
> Whether we need a target customization of this threshold, or just a way
> for a target to supplement the fine inlining decision, is unclear to me. It
> is also true that a the result of a bunch of locally-optimal decisions
> might be far from the global optimum. Maybe the target has something to say
> about that?
>
> In short, I'm fine with what you're proposing, but to the extent
possible,
> I want the numbers provided by the target to mean something. Replacing a
> global set of somewhat-arbitrary magic numbers, with target-specific sets
> of somewhat-arbitrary magic numbers should be our last choice.
>
> Thanks again,
> Hal
>
>
> >
> > Thanks,
> > --
> >
> >
> > --Artem Belevich
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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On Thu, Mar 10, 2016 at 6:49 AM, Chandler Carruth <chandlerc at
google.com>
wrote:
> IMO, the appropriate thing for TTI to inform the inliner about is how
> costly the actual act of a "call" is likely to be. I would hope
that this
> would only be used on targets where there is some really dramatic overhead
> of actually doing a function call such that the code size cost incurred by
> inlining is completely dwarfed by the improvements. GPUs are one of the few
> platforms that exhibit this kind of behavior, although I don't think
> they're truly unique, just a common example.
>
> This isn't quite the same thing as the cost of the call instruction,
which
> has much more to do with the size. Instead, it has to do with the expected
> consequences of actually leaving a call edge in the program.
>
>
> To me, this pretty accurately reflects the TTI hook we have for
> customizing loop unrolling where the cost of having a cyclic CFG is modeled
> to help indicate that on some targets (also GPUs) it is worth a very large
> amount of code size growth to simplify the control flow in a particular
way.
>
>
>From 10000 foot, the LLVM inliner implements a size based heuristic :  if
the inline instance's size*/cost after simplification via propagating the
call context (actually the relative size -- the callsite cost is subtracted
from it), is smaller than a threshold (adjusted from a base value), then
the callsite is considered an inline candidate. In most cases, the decision
is made locally due to the bottom-up order (there are tweaks to bypass it).
  The size/cost can be remotely tied and serves a proxy to represent the
real runtime cost due to icache/itlb effect, but it seems the
size/threshold scheme is mainly used to model the runtime speedup vs
compile time/binary size tradeoffs.

Set aside what we need longer term for the inliner, the GPU specific
problems can be addressed by
1) if the call overhead is really large, define a target specific
getCallCost and subtract it from the initial Cost when analyzing a callsite
(this will help boost all targets with high call costs)
2) if not, but instead GPU users can tolerate large code growth, then it is
better to this by adjusting the threshold -- perhaps have a user level
option -finline-limit=?

thanks,

David


* some target dependent info may be used: TTI.getUserCost

> Does that make sense to you Hal? Based on that, it would really just be a
> scaling factor of the inline heuristics. Unsure of how to more
> scientifically express this construct.
>
> -Chandler
>
> On Thu, Mar 10, 2016 at 3:42 PM Hal Finkel via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> ----- Original Message -----
>> > From: "Artem Belevich via llvm-dev" <llvm-dev at
lists.llvm.org>
>> > To: "llvm-dev" <llvm-dev at lists.llvm.org>
>> > Sent: Tuesday, March 1, 2016 6:31:06 PM
>> > Subject: [llvm-dev] [RFC] Target-specific parametrization of
function
>> inliner
>> >
>> > Hi,
>> >
>> >
>> > I propose to make function inliner parameters adjustable for
specific
>> > target.
>> >
>> > Currently function inlining pass appears to be target-agnostic
with
>> > various constants for calculating call cost hardcoded. While it
>> > works reasonably well for general purpose CPUs, some quirkier
>> > targets like NVPTX would benefit from target-specific tuning.
>> >
>> >
>> > Currently it appears that there are two things that need to be
done:
>> >
>> >
>> > * add Inliner preferences to TargetTransformInfo in a way similar
to
>> > how we customize loop unrolling. Use it to provide inliner with
>> > target-specific thresholds and other parameters.
>> > * augment Inliner pass to use existing TargetTransformInfo API to
>> > figure out cost of particular call on a given target.
>> > TargetTransforInfo already has getCallCost(), though it does not
>> > look like anything uses it.
>> >
>> >
>> > Comments? Concerns? Suggestions?
>> >
>>
>> Hi Art,
>>
>> I've long thought that we should have a more principled way of
doing
>> inline profitability. There is obviously some cost to executing a
function
>> body, some call site overhead, and some cost reduction associated with
any
>> post-inlining simplifications. If inlining reduces the overall call
site
>> cost by more than some factor, say 1% (this should probably depend on
the
>> optimization level), then we should inline. With profiling information,
we
>> might even use global speedup instead of local speedup.
>>
>> Whether we need a target customization of this threshold, or just a way
>> for a target to supplement the fine inlining decision, is unclear to
me. It
>> is also true that a the result of a bunch of locally-optimal decisions
>> might be far from the global optimum. Maybe the target has something to
say
>> about that?
>>
>> In short, I'm fine with what you're proposing, but to the
extent
>> possible, I want the numbers provided by the target to mean something.
>> Replacing a global set of somewhat-arbitrary magic numbers, with
>> target-specific sets of somewhat-arbitrary magic numbers should be our
last
>> choice.
>>
>> Thanks again,
>> Hal
>>
>>
>> >
>> > Thanks,
>> > --
>> >
>> >
>> > --Artem Belevich
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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----- Original Message -----
> From: "Xinliang David Li" <davidxl at google.com>
> To: "Chandler Carruth" <chandlerc at google.com>
> Cc: "Hal Finkel" <hfinkel at anl.gov>, "Artem
Belevich"
> <tra at google.com>, "llvm-dev" <llvm-dev at
lists.llvm.org>
> Sent: Thursday, March 10, 2016 12:34:07 PM
> Subject: Re: [llvm-dev] [RFC] Target-specific parametrization of
> function inliner
> On Thu, Mar 10, 2016 at 6:49 AM, Chandler Carruth <
> chandlerc at google.com > wrote:
> > IMO, the appropriate thing for TTI to inform the inliner about is
> > how
> > costly the actual act of a "call" is likely to be. I would
hope
> > that
> > this would only be used on targets where there is some really
> > dramatic overhead of actually doing a function call such that the
> > code size cost incurred by inlining is completely dwarfed by the
> > improvements. GPUs are one of the few platforms that exhibit this
> > kind of behavior, although I don't think they're truly unique,
just
> > a common example.
> 
> > This isn't quite the same thing as the cost of the call
> > instruction,
> > which has much more to do with the size. Instead, it has to do with
> > the expected consequences of actually leaving a call edge in the
> > program.
> 
> > To me, this pretty accurately reflects the TTI hook we have for
> > customizing loop unrolling where the cost of having a cyclic CFG is
> > modeled to help indicate that on some targets (also GPUs) it is
> > worth a very large amount of code size growth to simplify the
> > control flow in a particular way.
> 
> From 10000 foot, the LLVM inliner implements a size based heuristic :
> if the inline instance's size*/cost after simplification via
> propagating the call context (actually the relative size -- the
> callsite cost is subtracted from it), is smaller than a threshold
> (adjusted from a base value), then the callsite is considered an
> inline candidate. In most cases, the decision is made locally due to
> the bottom-up order (there are tweaks to bypass it). The size/cost
> can be remotely tied and serves a proxy to represent the real
> runtime cost due to icache/itlb effect, but it seems the
> size/threshold scheme is mainly used to model the runtime speedup vs
> compile time/binary size tradeoffs.
Yes, is kind of gets this. But sometimes not very well. 

Part of the problem is that we try to make local decisions to control global
code size. I understand why we do this, but we shouldn't. The local metric
should purely be based on local speedup. That metric itself can be modulated by
a global heuristic to control costs from itlb/icache misses, etc. - and
there's not too much we can do here without profiling data with temporal
correlation information.
> Set aside what we need longer term for the inliner, the GPU specific
> problems can be addressed by
> 1) if the call overhead is really large, define a target specific
> getCallCost and subtract it from the initial Cost when analyzing a
> callsite (this will help boost all targets with high call costs)
Yes, this makes sense. 
> 2) if not, but instead GPU users can tolerate large code growth, then
> it is better to this by adjusting the threshold -- perhaps have a
> user level option -finline-limit=?
Providing a user option makes sense, but as I said in the other e-mail, we
should phrase it in terms of something not in arbitrary units. I think that %
speedup makes the most sense. As it turns out, inlining a large function will
probably produce a low speedup (especially if we have partial inlining where we
only inline the hot regions), so this ends up correlated with code size too.

-Hal 
> thanks,
> David
> * some target dependent info may be used: TTI.getUserCost
> > Does that make sense to you Hal? Based on that, it would really
> > just
> > be a scaling factor of the inline heuristics. Unsure of how to more
> > scientifically express this construct.
> 
> > -Chandler
> 
> > On Thu, Mar 10, 2016 at 3:42 PM Hal Finkel via llvm-dev <
> > llvm-dev at lists.llvm.org > wrote:
> 
> > > ----- Original Message -----
> > 
> 
> > > > From: "Artem Belevich via llvm-dev" < llvm-dev
at lists.llvm.org >
> > 
> 
> > > > To: "llvm-dev" < llvm-dev at lists.llvm.org
>
> > 
> 
> > > > Sent: Tuesday, March 1, 2016 6:31:06 PM
> > 
> 
> > > > Subject: [llvm-dev] [RFC] Target-specific parametrization of
> > > > function inliner
> > 
> 
> > > >
> > 
> 
> > > > Hi,
> > 
> 
> > > >
> > 
> 
> > > >
> > 
> 
> > > > I propose to make function inliner parameters adjustable for
> > > > specific
> > 
> 
> > > > target.
> > 
> 
> > > >
> > 
> 
> > > > Currently function inlining pass appears to be
target-agnostic
> > > > with
> > 
> 
> > > > various constants for calculating call cost hardcoded. While
it
> > 
> 
> > > > works reasonably well for general purpose CPUs, some
quirkier
> > 
> 
> > > > targets like NVPTX would benefit from target-specific
tuning.
> > 
> 
> > > >
> > 
> 
> > > >
> > 
> 
> > > > Currently it appears that there are two things that need to
be
> > > > done:
> > 
> 
> > > >
> > 
> 
> > > >
> > 
> 
> > > > * add Inliner preferences to TargetTransformInfo in a way
> > > > similar
> > > > to
> > 
> 
> > > > how we customize loop unrolling. Use it to provide inliner
with
> > 
> 
> > > > target-specific thresholds and other parameters.
> > 
> 
> > > > * augment Inliner pass to use existing TargetTransformInfo
API
> > > > to
> > 
> 
> > > > figure out cost of particular call on a given target.
> > 
> 
> > > > TargetTransforInfo already has getCallCost(), though it does
> > > > not
> > 
> 
> > > > look like anything uses it.
> > 
> 
> > > >
> > 
> 
> > > >
> > 
> 
> > > > Comments? Concerns? Suggestions?
> > 
> 
> > > >
> > 
> 
> > > Hi Art,
> > 
> 
> > > I've long thought that we should have a more principled way
of
> > > doing
> > > inline profitability. There is obviously some cost to executing a
> > > function body, some call site overhead, and some cost reduction
> > > associated with any post-inlining simplifications. If inlining
> > > reduces the overall call site cost by more than some factor, say
> > > 1%
> > > (this should probably depend on the optimization level), then we
> > > should inline. With profiling information, we might even use
> > > global
> > > speedup instead of local speedup.
> > 
> 
> > > Whether we need a target customization of this threshold, or just
> > > a
> > > way for a target to supplement the fine inlining decision, is
> > > unclear to me. It is also true that a the result of a bunch of
> > > locally-optimal decisions might be far from the global optimum.
> > > Maybe the target has something to say about that?
> > 
> 
> > > In short, I'm fine with what you're proposing, but to the
extent
> > > possible, I want the numbers provided by the target to mean
> > > something. Replacing a global set of somewhat-arbitrary magic
> > > numbers, with target-specific sets of somewhat-arbitrary magic
> > > numbers should be our last choice.
> > 
> 
> > > Thanks again,
> > 
> 
> > > Hal
> > 
> 
> > > >
> > 
> 
> > > > Thanks,
> > 
> 
> > > > --
> > 
> 
> > > >
> > 
> 
> > > >
> > 
> 
> > > > --Artem Belevich
> > 
> 
> > > > _______________________________________________
> > 
> 
> > > > LLVM Developers mailing list
> > 
> 
> > > > llvm-dev at lists.llvm.org
> > 
> 
> > > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> > 
> 
> > > >
> > 
> 
> > > --
> > 
> 
> > > Hal Finkel
> > 
> 
> > > Assistant Computational Scientist
> > 
> 
> > > Leadership Computing Facility
> > 
> 
> > > Argonne National Laboratory
> > 
> 
> > > _______________________________________________
> > 
> 
> > > LLVM Developers mailing list
> > 
> 
> > > llvm-dev at lists.llvm.org
> > 
> 
> > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> > 
> 
-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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> On Mar 10, 2016, at 10:34 AM, Xinliang David Li via llvm-dev <llvm-dev
at lists.llvm.org> wrote:
> 
> 
> 
> On Thu, Mar 10, 2016 at 6:49 AM, Chandler Carruth <chandlerc at
google.com <mailto:chandlerc at google.com>> wrote:
> IMO, the appropriate thing for TTI to inform the inliner about is how
costly the actual act of a "call" is likely to be. I would hope that
this would only be used on targets where there is some really dramatic overhead
of actually doing a function call such that the code size cost incurred by
inlining is completely dwarfed by the improvements. GPUs are one of the few
platforms that exhibit this kind of behavior, although I don't think
they're truly unique, just a common example.
> 
> This isn't quite the same thing as the cost of the call instruction,
which has much more to do with the size. Instead, it has to do with the expected
consequences of actually leaving a call edge in the program.
>  
> 
> To me, this pretty accurately reflects the TTI hook we have for customizing
loop unrolling where the cost of having a cyclic CFG is modeled to help indicate
that on some targets (also GPUs) it is worth a very large amount of code size
growth to simplify the control flow in a particular way.
> 
> 
> From 10000 foot, the LLVM inliner implements a size based heuristic :  if
the inline instance's size*/cost after simplification via propagating the
call context (actually the relative size -- the callsite cost is subtracted from
it), is smaller than a threshold (adjusted from a base value), then the callsite
is considered an inline candidate. In most cases, the decision is made locally
due to the bottom-up order (there are tweaks to bypass it).   The size/cost can
be remotely tied and serves a proxy to represent the real runtime cost due to
icache/itlb effect, but it seems the size/threshold scheme is mainly used to
model the runtime speedup vs compile time/binary size tradeoffs.
Other than the call cost itself, I've been surprised that the TTI is not
more involved when it comes to this tradeoff: instructions don't have the
same tradeoff depending on the platform (oh this operation is not legal on this
type and will be expanded in multiple instructions in SDAG, too bad..).

-- 
Mehdi

> 
> Set aside what we need longer term for the inliner, the GPU specific
problems can be addressed by
> 1) if the call overhead is really large, define a target specific
getCallCost and subtract it from the initial Cost when analyzing a callsite
(this will help boost all targets with high call costs)
> 2) if not, but instead GPU users can tolerate large code growth, then it is
better to this by adjusting the threshold -- perhaps have a user level option
-finline-limit=?
> 
> thanks,
> 
> David
> 
> 
> * some target dependent info may be used: TTI.getUserCost
>  
> Does that make sense to you Hal? Based on that, it would really just be a
scaling factor of the inline heuristics. Unsure of how to more scientifically
express this construct.
> 
> -Chandler
> 
> On Thu, Mar 10, 2016 at 3:42 PM Hal Finkel via llvm-dev <llvm-dev at
lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> ----- Original Message -----
> > From: "Artem Belevich via llvm-dev" <llvm-dev at
lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
> > To: "llvm-dev" <llvm-dev at lists.llvm.org
<mailto:llvm-dev at lists.llvm.org>>
> > Sent: Tuesday, March 1, 2016 6:31:06 PM
> > Subject: [llvm-dev] [RFC] Target-specific parametrization of function
inliner
> >
> > Hi,
> >
> >
> > I propose to make function inliner parameters adjustable for specific
> > target.
> >
> > Currently function inlining pass appears to be target-agnostic with
> > various constants for calculating call cost hardcoded. While it
> > works reasonably well for general purpose CPUs, some quirkier
> > targets like NVPTX would benefit from target-specific tuning.
> >
> >
> > Currently it appears that there are two things that need to be done:
> >
> >
> > * add Inliner preferences to TargetTransformInfo in a way similar to
> > how we customize loop unrolling. Use it to provide inliner with
> > target-specific thresholds and other parameters.
> > * augment Inliner pass to use existing TargetTransformInfo API to
> > figure out cost of particular call on a given target.
> > TargetTransforInfo already has getCallCost(), though it does not
> > look like anything uses it.
> >
> >
> > Comments? Concerns? Suggestions?
> >
> 
> Hi Art,
> 
> I've long thought that we should have a more principled way of doing
inline profitability. There is obviously some cost to executing a function body,
some call site overhead, and some cost reduction associated with any
post-inlining simplifications. If inlining reduces the overall call site cost by
more than some factor, say 1% (this should probably depend on the optimization
level), then we should inline. With profiling information, we might even use
global speedup instead of local speedup.
> 
> Whether we need a target customization of this threshold, or just a way for
a target to supplement the fine inlining decision, is unclear to me. It is also
true that a the result of a bunch of locally-optimal decisions might be far from
the global optimum. Maybe the target has something to say about that?
> 
> In short, I'm fine with what you're proposing, but to the extent
possible, I want the numbers provided by the target to mean something. Replacing
a global set of somewhat-arbitrary magic numbers, with target-specific sets of
somewhat-arbitrary magic numbers should be our last choice.
> 
> Thanks again,
> Hal
> 
> 
> >
> > Thanks,
> > --
> >
> >
> > --Artem Belevich
> > _______________________________________________
> > 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>
> >
> 
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> _______________________________________________
> 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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