llvm dev - Nov 2017 - RFC: [GlobalISel] Towards a generic MI combiner framework

> On Nov 13, 2017, at 11:53 AM, Vedant Kumar via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi Amara,
> 
>> On Nov 10, 2017, at 9:12 AM, Amara Emerson via llvm-dev <llvm-dev at
lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> Hi everyone,
>> 
>> This RFC concerns the design and architecture of a generic machine
instruction combiner/optimizer framework to be developed as part of the GISel
pipeline. As we transition from correctness and reducing the fallback rate to
SelectionDAG at -O0, we’re now starting to think about using GlobalISel with
optimizations enabled. There are obviously many parts to this story as
optimizations happen at various stages of the codegen pipeline. The focus of
this RFC is the replacement of the equivalent of the DAGCombiner in SDAG land.
Despite the focus on the DAGCombiner, since there aren’t perfect 1-1 mappings
between SDAG and GlobalISel components, this may also include features that are
currently implemented as part of the target lowerings, and tablegen isel
patterns. As we’re starting from a blank slate, we have an opportunity here to
think about what we might need from such a framework without the legacy cruft
(although we still have the high performance bar to meet).
>> 
>> I want to poll the community about what future requirements we have for
the GISel G_MI optimizer/combiner. The following are the general requirements we
have so far:
>> 
>> It should have at least equivalent, but hopefully better
runtime/compile time trade off than the DAGCombiner.
>> There needs to be flexibility in the design to allow targets to run
subsets of the overall optimizer. For example, some targets may want to avoid
trying to run certain types of optimizations like vector or FP combines if
they’re either not applicable, or not worth the compile time.
>> Have a reasonably concise way to write most optimizations. Hand written
C++ will always be an option, but there’s value in having easy to read and
reason about descriptions of transforms.
>> 
>> These requirements aren’t set in stone nor complete, but using them as
a starting point: a single monolithic “Generic MI combiner” component doesn’t
look like the right approach. Our current thinking is that, like we’ve done with
the Legalizer, the specific mechanics of the actual optimization should be
separated into it’s own unit. This would allow the combines to be re-used at
different stages of the pipeline according to target needs. Using the current
situation with instcombine as an example, there is no way to explicitly pick and
choose a specific subset of IC, it’s only available as a whole pass with all the
costs that entails.
>> 
>> The reasoning behind req 3 is that there may be compile time savings
available if we can describe in a declarative style the combines we want to do,
like it’s currently possible with tablegen patterns. This hasn’t been proven out
yet, but consider an alternative where we use the machine instruction equivalent
of the IR/PatternMatch tooling which allows easy and expressive matching of IR
sub-trees. A concern I have with using that as the main approach to writing
combines is that it’s easy to add new matchers in an routine which re-computes
information that’s previously been computed in previous match() attempts. This
form of back-tracking might be avoided if we can reason about a group of
combines together automatically (or perhaps we could add caching capabilities to
PatternMatch).
>> 
>> What would everyone else like to see from this?
> 
> It would be great to provide first-class support for maintaining debug
value information as a part of the new combine framework.
> 
> With SelectionDAG, we don't have a systematic way of preserving debug
locations and values across combines. This is a source of optimized debugging
bugs. If, as a part of the new framework, we could concisely express that a
RAUW-style combine simply transfers debug values from A to B, we might define
away some of these bugs [1].
> 
> Adrian put in place some infrastructure to do this in SelectionDAG
(r317825). However, auditing/fixing debug value transfer issues in hand-written
combines is time-consuming. I think it should be a goal of the new framework to
make this a bit easier.
+1 

Do you and/or Adrian also have thought on testing debug values? Verification and
testing strategies should/could be part of the design
also.
> 
> best,
> vedant
> 
> [1] To pick one at random, the '(zext (zextload x)) -> (zext
(truncate (zextload x)))' combine should transfer debug values from N to the
new ExtLoad, and from N0 to the new (trunc ExtLoad), but currently doesn't.
> 
>> 
>> Thanks,
>> Amara
>> _______________________________________________
>> 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>
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> On Nov 17, 2017, at 4:41 PM, Gerolf Hoflehner <ghoflehner at
apple.com> wrote:
> 
> 
> 
>> On Nov 13, 2017, at 11:53 AM, Vedant Kumar via llvm-dev <llvm-dev at
lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> Hi Amara,
>> 
>>> On Nov 10, 2017, at 9:12 AM, Amara Emerson via llvm-dev
<llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
wrote:
>>> 
>>> Hi everyone,
>>> 
>>> This RFC concerns the design and architecture of a generic machine
instruction combiner/optimizer framework to be developed as part of the GISel
pipeline. As we transition from correctness and reducing the fallback rate to
SelectionDAG at -O0, we’re now starting to think about using GlobalISel with
optimizations enabled. There are obviously many parts to this story as
optimizations happen at various stages of the codegen pipeline. The focus of
this RFC is the replacement of the equivalent of the DAGCombiner in SDAG land.
Despite the focus on the DAGCombiner, since there aren’t perfect 1-1 mappings
between SDAG and GlobalISel components, this may also include features that are
currently implemented as part of the target lowerings, and tablegen isel
patterns. As we’re starting from a blank slate, we have an opportunity here to
think about what we might need from such a framework without the legacy cruft
(although we still have the high performance bar to meet).
>>> 
>>> I want to poll the community about what future requirements we have
for the GISel G_MI optimizer/combiner. The following are the general
requirements we have so far:
>>> 
>>> It should have at least equivalent, but hopefully better
runtime/compile time trade off than the DAGCombiner.
>>> There needs to be flexibility in the design to allow targets to run
subsets of the overall optimizer. For example, some targets may want to avoid
trying to run certain types of optimizations like vector or FP combines if
they’re either not applicable, or not worth the compile time.
>>> Have a reasonably concise way to write most optimizations. Hand
written C++ will always be an option, but there’s value in having easy to read
and reason about descriptions of transforms.
>>> 
>>> These requirements aren’t set in stone nor complete, but using them
as a starting point: a single monolithic “Generic MI combiner” component doesn’t
look like the right approach. Our current thinking is that, like we’ve done with
the Legalizer, the specific mechanics of the actual optimization should be
separated into it’s own unit. This would allow the combines to be re-used at
different stages of the pipeline according to target needs. Using the current
situation with instcombine as an example, there is no way to explicitly pick and
choose a specific subset of IC, it’s only available as a whole pass with all the
costs that entails.
>>> 
>>> The reasoning behind req 3 is that there may be compile time
savings available if we can describe in a declarative style the combines we want
to do, like it’s currently possible with tablegen patterns. This hasn’t been
proven out yet, but consider an alternative where we use the machine instruction
equivalent of the IR/PatternMatch tooling which allows easy and expressive
matching of IR sub-trees. A concern I have with using that as the main approach
to writing combines is that it’s easy to add new matchers in an routine which
re-computes information that’s previously been computed in previous match()
attempts. This form of back-tracking might be avoided if we can reason about a
group of combines together automatically (or perhaps we could add caching
capabilities to PatternMatch).
>>> 
>>> What would everyone else like to see from this?
>> 
>> It would be great to provide first-class support for maintaining debug
value information as a part of the new combine framework.
>> 
>> With SelectionDAG, we don't have a systematic way of preserving
debug locations and values across combines. This is a source of optimized
debugging bugs. If, as a part of the new framework, we could concisely express
that a RAUW-style combine simply transfers debug values from A to B, we might
define away some of these bugs [1].
>> 
>> Adrian put in place some infrastructure to do this in SelectionDAG
(r317825). However, auditing/fixing debug value transfer issues in hand-written
combines is time-consuming. I think it should be a goal of the new framework to
make this a bit easier.
> 
> +1 
> 
> Do you and/or Adrian also have thought on testing debug values?
Verification and testing strategies should/could be part of the design also.
I have two ideas on how to approach testing.

1) Recycle existing targeted lit tests to test debug info preservation.

Consider a targeted lit test which look like this:
RUN: opt -S -loop-reduce addrec-gep.ll -o - | FileCheck %s

We can repurpose this test by attaching synthetic debug information to the IR,
and then checking how much of it survives LSR. I prototyped this idea for
IR-level tests over the break. Adding a debug info test looks like this:
RUN: opt -S -debugify -loop-reduce -check-debugify addrec-gep.ll -o - |
FileCheck ...

The check-debugify pass can determine which DILocations and DIVariables went
missing:
> CheckDebugify: Instruction with empty DebugLoc --   %lsr.iv1 = bitcast
double* %lsr.iv to i1*
> CheckDebugify: Missing line 3
> CheckDebugify: Missing line 4
> ...
> CheckDebugify: Missing line 33
This could be a handy way to create new targeted test cases at the IR/MIR level.
Something like this would have helped triage issues like llvm.org/PR25630 as
well.

2) Assert that combines preserve debug info.

If there's an important API like "CombineTo(From, To)", it could
be useful to assert that the To node has at least as much debug info as the From
node. I'm experimenting with this in SelectionDAG (llvm.org/PR35338
<http://llvm.org/PR35338>). I don't yet know where these asserts
belong, how strict they should be, or if they need exception lists for certain
combines.

---

Adrian and others have thought about these issues much more, so take these ideas
with a grain of salt :).

thanks,
vedant

>> 
>> best,
>> vedant
>> 
>> [1] To pick one at random, the '(zext (zextload x)) -> (zext
(truncate (zextload x)))' combine should transfer debug values from N to the
new ExtLoad, and from N0 to the new (trunc ExtLoad), but currently doesn't.
>> 
>>> 
>>> Thanks,
>>> Amara
>>> _______________________________________________
>>> 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>
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Thanks for the suggestions Vedant. Synthetic debug info is an interesting idea
that sounds worthwhile.  Could this be implemented as a “wrapper” pass that
automatically decorates debug info before and after a specific pass run in opt
(or pipeline of passes)? It might be useful to be able to easily enable this for
a wide range of tests without having to manually modify each run line, perhaps
as an environment variable/build time flag.

Cheers,
Amara
> On Nov 27, 2017, at 6:18 PM, Vedant Kumar <vsk at apple.com> wrote:
> 
>> 
>> On Nov 17, 2017, at 4:41 PM, Gerolf Hoflehner <ghoflehner at
apple.com <mailto:ghoflehner at apple.com>> wrote:
>> 
>> 
>> 
>>> On Nov 13, 2017, at 11:53 AM, Vedant Kumar via llvm-dev
<llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
wrote:
>>> 
>>> Hi Amara,
>>> 
>>>> On Nov 10, 2017, at 9:12 AM, Amara Emerson via llvm-dev
<llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
wrote:
>>>> 
>>>> Hi everyone,
>>>> 
>>>> This RFC concerns the design and architecture of a generic
machine instruction combiner/optimizer framework to be developed as part of the
GISel pipeline. As we transition from correctness and reducing the fallback rate
to SelectionDAG at -O0, we’re now starting to think about using GlobalISel with
optimizations enabled. There are obviously many parts to this story as
optimizations happen at various stages of the codegen pipeline. The focus of
this RFC is the replacement of the equivalent of the DAGCombiner in SDAG land.
Despite the focus on the DAGCombiner, since there aren’t perfect 1-1 mappings
between SDAG and GlobalISel components, this may also include features that are
currently implemented as part of the target lowerings, and tablegen isel
patterns. As we’re starting from a blank slate, we have an opportunity here to
think about what we might need from such a framework without the legacy cruft
(although we still have the high performance bar to meet).
>>>> 
>>>> I want to poll the community about what future requirements we
have for the GISel G_MI optimizer/combiner. The following are the general
requirements we have so far:
>>>> 
>>>> It should have at least equivalent, but hopefully better
runtime/compile time trade off than the DAGCombiner.
>>>> There needs to be flexibility in the design to allow targets to
run subsets of the overall optimizer. For example, some targets may want to
avoid trying to run certain types of optimizations like vector or FP combines if
they’re either not applicable, or not worth the compile time.
>>>> Have a reasonably concise way to write most optimizations. Hand
written C++ will always be an option, but there’s value in having easy to read
and reason about descriptions of transforms.
>>>> 
>>>> These requirements aren’t set in stone nor complete, but using
them as a starting point: a single monolithic “Generic MI combiner” component
doesn’t look like the right approach. Our current thinking is that, like we’ve
done with the Legalizer, the specific mechanics of the actual optimization
should be separated into it’s own unit. This would allow the combines to be
re-used at different stages of the pipeline according to target needs. Using the
current situation with instcombine as an example, there is no way to explicitly
pick and choose a specific subset of IC, it’s only available as a whole pass
with all the costs that entails.
>>>> 
>>>> The reasoning behind req 3 is that there may be compile time
savings available if we can describe in a declarative style the combines we want
to do, like it’s currently possible with tablegen patterns. This hasn’t been
proven out yet, but consider an alternative where we use the machine instruction
equivalent of the IR/PatternMatch tooling which allows easy and expressive
matching of IR sub-trees. A concern I have with using that as the main approach
to writing combines is that it’s easy to add new matchers in an routine which
re-computes information that’s previously been computed in previous match()
attempts. This form of back-tracking might be avoided if we can reason about a
group of combines together automatically (or perhaps we could add caching
capabilities to PatternMatch).
>>>> 
>>>> What would everyone else like to see from this?
>>> 
>>> It would be great to provide first-class support for maintaining
debug value information as a part of the new combine framework.
>>> 
>>> With SelectionDAG, we don't have a systematic way of preserving
debug locations and values across combines. This is a source of optimized
debugging bugs. If, as a part of the new framework, we could concisely express
that a RAUW-style combine simply transfers debug values from A to B, we might
define away some of these bugs [1].
>>> 
>>> Adrian put in place some infrastructure to do this in SelectionDAG
(r317825). However, auditing/fixing debug value transfer issues in hand-written
combines is time-consuming. I think it should be a goal of the new framework to
make this a bit easier.
>> 
>> +1 
>> 
>> Do you and/or Adrian also have thought on testing debug values?
Verification and testing strategies should/could be part of the design also.
> 
> I have two ideas on how to approach testing.
> 
> 1) Recycle existing targeted lit tests to test debug info preservation.
> 
> Consider a targeted lit test which look like this:
> RUN: opt -S -loop-reduce addrec-gep.ll -o - | FileCheck %s
> 
> We can repurpose this test by attaching synthetic debug information to the
IR, and then checking how much of it survives LSR. I prototyped this idea for
IR-level tests over the break. Adding a debug info test looks like this:
> RUN: opt -S -debugify -loop-reduce -check-debugify addrec-gep.ll -o - |
FileCheck ...
> 
> The check-debugify pass can determine which DILocations and DIVariables
went missing:
> 
>> CheckDebugify: Instruction with empty DebugLoc --   %lsr.iv1 = bitcast
double* %lsr.iv to i1*
>> CheckDebugify: Missing line 3
>> CheckDebugify: Missing line 4
>> ...
>> CheckDebugify: Missing line 33
> 
> This could be a handy way to create new targeted test cases at the IR/MIR
level. Something like this would have helped triage issues like llvm.org/
<http://llvm.org/>PR25630 as well.
> 
> 2) Assert that combines preserve debug info.
> 
> If there's an important API like "CombineTo(From, To)", it
could be useful to assert that the To node has at least as much debug info as
the From node. I'm experimenting with this in SelectionDAG (llvm.org/PR35338
<http://llvm.org/PR35338>). I don't yet know where these asserts
belong, how strict they should be, or if they need exception lists for certain
combines.
> 
> ---
> 
> Adrian and others have thought about these issues much more, so take these
ideas with a grain of salt :).
> 
> thanks,
> vedant
> 
> 
>>> 
>>> best,
>>> vedant
>>> 
>>> [1] To pick one at random, the '(zext (zextload x)) -> (zext
(truncate (zextload x)))' combine should transfer debug values from N to the
new ExtLoad, and from N0 to the new (trunc ExtLoad), but currently doesn't.
>>> 
>>>> 
>>>> Thanks,
>>>> Amara
>>>> _______________________________________________
>>>> 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>
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