Re: [PATCH 2/4] ipa-cp: Propagation boost for recursion generated values

[Jan Hubicka <hubicka@ucw.cz>]

> Hi,
> >
> > If you boost every self fed value by factor of 6, I wonder how quickly
> > we run into exponential explosion of the cost (since the frequency
> > should be close to 1 and 6^9=10077696....
> 
> The factor of six is applied once for an entire SCC, so we'd reach this
> huge number only if there was a chain of nine different recursive
> functions - with this patch we assume each one will recurse six times,
> so the result is indeed a huge execution count estimate.
> 
> The constant is not used for the "self generated" values like those in
> exchange, those are handled by the else branch below.  For those we
> expect the recursion happens 8 times, because that is how many values we
> generate, but the boost is different depending on the recursion depth.
> 
> >
> > I think it would be more robust to simply assume that the job will
> >distribute evenly across the clones.  How hard is to implement that?
> 
> This is not an update of counters.  The code tries to estimate execution
> time improvement that is will be possible in callees if we clone for
> this particular value and so is based on call graph edge frequencies (so
> that if in a callee we can save 5 units of time and the frequency is 5,
> we estimate we will save 25).  The code has the advantage that it is
> universal for both situations when profile feedback is and is not
> available.

I guess the patch is OK then.

Thanks,
Honza
> 
> Martin
>