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

[Mehdi Amini via llvm-dev]

> On Sep 2, 2016, at 4:13 PM, Piotr Padlewski <piotr.padlewski at gmail.com> wrote:
> 
> The profile summary is saved in the global metadata ASAIK. If we want to calculate if something is hot/cold while choosing functions for importing, we would either need to read whole Module (which we clearly don't want to do)
> or duplicate this information in the summary, so we could get it without reading Module.

When I say “explain a bit more” I meant:

- what is the profile summary?
- how do you compute the hot/cold information from it and from the count we attache to edges?

— 
Mehdi

> 
> 2016-09-02 15:49 GMT-07:00 Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>>:
> 
>> On Sep 2, 2016, at 3:16 PM, Piotr Padlewski <piotr.padlewski at gmail.com <mailto:piotr.padlewski at gmail.com>> wrote:
>> 
>> 
>> 
>> 2016-09-02 15:04 GMT-07:00 Xinliang David Li <davidxl at google.com <mailto:davidxl at google.com>>:
>> On Fri, Sep 2, 2016 at 2:58 PM, Piotr Padlewski
>> <piotr.padlewski at gmail.com <mailto: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. 
> 
> Can you explain a bit more the issue here?
> 
> Thanks,
> 
> Mehid
> 
> 
>> If we will follow 2) then we won't have to duplicate the data.
>>  
>> >
>> > 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
> 
> 

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