On Fri, Mar 7, 2025 at 8:42 PM Jan Hubicka <hubi...@ucw.cz> wrote:
>
> > > This is OK. In general, I think we could also go with assert on
> > > mem_cost <= 2, since that is kind of bogus setting (I don't think we
> > > will ever need to support x86 CPU with memory stores being as cheap as
> > > reg-reg moves), but current form is good.
> >
> > Unless the loading/storing integer costs in x86-tune-costs.h are unrelated,
> > there are tons of tunings which have cost 2:
> >
> > grep
> > '\(processor_costs\|2.*cost.of.loading.integer\|2.*cost.of.storing.integer\)'
> > x86-tune-costs.h
> > struct processor_costs ix86_size_cost = {/* costs for tuning for size */
> > {2, 2, 2}, /* cost of loading integer registers
> > {2, 2, 2}, /* cost of storing integer registers
> > */
> > {2, 2, 2}, /* cost of loading integer registers
> > {2, 2, 2}, /* cost of storing integer registers
> > */
> > struct processor_costs i386_cost = { /* 386 specific costs */
> > {2, 4, 2}, /* cost of loading integer registers
> > {2, 4, 2}, /* cost of storing integer registers
> > */
> > {2, 4, 2}, /* cost of loading integer registers
> > {2, 4, 2}, /* cost of storing integer registers
> > */
> > struct processor_costs i486_cost = { /* 486 specific costs */
> > {2, 4, 2}, /* cost of loading integer registers
> > {2, 4, 2}, /* cost of storing integer registers
> > */
> > {2, 4, 2}, /* cost of loading integer registers
> > {2, 4, 2}, /* cost of storing integer registers
> > */
> > struct processor_costs pentium_cost = {
> > {2, 4, 2}, /* cost of loading integer registers
> > {2, 4, 2}, /* cost of storing integer registers
> > */
> > {2, 4, 2}, /* cost of loading integer registers
> > {2, 4, 2}, /* cost of storing integer registers
> > */
> > struct processor_costs lakemont_cost = {
> > {2, 4, 2}, /* cost of loading integer registers
> > {2, 4, 2}, /* cost of storing integer registers
> > */
> > {2, 4, 2}, /* cost of loading integer registers
> > {2, 4, 2}, /* cost of storing integer registers
> > */
> > struct processor_costs pentiumpro_cost = {
> > {2, 2, 2}, /* cost of storing integer registers
> > */
> > {2, 2, 2}, /* cost of storing integer registers
> > */
> > struct processor_costs geode_cost = {
> > {2, 2, 2}, /* cost of loading integer registers
> > {2, 2, 2}, /* cost of storing integer registers
> > */
> > {2, 2, 2}, /* cost of loading integer registers
> > {2, 2, 2}, /* cost of storing integer registers
> > */
> > struct processor_costs k6_cost = {
> > {2, 3, 2}, /* cost of storing integer registers
> > */
> > {2, 3, 2}, /* cost of storing integer registers
> > */
>
> Hmm, this is bit odd, indeed. ix86_register_move_cost return 2 for
> integer-integer and all costs should be relative to that to get
> meaningful register class preferences.
>
> This is even documented:
>
> /* Start of register allocator costs. integer->integer move cost is 2. */
> 4, /* cost for loading QImode using movzbl
> */
> {2, 4, 2}, /* cost of loading integer registers
> in QImode, HImode and SImode.
> Relative to reg-reg move (2). */
> {2, 4, 2}, /* cost of storing integer registers
> */
>
>
> Thus I think all loads/stores should be more expensive than that (at
> least by 1 since they are longer). I can not claim it predates me since
> I added k6_cost.
>
>
> I will see how changing those costs to something more realistic changes
> the codegeon. these costs were tuned for old RA and I do not think
> (possibly except for lakemont) they were retuned for IRA. But even old
> RA was comparing relative cost of a spill to reg-reg move...
>
> So the patch is OK without an assert.
> Honza
I will check it in tomorrow.
Thanks.
--
H.J.
