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

[Artem Belevich via llvm-dev]

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?

Thanks,

--

--Artem Belevich

[Hal Finkel via llvm-dev]

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...@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
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[Chandler Carruth via llvm-dev]

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

[Xinliang David Li via llvm-dev]

IMO, a good inliner with a precise cost/benefit model will eventually need what Art is proposing here. 

Giving the function call overhead as an example. It depends on a couple of factors: 1) call/return instruction latency; 2) function epilogue/prologue; 3) calling convention (argument parsing, using registers or not, what register classes etc).  All these factors depend on target information.  If we want go deeper, we know certain micro architectures uses a stack of call/return pairs to help branch prediction of ret instructions -- such stack has a target specific limit which can be triggered when a callsite is deep in the callchain.   Register file size and register pressure increase due to inline comes as another example.

Another relevant example is the icache/itlb sizes. To do a more precise analysis of the cost to 'speed' due to icache/itlb pressure increase requires target information, profile information as well as some global analysis. Easwaran has done some research in this area in the past and can share the analysis design when other things are ready.

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.

yes -- with target specific cost information, global speedup analysis can be more precise :)

 

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?

The concept of threshold can be a topic of another discussion.  In current design, I think the threshold should remain target independent.  It is the cost that is target specific.

thanks,

David

[Xinliang David Li via llvm-dev]

On Thu, Mar 10, 2016 at 6:49 AM, Chandler Carruth <chan...@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

[Hal Finkel via llvm-dev]

From: "Xinliang David Li" <dav...@google.com>
To: "Hal Finkel" <hfi...@anl.gov>
Cc: "Artem Belevich" <t...@google.com>, "llvm-dev" <llvm...@lists.llvm.org>, "chandlerc" <chan...@gmail.com>, "Easwaran Raman" <era...@google.com>
Sent: Thursday, March 10, 2016 11:00:30 AM
Subject: Re: [llvm-dev] [RFC] Target-specific parametrization of function inliner

IMO, a good inliner with a precise cost/benefit model will eventually need what Art is proposing here. 

Giving the function call overhead as an example. It depends on a couple of factors: 1) call/return instruction latency; 2) function epilogue/prologue; 3) calling convention (argument parsing, using registers or not, what register classes etc).  All these factors depend on target information.  If we want go deeper, we know certain micro architectures uses a stack of call/return pairs to help branch prediction of ret instructions -- such stack has a target specific limit which can be triggered when a callsite is deep in the callchain.   Register file size and register pressure increase due to inline comes as another example.

Another relevant example is the icache/itlb sizes. To do a more precise analysis of the cost to 'speed' due to icache/itlb pressure increase requires target information, profile information as well as some global analysis. Easwaran has done some research in this area in the past and can share the analysis design when other things are ready.

I don't know what you mean by "when other things are ready", but what you say above sounds exactly right. I'm certainly curious what Easwaran has found.

Generally, there seem to be two categories here:

 1. Locally decidable issues, for which there are (or can be) good static heuristics (call latencies, costs associated with parameter passing, stack spilling, etc.)
 2. Globally decidable issues, like reducing the number of pages consumed by temporally-correlated hot code regions - profiling data likely necessary for good decision-making (although it might be possible to make a reasonable function-local threshold based on page size without it)

and then there are things like icache/itlb effects due to multiple applications running simultaneously, for which profiling might help, but are also policy-level decisions over which users may need more-direct control.

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.

yes -- with target specific cost information, global speedup analysis can be more precise :)

 

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?

The concept of threshold can be a topic of another discussion.  In current design, I think the threshold should remain target independent.  It is the cost that is target specific.

That's fine, but the units are important here. Having a target independent threshold in terms of, roughly, instruction count makes little sense. How instruction count is correlated with either performance or code size is highly target specific (although it is certainly closer for code size). That, however, is, roughly what our TTI.getUserCost gives us. Having target-independent thresholds like % speedup (e.g. inlining should be done when the speedup is > some %) or code-size thresholds (e.g. functions spanning more than a 4 KB are bad) makes sense.

 -Hal

[Hal Finkel via llvm-dev]

From: "Xinliang David Li" <dav...@google.com>

To: "Chandler Carruth" <chan...@google.com>
Cc: "Hal Finkel" <hfi...@anl.gov>, "Artem Belevich" <t...@google.com>, "llvm-dev" <llvm...@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 <chan...@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...@lists.llvm.org> wrote:

[Mehdi Amini via llvm-dev]

On Mar 10, 2016, at 10:34 AM, Xinliang David Li via llvm-dev <llvm...@lists.llvm.org> wrote:

On Thu, Mar 10, 2016 at 6:49 AM, Chandler Carruth <chan...@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

[Hal Finkel via llvm-dev]

From: "Mehdi Amini via llvm-dev" <llvm...@lists.llvm.org>
To: "Xinliang David Li" <dav...@google.com>
Cc: "llvm-dev" <llvm...@lists.llvm.org>
Sent: Friday, April 1, 2016 2:26:27 PM
Subject: Re: [llvm-dev] [RFC] Target-specific parametrization of function inliner

On Mar 10, 2016, at 10:34 AM, Xinliang David Li via llvm-dev <llvm...@lists.llvm.org> wrote:

On Thu, Mar 10, 2016 at 6:49 AM, Chandler Carruth <chan...@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..).

I think that doing this was intended, but we've not done it yet (as we did for the throughput model used for vectorization). I think we should (I also think we should combine the cost models so that we have a single model that returns multiple kinds of costs (throughput, size, latency, etc.)).

 -Hal

-- 

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...@lists.llvm.org> wrote:

[Xinliang David Li via llvm-dev]

On Fri, Apr 1, 2016 at 12:10 PM, Hal Finkel <hfi...@anl.gov> wrote:

---

From: "Xinliang David Li" <dav...@google.com>
To: "Hal Finkel" <hfi...@anl.gov>
Cc: "Artem Belevich" <t...@google.com>, "llvm-dev" <llvm...@lists.llvm.org>, "chandlerc" <chan...@gmail.com>, "Easwaran Raman" <era...@google.com>
Sent: Thursday, March 10, 2016 11:00:30 AM
Subject: Re: [llvm-dev] [RFC] Target-specific parametrization of function inliner

IMO, a good inliner with a precise cost/benefit model will eventually need what Art is proposing here. 

Giving the function call overhead as an example. It depends on a couple of factors: 1) call/return instruction latency; 2) function epilogue/prologue; 3) calling convention (argument parsing, using registers or not, what register classes etc).  All these factors depend on target information.  If we want go deeper, we know certain micro architectures uses a stack of call/return pairs to help branch prediction of ret instructions -- such stack has a target specific limit which can be triggered when a callsite is deep in the callchain.   Register file size and register pressure increase due to inline comes as another example.

Another relevant example is the icache/itlb sizes. To do a more precise analysis of the cost to 'speed' due to icache/itlb pressure increase requires target information, profile information as well as some global analysis. Easwaran has done some research in this area in the past and can share the analysis design when other things are ready.

I don't know what you mean by "when other things are ready", but what you say above sounds exactly right. I'm certainly curious what Easwaran has found.

By readiness, I mean the basic infrastructure support (such as making profile data available for inliner) and related tuning based on simple heuristics. The former will be revisited very soon.  Those work will be useful to setup a good baseline before we start engaging in more sophisticated analysis.

 

Generally, there seem to be two categories here:

 1. Locally decidable issues, for which there are (or can be) good static heuristics (call latencies, costs associated with parameter passing, stack spilling, etc.)
 2. Globally decidable issues, like reducing the number of pages consumed by temporally-correlated hot code regions - profiling data likely necessary for good decision-making (although it might be possible to make a reasonable function-local threshold based on page size without it)

 Program level static analysis needs to be combined with profile data to form independent or nested hot regions (with cache/tlb reuse). For global inlining decisions,  there won't be a single budget to be used. The decision will highly depend on the region nest where the callsite sits in. 

Another side effect is that we may need more flexible inlining order (based on net benefit of inlining a callsite) than the current bottom up order scheme.

 

and then there are things like icache/itlb effects due to multiple applications running simultaneously, for which profiling might help, but are also policy-level decisions over which users may need more-direct control.

Some thread level profiling may also be useful in guiding the decision -- i.e., use information about the shared instruction footprint across different threads - e.g. Programs running heterogeneous threads vs programs with homogeneous worker threads running identical code.

thanks,

David

[Xinliang David Li via llvm-dev]

On Fri, Apr 1, 2016 at 12:35 PM, Hal Finkel <hfi...@anl.gov> wrote:

---

From: "Mehdi Amini via llvm-dev" <llvm...@lists.llvm.org>
To: "Xinliang David Li" <dav...@google.com>
Cc: "llvm-dev" <llvm...@lists.llvm.org>
Sent: Friday, April 1, 2016 2:26:27 PM
Subject: Re: [llvm-dev] [RFC] Target-specific parametrization of function inliner

On Mar 10, 2016, at 10:34 AM, Xinliang David Li via llvm-dev <llvm...@lists.llvm.org> wrote:

On Thu, Mar 10, 2016 at 6:49 AM, Chandler Carruth <chan...@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..).

I think that doing this was intended, but we've not done it yet (as we did for the throughput model used for vectorization). I think we should (I also think we should combine the cost models so that we have a single model that returns multiple kinds of costs (throughput, size, latency, etc.)).

yes -- the time/speed up estimate should be independent of size increase estimate.

David