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

[Philip Reames via llvm-dev]

On 04/18/2016 04:05 PM, Easwaran Raman via llvm-dev wrote:
>
>
> On Mon, Apr 18, 2016 at 3:25 PM, Chandler Carruth <chandlerc at gmail.com 
> <mailto:chandlerc at gmail.com>> wrote:
>
>     On Mon, Apr 18, 2016 at 3:20 PM Easwaran Raman <eraman at google.com
>     <mailto:eraman at google.com>> wrote:
>
>         On Mon, Apr 18, 2016 at 3:00 PM, Chandler Carruth via llvm-dev
>         <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>
>             On Mon, Apr 18, 2016 at 2:48 PM Hal Finkel
>             <hfinkel at anl.gov <mailto:hfinkel at anl.gov>> wrote:
>
>
>
>                 ------------------------------------------------------------------------
>
>                     *From: *"Xinliang David Li" <davidxl at google.com
>                     <mailto:davidxl at google.com>>
>
>                     On Mon, Apr 18, 2016 at 2:33 PM, Mehdi Amini
>                     <mehdi.amini at apple.com
>                     <mailto:mehdi.amini at apple.com>> wrote:
>
>                         In the current case at stake: the issue is
>                         that we can't make the Analysis library using
>                         anything from the ProfileData library.
>                         Conceptually there is a problem IMO.
>
>
>
>                     Yes -- this is a very good point.
>
>                 Independent of anything else, +1.
>
>
>             The design of ProfileData and reading profile information
>             in the entire middle end had a really fundamental
>             invariant that folks seem to have lost track of:
>
>             a) There is exactly *one* way to get at profile
>             information from general analyses and transforms: a
>             dedicated analysis pass that manages access to the profile
>             info.
>
>             b) There is exactly *one* way for this analysis to compute
>             this information from an *external* profile source:
>             profile metadata attached to the IR.
>
>             c) There could be many external profile sources, but all
>             of them should be read and then translated into metadata
>             annotations on the IR so that serialization /
>             deserialization preserve them in a common format and we
>             can reason about how they work.
>
>
>             This layering is why it is only a transform that accesses
>             ProfileData -- it is responsible for annotating the IR and
>             nothing else. Then the analysis uses these annotations and
>             never reads the data directly.
>
>             I think this is a really important separation of concerns
>             as it ensures that we don't get an explosion of different
>             analyses supporting various different subsets of profile
>             sources.
>
>
>             Now, the original design only accounted for profile
>             information *within* a function body, clearly it needs to
>             be extended to support intraprocedural information. But I
>             would still expect that to follow a similar layering where
>             we first read the data into IR annotations, then have an
>             analysis pass (this time a module analysis pass in all
>             likelihood) that brokers access to these annotations
>             through an API that can do intelligent things like
>             synthesizing it from the "cold" attribute or whatever when
>             missing.
>
>
>         Invariants b) and c) above are still true, but a) is not since
>         InlineCost directly calls ProfileSummary instead of through an
>         analysis pass.
>
>
>     Not quite -- ProfileSummary seems to only exist in the profile
>     *reading* code, so it isn't *just* an annotation on the IR.
>
> Sorry, I'm lost here. There is an IR annotation (module flag) called 
> ProfileSummary and this data is represented in memory by the 
> ProfileSummary class. . This class  provides methods to 
> serialize/de-serialize  this data into/from metadata. This class has 
> methods to compute this summary, but this is used only by the profile 
> readers and writers. I am not sure what you mean by "it isn't just* an 
> annotation on the IR".
If this is true, why is this class currently part of ProfileData? 
Shouldn't this live in IR?  Or to phrase this differently, why is an 
"analysis" over IR mixed in with the parsing code?
>
> - Easwaran
>
>
>     I think the problem here is that we have failed to build a proper
>     separate abstraction around the interprocedural profile data that
>     is extracted from IR annotations. That abstraction should have
>     nothing to do with reading profile data and so shouldn't live in
>     the ProifileData library, it should (IMO) live in the Analysis
>     library.
>
>           I'll add a module level pass as you suggest, but that still
>         needs breaking the ProfileData->Analysis dependence chain.
>
>
>     Well, this will also re-highlight the fundamental pass manager
>     problem as you won't have easy access to this analysis pass.
>
>
>         - Easwaran
>
>             -Chandler
>
>
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