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

[Chandler Carruth via llvm-dev]

On Mon, Apr 18, 2016 at 3:20 PM Easwaran Raman <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> wrote:
>
>> On Mon, Apr 18, 2016 at 2:48 PM Hal Finkel <hfinkel at anl.gov> wrote:
>>
>>>
>>>
>>> ------------------------------
>>>
>>> *From: *"Xinliang David Li" <davidxl at google.com>
>>>
>>> On Mon, Apr 18, 2016 at 2:33 PM, Mehdi Amini <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.

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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>>
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