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

[Piotr Padlewski via llvm-dev]

2016-09-02 15:11 GMT-07:00 Teresa Johnson <tejohnson 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
>>
>
> How much PGO data would need to be copied to summary?
>

AFAI got information form Easwaran - not much, about 3*15 integers.

>
>
>> 2. Replace CalleeInfo::ProfileCount with enum {None, Cold, Hot} computed
>> during computing summary.
>>
>
> It depends on how binary the inlining decisions that (will) use this are -
> do we want to give different thresholds to medium hot/cold vs very hot/cold?
>

We could also just later add MediumHot, MediumCold to the enum. But I am
not sure if we want to have that specific information. We can always mark
medium hot
as hot, and then maybe import more stuff, which should not be that harrmful.


>
>>
>> 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.
>>
>
> It was released in 3.9 and in Xcode so it has external users and I believe
> will need to support old versions going forward.
>

Is this really needed? I think it is important to be able to distinguish
between versions, so combining old summary with new one won't lead to some
weird behaviour and user will get an error. The user will maybe have to
recompile whole project, but on the other hand we won't spend
time on trying to figure out support for all the versions.



>
> Teresa
>
>
>>
>> Piotr
>>
>
>
>
> --
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
> 408-460-2413
>
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