[llvm-dev] Move InlineCost.cpp out of Analysis?

[Hal Finkel via llvm-dev]

----- Original Message -----

> From: "Chandler Carruth" <chandlerc at gmail.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Mehdi Amini" <mehdi.amini at apple.com>, "via llvm-dev"
> <llvm-dev at lists.llvm.org>, "Xinliang David Li" <davidxl at google.com>
> Sent: Monday, April 18, 2016 4:54:36 PM
> Subject: Re: [llvm-dev] Move InlineCost.cpp out of Analysis?

> On Mon, Apr 18, 2016 at 2:46 PM Hal Finkel < hfinkel at anl.gov > wrote:

> > > From: "Chandler Carruth" < chandlerc at gmail.com >
> > 
> 
> > > To: "Xinliang David Li" < davidxl at google.com >
> > 
> 
> > > Cc: "Mehdi Amini" < mehdi.amini at apple.com >, "Hal Finkel" <
> > > hfinkel at anl.gov >, "via llvm-dev" < llvm-dev at lists.llvm.org >
> > 
> 
> > > Sent: Monday, April 18, 2016 4:31:05 PM
> > 
> 
> > > Subject: Re: [llvm-dev] Move InlineCost.cpp out of Analysis?
> > 
> 
> > > On Mon, Apr 18, 2016 at 2:28 PM Xinliang David Li <
> > > davidxl at google.com > wrote:
> > 
> 

> > > > On Mon, Apr 18, 2016 at 2:18 PM, Chandler Carruth <
> > > > chandlerc at gmail.com > wrote:
> > > 
> > 
> 

> > > > > The difference between Analysis and Transforms is *not* about
> > > > > passes,
> > > > > but about what the code *does*.
> > > > 
> > > 
> > 
> 

> > > > > Code for mutating the IR should be in Transforms, and code
> > > > > that
> > > > > analyzes the IR without mutating it should be in Analysis.
> > > > > This
> > > > > is
> > > > > why, for example, InstructionSimplify is in Analysis -- it
> > > > > does
> > > > > not
> > > > > mutate the IR in any way.
> > > > 
> > > 
> > 
> 

> > > > > So I think InlineCost and similar things should stay in the
> > > > > Analysis
> > > > > library regardless of whether they are passes or not.
> > > > 
> > > 
> > 
> 
> > > > Is that the only criteria (IR mod or not) ?
> > > 
> > 
> 
> > > Given a public API, yes, that should be the only criteria IMO.
> > 
> 

> > > > Most of the transformations have pass specific analysis (that
> > > > are
> > > > not
> > > > shared with other clients) -- those code stay with the
> > > > transformation -- it does not make sense to move those code in
> > > > to
> > > > Analysis.
> > > 
> > 
> 
> > > But I would expect these to also not expose public APIs to the
> > > analyses. Provided the public API is limited to the
> > > transformation,
> > > I think the code be closely located makes sense.
> > 
> 
> > I'm not sure the situation is as clear cut as you make it out to
> > be.
> > We can factor common components out of different transformations
> > (e.g. out of different inliner implementations), including common
> > components that don't modify the IR, without that component forming
> > what I would generally consider an analysis. In this case, we're
> > really talking about the encapsulation of the inliner's cost
> > heuristic, and the generality of this information is questionable.
> > To put it another way, while I'm fine with someone reusing this
> > logic to build a new inliner, I'd be very skeptical of using it for
> > any other purpose. That's why it does not really feel like an
> > analysis to me. This is not a strong feeling, however, and I'm also
> > fine with it staying in Analysis.
> 
> I don't think that everything that is an analysis needs to be
> completely general though. Certainly, we need to be very clear and
> careful in what terms we describe the API to make the quality and
> nature of the results unsurprising, but I think that the API around
> the inline cost is reasonably good about that -- it seems to pretty
> clearly indicate that this is a threshold determining mechanism and
> not something that provides a holistic view of the impact of a
> potential inlining decision.

> I actually think it would be good if essentially *everything* we can
> put a reasonable public API around and which doesn't mutate the IR
> were moved to the analysis library. As an example of why I think
> this, without this it is very hard to know what code can be re-used
> in analysis passes. And the constraint there is really exactly this:
> it has a public API that can be re-used, and it doesn't mutate the
> IR. Nothing more or less that I see.

> Naturally, we don't need to go on a campaign to move everything
> *right now*, and we can even be fairly lazy about it, but I do think
> that this direction is the correct long-term design of the libraries
> here.
In general I agree, but the inliner's cost heuristic is a bit special in this regard. It is special because it contains tuned thresholds that are meaningful only in the context of inlining as performed by the current inliner. Were we to change the inliner's overall scheme (e.g. made it more top-down) those would need to change. If someone wanted to build some higher-level analysis that made use of the InlineCost information, the fact that they had to move things from Transforms/Utils to Anslysis would be a good red flag that they're probably using the wrong tool. Obviously it might be the right tool, but it is probably something we'd want to think about. A second inliner, however, might be sufficiently close in design to the current one that the tuned thresholds might be applicable. 

In short, I like your proposal, but I'd further propose that we say that anything that does not mutate the IR and does not make assumptions about its ultimate client should be an analysis. 

-Hal 

-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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