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

[Xinliang David Li via llvm-dev]

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


>
> 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