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

[Piotr Padlewski via llvm-dev]

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.

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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