[PATCH] D54175: [PGO] context sensitive PGO

[Rong Xu via llvm-commits]

On Thu, Nov 8, 2018 at 11:16 AM Vedant Kumar via Phabricator <
reviews at reviews.llvm.org> wrote:

> vsk added subscribers: tmsriram, tejohnson, rnk.
> vsk added a comment.
>
> Hi Rong, at a high-level, I like what this patch is doing. I'll try to
> leave in-depth comments by next week -- please ping the review otherwise.
>

Sounds good. Looking forward to your reviews.


>
> > These suboptimal profile can greatly affect some downstream
> optimizations, in particular for machine basic block placement optimization.
>
> If it's possible to get these numbers, it'd be interesting to know how the
> improvements from this patch compare to link-time or post-link block
> ordering tools (Bolt).
>

We mainly use a key google benchmark for performance evaluation. Note that
this benchmark has been highly tuned. A performance gain of 0.5% is
considered significant.

Our experiments shows 1.5% to 2% from CSPGO.  The improvement is mainly
from machine basicblock placement. CSPGO also sets the accurate function
entry count (for the cases that inline are cross module). That will make
the hot-text section more dense. This can be seen from instruction heat
map. But the performance improvement was small.

BOLT can have similar performance boosts from  basic block reordering if
it's applied to regular PGO binary (i.e. ~2%)
BOLT can improve the CSPGO binary by 0.5% and that is from function
splitting, which is not currently enabled in llvm.


>
> https://reviews.llvm.org/D54175
>
>
>
>
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