llvm dev - Jul 2016 - [PM] I think that the new PM needs to learn about inter-analysis dependencies...

----- Original Message -----
> From: "Sean Silva" <chisophugis at gmail.com>
> To: "Chandler Carruth" <chandlerc at gmail.com>
> Cc: "Xinliang David Li" <davidxl at google.com>,
"llvm-dev"
> <llvm-dev at lists.llvm.org>, "Davide Italiano"
> <dccitaliano at gmail.com>, "Tim Amini Golling"
> <mehdi.amini at apple.com>, "Hal Finkel" <hfinkel at
anl.gov>, "Sanjoy
> Das" <sanjoy at playingwithpointers.com>, "Pete
Cooper"
> <peter_cooper at apple.com>
> Sent: Wednesday, July 13, 2016 2:25:52 AM
> Subject: Re: [PM] I think that the new PM needs to learn about
> inter-analysis dependencies...
> On Tue, Jul 12, 2016 at 11:39 PM, Chandler Carruth <
> chandlerc at gmail.com > wrote:
> > On Tue, Jul 12, 2016 at 11:34 PM Sean Silva < chisophugis at
gmail.com
> > >
> > wrote:
> 
> > > On Tue, Jul 12, 2016 at 11:32 PM, Xinliang David Li <
> > > davidxl at google.com > wrote:
> > 
> 
> > > > On Tue, Jul 12, 2016 at 10:57 PM, Chandler Carruth <
> > > > chandlerc at gmail.com > wrote:
> > > 
> > 
> 
> > > > > Yea, this is a nasty problem.
> > > > 
> > > 
> > 
> 
> > > > > One important thing to understand is that this is
specific to
> > > > > analyses which hold references to other analyses. While
this
> > > > > isn't
> > > > > unheard of, it isn't as common as it could be.
Still,
> > > > > definitely
> > > > > something we need to address.
> > > > 
> > > 
> > 
> 
> > > > We can call this type of dependencies (holding references)
> > > > hard-dependency. The soft dependency refers to the case
where
> > > > analysis 'A' depends on 'B' during
computation, but does not
> > > > need
> > > > 'B' once it is computed.
> > > 
> > 
> 
> > > > There are actually quite a few examples of hard-dependency
> > > > case.
> > > > For
> > > > instance LoopAccessInfo, LazyValueInfo etc which hold
> > > > references
> > > > to
> > > > other analyses.
> > > 
> > 
> 
> > > > Problem involving hard-dependency is actually easier to
detect,
> > > > as
> > > > it
> > > > is usually a compile time problem. Issues involving soft
> > > > dependencies are more subtle and can lead to wrong code gen.
> > > 
> > 
> 
> > > Did you mean to say that soft-dependency problems are easier to
> > > detect? At least my intuition is that soft-dependency is easier
> > > because there is no risk of dangling pointers to other analyses.
> > 
> 
> > The issue is that the fact that there is *any* dependency isn't
> > clear.
> 
> > However, I think the only real problem here are these "hard
> > dependencies" (I don't really like that term though). For
others,
> > only an analysis that is *explicitly* preserved survives. So I'm
> > not
> > worried about the fact that people have to remember this.
> 
> > The question is how often there are cross-data-structure
> > references.
> > David mentions a few examples, and I'm sure there are more, but it
> > isn't clear to me yet whether this is pervasive or occasional.
> 
> I just did a quick run-through of PassRegistry.def and this is what I
> found:
> Module analyses: 0/5 hold pointers to other analyses
> CallGraph: No pointers to other analyses.
> LazyCallGraph: No pointers to other analyses.
> ProfileSummaryAnalysis: No pointers to other analyses.
> TargetLibraryAnalysis: No pointers to other analyses.
> VerifierAnalysis: No pointers to other analyses.
> Module alias analyses: 1/1 keeps pointer to other analysis.
> GlobalsAA: Result keeps pointer to TLI (this is a function analysis).
> Function analyses: 9/17 keep pointers to other analysis
> AAManager: Its Result holds TLI pointer and pointers to individual AA
> result objects.
> AssumptionAnalysis: No pointers to other analyses.
> BlockFrequencyAnalysis: Its Result holds pointers to LoopInfo and
> BPI.
> BranchProbabilityAnalysis: Stores no pointers to other analyses.
> (uses LoopInfo to "recalculate" though)
> DominatorTreeAnalysis: Stores no pointers to other analyses.
> PostDominatorTreeAnalysis: Stores no pointers to other analyses.
> DemandedBitsAnalysis: Stores pointers to AssumptionCache and
> DominatorTree
> DominanceFrontierAnalysis: Stores no pointers to other analyses.
> (uses DominatorTreeAnalysis for "recalculate" though).
> LoopInfo: Uses DominatorTreeAnalysis for "recalculate" but stores
no
> pointers.
> LazyValueAnalysis: Stores pointers to AssumptionCache,
> TargetLibraryInfo, DominatorTree.
> DependenceAnalysis: Stores pointers to AliasAnalysis,
> ScalarEvolution, LoopInfo
> MemoryDependenceAnalysis: Stores pointers to AliasAnalysis,
> AssumptionCache, TargetLibraryInfo, DominatorTree
> MemorySSAAnalysis: Stores pointers to AliasAnalysis, DominatorTree
> RegionInfoAnalysis: Stores pointers to DomTree, PostDomTree,
> DomFrontier
> ScalarEvolutionAnalysis: Stores pointers to TargetLibraryInfo,
> AssumptionCache, DominatorTree, LoopInfo
> TargetLibraryAnalysis: Has no dependencies
> TargetIRAnalysis: Has no dependencies.
> Function alias analyses: 3/5 keep pointers to other analyses
> BasicAA: Keeps pointers to TargetLibraryInfo, AssumptionCache,
> DominatorTree, LoopInfo
> CFLAA: Keeps pointer to TargetLibraryInfo
> SCEVAA: Keeps pointer to ScalarEvolution
> ScopedNoAliasAA: No dependencies
> TypeBasedAA: No dependencies
> Total: 13/28 analyses (~50%) hold pointers to other analyses.
> Of the 15/28 analyses that don't hold pointers, 12/15 simply have no
> dependencies. Only 3/15 (BPI, LoopInfo, DominanceFrontier) have
> dependencies that are used just for a "recalculate" step that
> retains no pointers.
> So I think it is fair to say that analyses which hold pointers to
> other analyses is not an exceptional case. In fact, analyses that
> use other analyses just for a "recalculate" step seems to be the
> exceptional case (only 3/28 or about 10%)
Interesting. I'm not sure this is the right metric, however. There are lots
of analyses that hold pointers to other analyses but don't need to. The
analysis handle itself can be reacquired lazily if we care to do so. What's
truly problematic is holding pointers into another analysis's data
structures. To be concrete, holding a pointer to ScalarEvolution is not a
fundamental problem because we could make the analysis reacquire the pointer at
the start of every query. Holding SCEV* is the problem.

FWIW, I still think this is common enough to design a solution that makes it
easy to get this right.

-Hal 
> Since I like to visualize things, here is a quick graph of the
> dependencies between analyses which hold pointers to each other.
> Edge A -> B indicates "A can/does hold a pointer to B".
> dot file: http://reviews.llvm.org/P6603
> rendering: http://reviews.llvm.org/F2161058
> (I've been a bit loose with terminology here. A lot of the times that
> I say "analysis" I mean "the analysis' result
object" or I use the
> name of the analysis interchangeably with the analysis result
> object)
> -- Sean Silva
> > And even then it isn't clear how onerous explicitly managing this
> > in
> > invalidate overrides will be.
> 
> > > -- Sean Silva
> > 
> 
> > > > David
> > > 
> > 
> 
> > > > > Some ideas about mitigating and fixing it below.
> > > > 
> > > 
> > 
> 
> > > > > On Tue, Jul 12, 2016 at 6:15 PM Sean Silva <
> > > > > chisophugis at gmail.com
> > > > > >
> > > > > wrote:
> > > > 
> > > 
> > 
> 
> > > > > > How should we solve this? I see two potential
solutions:
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > 1. Analyses must somehow list the analyses they
depend on
> > > > > > (either
> > > > > > by
> > > > > > overriding "invalidate" to make sure
that they invalidate
> > > > > > them,
> > > > > > or
> > > > > > something "declarative" that would allow
the
> > > > > > AnalysisManager
> > > > > > to
> > > > > > walk
> > > > > > the transitive dependencies).
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > I think this is the right approach. I would personally
start
> > > > > by
> > > > > overriding the invalidate callback everywhere that it
is
> > > > > necessary,
> > > > > and see how bad that becomes.
> > > > 
> > > 
> > 
> 
> > > > > If it becomes common and burdensome, then we can change
the
> > > > > way
> > > > > invalidation works such that the analysis manager is
aware of
> > > > > the
> > > > > preserved analysis set in more detail, and have it
build up
> > > > > the
> > > > > necessary data structures to know in-advance whether it
must
> > > > > make
> > > > > an
> > > > > explicit invalidate call.
> > > > 
> > > 
> > 
> 
> > > > > However, I suspect this may not be *too* bad for two
reasons:
> > > > 
> > > 
> > 
> 
> > > > > a) As I mentioned above, I'm hoping there
aren't *too* many
> > > > > handles
> > > > > between different analyses. But I've not done a
careful
> > > > > examination,
> > > > > so we can check this.
> > > > 
> > > 
> > 
> 
> > > > > b) For many analyses that might trigger this, I think
we have
> > > > > a
> > > > > simpler option. If the analysis is *immutable* for any
reason
> > > > > --
> > > > > that is, it overrides its invalidate routine to always
return
> > > > > "false" the way TargetLibraryInfo should
(although I'm not
> > > > > sure
> > > > > it
> > > > > does currently), we shouldn't need to do this as it
shouldn't
> > > > > be
> > > > > getting cleared out. Does this make sense? Do others
see
> > > > > anything
> > > > > I'm missing with that approach?
> > > > 
> > > 
> > 
> 
> > > > > > 2. The AnalysisManager must do a somewhat
complicated dance
> > > > > > to
> > > > > > track
> > > > > > when analyses call back into it in order to get
other
> > > > > > analyses.
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > I would really rather avoid this, as currently the
analysis
> > > > > manager's
> > > > > logic here is very simple, and in many cases we only
need the
> > > > > analyses to *compute* our result, not to embed it.
I'm
> > > > > tihnking
> > > > > of
> > > > > stuff like Dominators is used to build LoopInfo, but
there
> > > > > isn't
> > > > > a
> > > > > stale handle there.
> > > > 
> > > 
> > 
> 
> > > > > There is another aspect of course in that if something
is
> > > > > preserving
> > > > > LoopInfo, it really should be preserving Dominators
too...
> > > > 
> > > 
> > 
> 
-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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On Wed, Jul 13, 2016 at 12:39 AM Hal Finkel <hfinkel at anl.gov> wrote:
> Interesting. I'm not sure this is the right metric, however. There are
> lots of analyses that hold pointers to other analyses but don't need
to.
> The analysis handle itself can be reacquired lazily if we care to do so.
> What's truly problematic is holding pointers into another
analysis's data
> structures. To be concrete, holding a pointer to ScalarEvolution is not a
> fundamental problem because we could make the analysis reacquire the
> pointer at the start of every query. Holding SCEV* is the problem.
>
Agreed. I suspect this is the dominant pattern.

FWIW, I still think this is common enough to design a solution that
makes
> it easy to get this right.
>
I somewhat suspect that it would be better to pass these in along the query
path rather than have results cache them. The reason is that if the user of
the analysis is aware of the other analyses it depends on, it will also be
aware of that cost and the reality of potentially wanting to preserve them.

I'd be interested in us spending some time trying to make this refactoring
and at least understanding how hard it is before we engineer a solution
that makes the analysis management more complex.

So as you seemed to agree in your other email (sorry for the heavily
branching thread) perhaps we should start with the potentially error prone
approach now, and see how much iteration can move toward "nothing to
do" by
lifting things into the query path rather than the result object.
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----- Original Message -----
> From: "Chandler Carruth" <chandlerc at gmail.com>
> To: "Hal Finkel" <hfinkel at anl.gov>, "Sean
Silva"
> <chisophugis at gmail.com>
> Cc: "Xinliang David Li" <davidxl at google.com>,
"llvm-dev"
> <llvm-dev at lists.llvm.org>, "Davide Italiano"
> <dccitaliano at gmail.com>, "Tim Amini Golling"
> <mehdi.amini at apple.com>, "Sanjoy Das"
> <sanjoy at playingwithpointers.com>, "Pete Cooper"
> <peter_cooper at apple.com>
> Sent: Wednesday, July 13, 2016 3:05:26 AM
> Subject: Re: [PM] I think that the new PM needs to learn about
> inter-analysis dependencies...
> On Wed, Jul 13, 2016 at 12:39 AM Hal Finkel < hfinkel at anl.gov >
> wrote:
> > Interesting. I'm not sure this is the right metric, however. There
> > are lots of analyses that hold pointers to other analyses but
don't
> > need to. The analysis handle itself can be reacquired lazily if we
> > care to do so. What's truly problematic is holding pointers into
> > another analysis's data structures. To be concrete, holding a
> > pointer to ScalarEvolution is not a fundamental problem because we
> > could make the analysis reacquire the pointer at the start of every
> > query. Holding SCEV* is the problem.
> 
> Agreed. I suspect this is the dominant pattern.
> > FWIW, I still think this is common enough to design a solution that
> > makes it easy to get this right.
> 
> I somewhat suspect that it would be better to pass these in along the
> query path rather than have results cache them. The reason is that
> if the user of the analysis is aware of the other analyses it
> depends on, it will also be aware of that cost and the reality of
> potentially wanting to preserve them.
> I'd be interested in us spending some time trying to make this
> refactoring and at least understanding how hard it is before we
> engineer a solution that makes the analysis management more complex.
I'm not too concerned about making analysis management more complex for this
purpose; handling transitive analysis invalidation was always going to have been
necessary.
> So as you seemed to agree in your other email (sorry for the heavily
> branching thread) perhaps we should start with the potentially error
> prone approach now, and see how much iteration can move toward
> "nothing to do" by lifting things into the query path rather than
> the result object.
Yes, I'm happy to see where this goes. The more I think about it, the more I
suspect that regardless of how easy the scheme is to use the main conceptual
challenge will be remembering/knowing that it needs to be used.

-Hal 

-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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On Wed, Jul 13, 2016 at 12:39 AM, Hal Finkel <hfinkel at anl.gov> wrote:
>
> ------------------------------
>
> *From: *"Sean Silva" <chisophugis at gmail.com>
> *To: *"Chandler Carruth" <chandlerc at gmail.com>
> *Cc: *"Xinliang David Li" <davidxl at google.com>,
"llvm-dev" <
> llvm-dev at lists.llvm.org>, "Davide Italiano" <dccitaliano
at gmail.com>, "Tim
> Amini Golling" <mehdi.amini at apple.com>, "Hal
Finkel" <hfinkel at anl.gov>,
> "Sanjoy Das" <sanjoy at playingwithpointers.com>,
"Pete Cooper" <
> peter_cooper at apple.com>
> *Sent: *Wednesday, July 13, 2016 2:25:52 AM
> *Subject: *Re: [PM] I think that the new PM needs to learn about
> inter-analysis dependencies...
>
>
>
>
> On Tue, Jul 12, 2016 at 11:39 PM, Chandler Carruth <chandlerc at
gmail.com>
> wrote:
>
>> On Tue, Jul 12, 2016 at 11:34 PM Sean Silva <chisophugis at
gmail.com>
>> wrote:
>>
>>> On Tue, Jul 12, 2016 at 11:32 PM, Xinliang David Li <davidxl at
google.com>
>>> wrote:
>>>
>>>>
>>>>
>>>> On Tue, Jul 12, 2016 at 10:57 PM, Chandler Carruth
<chandlerc at gmail.com
>>>> > wrote:
>>>>
>>>>> Yea, this is a nasty problem.
>>>>>
>>>>> One important thing to understand is that this is specific
to analyses
>>>>> which hold references to other analyses. While this
isn't unheard of, it
>>>>> isn't as common as it could be. Still, definitely
something we need to
>>>>> address.
>>>>>
>>>>
>>>> We can call this type of dependencies (holding references)
>>>> hard-dependency. The soft dependency refers to the case where
analysis 'A'
>>>> depends on 'B' during computation, but does not need
'B' once it is
>>>> computed.
>>>>
>>>> There are actually quite a few examples of hard-dependency
case. For
>>>> instance LoopAccessInfo, LazyValueInfo etc which hold
references to other
>>>> analyses.
>>>>
>>>> Problem involving hard-dependency is actually easier to detect,
as it
>>>> is usually a compile time problem. Issues involving soft
dependencies are
>>>> more subtle and can lead to wrong code gen.
>>>>
>>>
>>> Did you mean to say that soft-dependency problems are easier to
detect?
>>> At least my intuition is that soft-dependency is easier because
there is no
>>> risk of dangling pointers to other analyses.
>>>
>>
>> The issue is that the fact that there is *any* dependency isn't
clear.
>>
>> However, I think the only real problem here are these "hard
dependencies"
>> (I don't really like that term though). For others, only an
analysis that
>> is *explicitly* preserved survives. So I'm not worried about the
fact that
>> people have to remember this.
>>
>> The question is how often there are cross-data-structure references.
>> David mentions a few examples, and I'm sure there are more, but it
isn't
>> clear to me yet whether this is pervasive or occasional.
>>
>
>
> I just did a quick run-through of PassRegistry.def and this is what I
> found:
>
> Module analyses: 0/5 hold pointers to other analyses
> CallGraph: No pointers to other analyses.
> LazyCallGraph: No pointers to other analyses.
> ProfileSummaryAnalysis: No pointers to other analyses.
> TargetLibraryAnalysis: No pointers to other analyses.
> VerifierAnalysis: No pointers to other analyses.
>
> Module alias analyses: 1/1 keeps pointer to other analysis.
> GlobalsAA: Result keeps pointer to TLI (this is a function analysis).
>
> Function analyses: 9/17 keep pointers to other analysis
> AAManager: Its Result holds TLI pointer and pointers to individual AA
> result objects.
> AssumptionAnalysis: No pointers to other analyses.
> BlockFrequencyAnalysis: Its Result holds pointers to LoopInfo and BPI.
> BranchProbabilityAnalysis: Stores no pointers to other analyses. (uses
> LoopInfo to "recalculate" though)
> DominatorTreeAnalysis: Stores no pointers to other analyses.
> PostDominatorTreeAnalysis: Stores no pointers to other analyses.
> DemandedBitsAnalysis: Stores pointers to AssumptionCache and DominatorTree
> DominanceFrontierAnalysis: Stores no pointers to other analyses.
> (uses DominatorTreeAnalysis for "recalculate" though).
> LoopInfo: Uses DominatorTreeAnalysis for "recalculate" but stores
no
> pointers.
> LazyValueAnalysis: Stores pointers to AssumptionCache, TargetLibraryInfo,
> DominatorTree.
> DependenceAnalysis: Stores pointers to AliasAnalysis, ScalarEvolution,
> LoopInfo
> MemoryDependenceAnalysis: Stores pointers to AliasAnalysis,
> AssumptionCache, TargetLibraryInfo, DominatorTree
> MemorySSAAnalysis: Stores pointers to AliasAnalysis, DominatorTree
> RegionInfoAnalysis: Stores pointers to DomTree, PostDomTree, DomFrontier
> ScalarEvolutionAnalysis: Stores pointers to TargetLibraryInfo,
> AssumptionCache, DominatorTree, LoopInfo
> TargetLibraryAnalysis: Has no dependencies
> TargetIRAnalysis: Has no dependencies.
>
> Function alias analyses: 3/5 keep pointers to other analyses
> BasicAA: Keeps pointers to TargetLibraryInfo, AssumptionCache,
> DominatorTree, LoopInfo
> CFLAA: Keeps pointer to TargetLibraryInfo
> SCEVAA: Keeps pointer to ScalarEvolution
> ScopedNoAliasAA: No dependencies
> TypeBasedAA: No dependencies
>
>
> Total: 13/28 analyses (~50%) hold pointers to other analyses.
> Of the 15/28 analyses that don't hold pointers, 12/15 simply have no
> dependencies. Only 3/15 (BPI, LoopInfo, DominanceFrontier) have
> dependencies that are used just for a "recalculate" step that
retains no
> pointers.
> So I think it is fair to say that analyses which hold pointers to other
> analyses is not an exceptional case. In fact, analyses that use other
> analyses just for a "recalculate" step seems to be the
exceptional case
> (only 3/28 or about 10%)
>
> Interesting. I'm not sure this is the right metric, however. There are
> lots of analyses that hold pointers to other analyses but don't need
to.
> The analysis handle itself can be reacquired lazily if we care to do so.
>
Are you thinking of instead holding a pointer to the analysis manager?

> What's truly problematic is holding pointers into another
analysis's data
> structures. To be concrete, holding a pointer to ScalarEvolution is not a
> fundamental problem because we could make the analysis reacquire the
> pointer at the start of every query. Holding SCEV* is the problem.
>
Looks like SCEV* at least is held only by LoopAccessInfo. (Looks like LAA
holds Loop* too)
New updated rendering at http://reviews.llvm.org/F2161258
(DependenceAnalysis was missing some edges in my previous rendering and I
didn't have and I've added LoopAccessAnalysis; I've updated
http://reviews.llvm.org/P6603). Which other analyses vend data objects that
others might hold pointers to?

-- Sean Silva

> FWIW, I still think this is common enough to design a solution that makes
> it easy to get this right.
>
>  -Hal
>
>
> Since I like to visualize things, here is a quick graph of the
> dependencies between analyses which hold pointers to each other.
> Edge A -> B indicates "A can/does hold a pointer to B".
> dot file: http://reviews.llvm.org/P6603
> rendering: http://reviews.llvm.org/F2161058
>
> (I've been a bit loose with terminology here. A lot of the times that I
> say "analysis" I mean "the analysis' result object"
or I use the name of
> the analysis interchangeably with the analysis result object)
>
> -- Sean Silva
>
>
>>
>> And even then it isn't clear how onerous explicitly managing this
in
>> invalidate overrides will be.
>>
>>
>>>
>>> -- Sean Silva
>>>
>>>
>>>>
>>>> David
>>>>
>>>>
>>>>
>>>>>
>>>>> Some ideas about mitigating and fixing it below.
>>>>>
>>>>> On Tue, Jul 12, 2016 at 6:15 PM Sean Silva <chisophugis
at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> How should we solve this? I see two potential
solutions:
>>>>>> 1. Analyses must somehow list the analyses they depend
on (either by
>>>>>> overriding "invalidate" to make sure that
they invalidate them, or
>>>>>> something "declarative" that would allow the
AnalysisManager to walk the
>>>>>> transitive dependencies).
>>>>>>
>>>>>
>>>>> I think this is the right approach. I would personally
start by
>>>>> overriding the invalidate callback everywhere that it is
necessary, and see
>>>>> how bad that becomes.
>>>>>
>>>>> If it becomes common and burdensome, then we can change the
way
>>>>> invalidation works such that the analysis manager is aware
of the preserved
>>>>> analysis set in more detail, and have it build up the
necessary data
>>>>> structures to know in-advance whether it must make an
explicit invalidate
>>>>> call.
>>>>>
>>>>> However, I suspect this may not be *too* bad for two
reasons:
>>>>>
>>>>> a) As I mentioned above, I'm hoping there aren't
*too* many handles
>>>>> between different analyses. But I've not done a careful
examination, so we
>>>>> can check this.
>>>>>
>>>>> b) For many analyses that might trigger this, I think we
have a
>>>>> simpler option. If the analysis is *immutable* for any
reason -- that is,
>>>>> it overrides its invalidate routine to always return
"false" the way
>>>>> TargetLibraryInfo should (although I'm not sure it does
currently), we
>>>>> shouldn't need to do this as it shouldn't be
getting cleared out. Does this
>>>>> make sense? Do others see anything I'm missing with
that approach?
>>>>>
>>>>> 2. The AnalysisManager must do a somewhat complicated dance
to track
>>>>>> when analyses call back into it in order to get other
analyses.
>>>>>>
>>>>>
>>>>> I would really rather avoid this, as currently the analysis
manager's
>>>>> logic here is very simple, and in many cases we only need
the analyses to
>>>>> *compute* our result, not to embed it. I'm tihnking of
stuff like
>>>>> Dominators is used to build LoopInfo, but there isn't a
stale handle there.
>>>>>
>>>>>
>>>>>
>>>>> There is another aspect of course in that if something is
preserving
>>>>> LoopInfo, it really should be preserving Dominators too...
>>>>>
>>>>
>>>>
>
>
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
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On Wed, Jul 13, 2016 at 1:40 AM Sean Silva <chisophugis at gmail.com>
wrote:
> On Wed, Jul 13, 2016 at 12:39 AM, Hal Finkel <hfinkel at anl.gov>
wrote:
>
>> Interesting. I'm not sure this is the right metric, however. There
are
>> lots of analyses that hold pointers to other analyses but don't
need to.
>> The analysis handle itself can be reacquired lazily if we care to do
so.
>>
>
> Are you thinking of instead holding a pointer to the analysis manager?
>
I'm really concerned with using this approach as the common case. It
triggers the run of the analyses at very strange points (mid-query of some
other analysis) and forces us to pay the analysis manager lookup overhead
on every query. For many analyses, this overhead is larger than the actual
query.

There may be cases where this is the only sane way to manage things, but
this should be the exception rather than the rule IMO.

>
>
>> What's truly problematic is holding pointers into another
analysis's data
>> structures. To be concrete, holding a pointer to ScalarEvolution is not
a
>> fundamental problem because we could make the analysis reacquire the
>> pointer at the start of every query. Holding SCEV* is the problem.
>>
>
> Looks like SCEV* at least is held only by LoopAccessInfo. (Looks like LAA
> holds Loop* too)
>
Note that Loop (and SCC) are somewhat special as they are IRUnitTs and
might as a consequence be more reasonable to hold on to and expect
definitive invalidation to occur. But I say "might". I think this will
be
case-by-case depending on how they're being used.

> New updated rendering at http://reviews.llvm.org/F2161258
> (DependenceAnalysis was missing some edges in my previous rendering and I
> didn't have and I've added LoopAccessAnalysis; I've updated
> http://reviews.llvm.org/P6603). Which other analyses vend data objects
> that others might hold pointers to?
>
SCEV, Loop, SCC, DomTreeNode, and Region leap immediately to mind. and 3 of
those are what would be IRUnitTs (Region being the third, and its weird and
likely won't be in the new PM ever).
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