Re: [Patch,optimization]: Optimized changes in the estimate register pressure cost.

[Aaron Sawdey <acsawdey at linux dot vnet dot ibm dot com>]

On Sat, 2015-09-26 at 04:51 +0000, Ajit Kumar Agarwal wrote:
> I have made the following changes in the estimate_reg_pressure_cost function used 
> by the loop invariant and IVOPTS. 
> 
> Earlier the estimate_reg_pressure cost uses the cost of n_new variables that are generated by the Loop Invariant
>  and IVOPTS. These are not sufficient for register pressure calculation. The register pressure cost calculation should
> use the n_new + n_old (numbers) to consider the cost. n_old is the register  used inside the loops and the effect of
>  n_new new variables generated by loop invariant and IVOPTS on register pressure is based on how the new
> variables impact on register used inside the loops. The increase or decrease in register pressure is due to the impact
> of new variables on the register used  inside the loops. The register-register move cost or the spill cost should consider
> the cost associated with register used and the new variables generated. The movement  of new variables increases or 
> decreases the register pressure, which is based on  overall cost of n_new + n_old variables.
> 
> The increase and decrease in register pressure is based on the overall cost of n_new + n_old as the changes in the 
> register pressure caused due to new variables is based on how the changes behave with respect to the register used 
> in the loops.
> 
> Thus the register pressure caused to new variables is based on the new variables and its impact on register used inside
>  the loops and thus consider the overall  cost of n_new + n_old.
> 
> Bootstrap for i386 and reg tested on i386 with the change is fine.
> 
> SPEC CPU 2000 benchmarks are run and there is following impact on the performance
> and code size.
> 
> ratio with the optimization vs ratio without optimization for INT benchmarks
> (3807.632 vs 3804.661)
> 
> ratio with the optimization vs ratio without optimization for FP benchmarks
> ( 4668.743 vs 4778.741)
> 
> Code size reduction with respect to FP SPEC CPU 2000 benchmarks
> 
> Number of instruction with optimization = 1094117
> Number of instruction without optimization = 1094659
> 
> Reduction in number of instruction with the optimization = 542 instruction.
> 
> [Patch,optimization]: Optimized changes in the estimate
>  register pressure cost.
> 
> Earlier the estimate_reg_pressure cost uses the cost of n_new variables that
> are generated by the Loop Invariant and IVOPTS. These are not sufficient for
> register pressure calculation. The register pressure cost calculation should
> use the n_new + n_old (numbers) to consider the cost. n_old is the register
> used inside the loops and the affect of n_new new variables generated by
> loop invariant and IVOPTS on register pressure is based on how the new
> variables impact on register used inside the loops.
> 
> ChangeLog:
> 2015-09-26  Ajit Agarwal  <ajitkum@xilinx.com>
> 
> 	* cfgloopanal.c (estimate_reg_pressure_cost) : Add changes
> 	to consider the n_new plus n_old in the register pressure
> 	cost.
> 
> Signed-off-by:Ajit Agarwal ajitkum@xilinx.com

Ajit,
  It looks to me like your change doesn't do anything at all inside the
loop-invariant.c code. There it's doing a difference between two
estimate_reg_pressure_cost calls so adding n_old (regs_used) to both is
canceled out.

      size_cost = (estimate_reg_pressure_cost (new_regs[0] + regs_needed[0],
                                               regs_used, speed, call_p)
                   - estimate_reg_pressure_cost (new_regs[0],
                                                 regs_used, speed, call_p));

I'm not quite sure I understand the "why" of the heuristic you've added
here -- can you explain your reasoning further?

> 
> Thanks & Regards
> Ajit
> 

Thanks,
    Aaron

-- 
Aaron Sawdey, Ph.D.  acsawdey@linux.vnet.ibm.com
050-2/C113  (507) 253-7520 home: 507/263-0782
IBM Linux Technology Center - PPC Toolchain