[Bug tree-optimization/84037] [8 Regression] Speed regression of polyhedron benchmark since r256644

[amker at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84037

--- Comment #26 from amker at gcc dot gnu.org ---
(In reply to amker from comment #25)
> I tend to believe this is an register pressure based strength-reduction +
> lim problem than ivopts.
> 
> So given class of memory references like:
> 
>   reg = ...
> Loop:
>   MEM[iv_base + reg * 0];
>   MEM[iv_base + reg * 1];
>   MEM[iv_base + reg * 2];
>   MEM[iv_base + reg * 3];
>   MEM[iv_base + reg * 4];
>   MEM[iv_base + reg * 5];
>   MEM[iv_base + reg * 6];
>   MEM[iv_base + reg * 7];
> 
> The best arrangement probably would be:
> 
>   reg = ...
>   regX = reg * 3;
> Loop:
>   MEM[iv_base + reg * 0];
>   MEM[iv_base + reg * 1];
>   MEM[iv_base + reg * 2];
>   t = iv_base + regX;
>   MEM[t];
>   MEM[iv_base + reg * 4];
>   MEM[t + reg * 2];
>   MEM[iv_base + regX * 2];
>   MEM[t + reg * 4];
> 
> Depending on the register pressure, regX should be re-materialized in loop
> or hoisted as invariant.

Of course supported scales in addressing modes should be considered, but I
guess most targets only (if) support [base + reg * 2^x]?