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

[Xinliang David Li via llvm-dev]

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.

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
>
>