[Bug target/115934] [15 Regression] nvptx vs. ivopts: replace constant_multiple_of with aff_combination_constant_multiple_p [PR114932]

[tnfchris at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115934

--- Comment #7 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
(In reply to Thomas Schwinge from comment #6)
> Tamar, Richard, thanks for having a look.
> 
> (In reply to Tamar Christina from comment #4)
> > This one looks a bit like costing, [...]
> 
> I see.  So we (I) shall later re-visit this PR in context of
> <https://inbox.sourceware.org/
> 001101dad3b2$ef215730$cd640590$@nextmovesoftware.com> "[nvptx PATCH]
> Implement rtx_costs target hook for nvptx backend", and, if necessary,
> follow-up work:
> 

I believe so, it looks like try_improve_iv_set does nothing for nvptx because
it tries to look for TARGET_ADDRESS_COST and in it's absence tries to use
TARGET_RTX_COSTS both of which are missing.

Because of this it can't compare the different IVs and the costs all end up
being the same.

So basically it just ends up picking the first one from the list, which in this
case just happens to be worse off. 

> > I don't however see an implementation of TARGET_ADDRESS_COST for the target.