[llvm-dev] [ThinLTO] Importing based on PGO data

[Xinliang David Li via llvm-dev]

On Fri, Sep 2, 2016 at 3:44 PM, Easwaran Raman <eraman at google.com> wrote:

> IMO, multiple levels of hotness/coldness is not very useful.  If that
> proves to be beneficial, tweaking the hotness cutoffs and the inline
> threshold for hot/cold sites should get to similar performance on average
> without the additional complexity.
>
>
I think it can be useful -- even though the inliner's implementation may
choose to not use the information.

Suppose we have compile time/import size budget, having more fine grained
information helps the importer prioritize the callsites based on that
information -- similar to priority based inliner. Besides I don't see how
it adds more complexity.

David



> On Fri, Sep 2, 2016 at 3:32 PM, Teresa Johnson <tejohnson at google.com>
> wrote:
>
>>
>>
>> On Fri, Sep 2, 2016 at 3:30 PM, Xinliang David Li <davidxl at google.com>
>> wrote:
>>
>>> On Fri, Sep 2, 2016 at 3:16 PM, Piotr Padlewski
>>> <piotr.padlewski at gmail.com> wrote:
>>> >
>>> >
>>> > 2016-09-02 15:04 GMT-07:00 Xinliang David Li <davidxl at google.com>:
>>> >>
>>> >> On Fri, Sep 2, 2016 at 2:58 PM, Piotr Padlewski
>>> >> <piotr.padlewski at gmail.com> wrote:
>>> >> > Hi,
>>> >> > I am working right now on importing based on PGO/FDO data. There is
>>> one
>>> >> > issue that I found - when we calculate the list of imports, we
>>> can't get
>>> >> > the
>>> >> > ProfileSummaryInfo, which is the best and I
>>> >> > think only valid way of checking if callsite/callee is hot
>>> >> > (isHotCount()).
>>> >> > There are 2 solutions that I come up with Teresa and Easwaran:
>>> >> >
>>> >> > 1. Add PGO data to summary
>>> >> > 2. Replace CalleeInfo::ProfileCount with enum {None, Cold, Hot}
>>> computed
>>> >> > during computing summary.
>>> >>
>>> >>
>>> >> Don't we already have edge profile count in the callgraph summary?
>>> >> I think what is missing is the Profile SUmmary data itself -- that one
>>> >> should be copied over to thinLTO summary so that the importing
>>> >> analysis can use. However I we should not need to duplicate the
>>> >> information in every module.
>>> >>
>>> >> David
>>> >>
>>> > Yes we do have edge profile cout, but in order to compare it with
>>> global
>>> > couts we need Profile Summary as you said.
>>> > If we will follow 2) then we won't have to duplicate the data.
>>>
>>> Ok -- basically the profile summary is already consumed before
>>> writing.  If you go this route, I think you need more enum values for
>>> fine tuning: for instance hot_10 --> 10 percentile hotness, ... hot_90
>>> etc, and cold_99, cold_999 etc.
>>>
>>
>> Right, that relates to my question about how the inliner will eventually
>> use the same profile summary data, and whether it will be useful to
>> distinguish between various levels of hotness.
>>
>> Teresa
>>
>>
>>> David
>>>
>>>
>>> >
>>> >>
>>> >> >
>>> >> > I like the 2. much more. It will reduce the summary size slightly
>>> and I
>>> >> > don't think we will need ProfileCount anywhere else.
>>> >> >
>>> >> > The other thing I would like to mention is that I think we should
>>> start
>>> >> > using the summary versioning and drop support of old version.
>>> >> > ThinLTO doesn't have enough users right now and parsing many
>>> versions of
>>> >> > summary will just add additional cost, that will start to grow.
>>> >> >
>>> >> > Piotr
>>> >
>>> >
>>>
>>
>>
>>
>> --
>> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>> 408-460-2413
>>
>
>
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