[Bug tree-optimization/88760] GCC unrolling is suboptimal

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88760

--- Comment #33 from rguenther at suse dot de <rguenther at suse dot de> ---
On Fri, 11 Oct 2019, segher at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88760
> 
> --- Comment #31 from Segher Boessenkool <segher at gcc dot gnu.org> ---
> Gimple passes know a lot about machine details, too.
> 
> Irrespective of if this is "low-level" or "high-level", it should be done
> earlier than it is now.  It should either be done right after expand, or
> somewhere before it.  Doing it in gimple seems easiest?

Sure it's easiest in GIMPLE.  But I'd say it shouldn't be done earlier,
it should be done correctly - RTL unroll simply unrolls everything
without any good costing.  That is what needs to be fixed.

Other than that, where we do cunroll on GIMPLE is a good spot I guess
(need to do it _before_ IVOPTs at least, though I wanted to push that
one later as well).