Consideration of Cost associated with SEME regions.

[Vladimir Makarov]

On 07/02/2015 07:06 AM, Ajit Kumar Agarwal wrote:
> Sorry for the typo error. I meant exits instead of exists.
>
> The below is corrected.
>
> The Cost Calculation for a candidate to Spill in the Integrated Register Allocator(IRA) considers only the SESE regions.
> The Cost Calculation in the IRA should consider the SEME regions into consideration for spilling decisions.
IRA is a regional allocator.  It spills pseudo in a region. Currently 
regions are loops as the most important for optimizations.  Loops can 
have more one exit.  So your assumption that IRA works on SESE region is 
not accurate.

IRA with some work can be extended non-loop regions too.  I am not sure 
it will give significant improvement (LRA compensates partially this by 
inheritance in EBBs) but it definitely will slowdown IRA whose speed 
heavily depends on the number of regions (therefore a lot was done in 
IRA to decrease regions number by merging regions with low register 
pressure).

Of course, it would be interesting to implement non-loop regions in IRA 
and see the results.
> The Cost associated with the path that has un-matured exits should be less, thus making the more chances of spilling decision
> In the path of  un-matured exits. The path that has normal exit should be having a higher cost than the cost of un-matured
> exit and Spilling decisions has to made accordingly in order to spill inside the less frequency path with the un-matured exits
> than the high frequency Path with the normal exits.
>
> I would like to propose the above for consideration of cost associated with SEME regions in IRA.
IRA uses standard GCC evaluation of edge and BB execution frequencies, 
static (see predict.c) or based on execution profile (see *profile.c).  
Without profile usage it might be inaccurate. May be it can be 
improved.  For me it has more sense to work on this code instead of 
working on IRA code only.