Re: Unrolling factor heuristics for Loop Unrolling

[Oleg Endo <oleg dot endo at t-online dot de>]

On Thu, 2015-02-12 at 10:09 +0000, Ajit Kumar Agarwal wrote:
> Hello All:
> 
> The Loop unrolling without good unrolling factor heuristics becomes the performance bottleneck. The Unrolling factor heuristics based on minimum 
> Initiation interval is quite useful with respect to better ILP.  The minimum Initiation interval based on recurrence and resource calculation on Data 
> Dependency Graph  along with the register pressure can be used to add the unrolling factor heuristics. To achieve better ILP with the given schedule,
> the Loops unrolling and the scheduling are inter dependent and has been widely used in Software Pipelining Literature along with the more granular
> List and Trace Scheduling.
> 
> The recurrence calculation based on the Loop carried dependencies and the resource allocation based on the simultaneous access of the resources 
> Using the reservation table will give good heuristics with respect to calculation of unrolling factor. This has been taken care in the
> MII interval Calculation.
> 
> Along with MII, the register pressure should also be  considered in the calculation of heuristics for unrolling factor.
> 
> This enable better heuristics with respect to unrolling factor. The main advantage of the above heuristics for unrolling factor is that it can be 
> Implemented in the Code generation Level. Currently Loop unrolling is done much before the code generation. Let's go by the current implementation
> Of doing Loop unrolling optimization at the Loop optimizer level and unrolling happens. After the Current unrolling at the optimizer level the above heuristics
> Can be  used to do the unrolling at the Code generation Level with the accurate Register pressure calculation as done in the register allocator and the
> Unrolling is done at the code generation level. This looks feasible solution which I am going to propose for the above unrolling heuristics.
> 
> This enables the Loop unrolling done at the Optimizer Level  +  at the Code Generation Level. This double level of Loop unrolling is quite useful.
> This will overcome the shortcomings of the Loop unrolling at the optimizer level.
> 
> The SPEC benchmarks are the better candidates for the above heuristics instead of Mibench and EEMBC.

Not taking register pressure into account when unrolling (and doing
other optimizations/choices) is an old problem.  See also:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=20969

Cheers,
Oleg