Unrolling factor heuristics for Loop Unrolling

[Ajit Kumar Agarwal <ajit dot kumar dot agarwal at xilinx dot com>]

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.

Thanks & Regards
Ajit