[LLVMdev] RFC - Improvements to PGO profile support

Chandler Carruth chandlerc at google.com
Tue Mar 24 11:29:59 PDT 2015

Sorry I haven't responded earlier, but one point here still doesn't make
sense to me:

On Tue, Mar 24, 2015 at 10:27 AM, Xinliang David Li <davidxl at google.com>

> Diego and I have discussed this according to the feedback received. We
> have revised plan for this (see Diego's last reply).  Here is a more
> detailed re-cap:
> 1) keep MD_prof definition as it is today; also keep using the
> frequency propagation as it is (assuming programs with irreducible
> loops are not common and not important. If it turns out to be
> otherwise, we will revisit this).
> 2) fix all problems that lead to wrong 'frequency/count' computed from
> the frequency propagation algorithm
>    2.1) relax 32bit limit

I still don't understand why this is important or useful.... Maybe I'm just
missing something.

Given the current meaning of MD_prof, it seems like the result of limiting
this to 32-bits is that the maximum relative ratio of probabilities between
two successors of a basic block with N successors is (2 billion / N):1 --
what is the circumstance that makes this resolution insufficient?

It also doesn't seem *bad* per-se, I just don't see what it improves, and
it does cost memory...
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