Cost Calculation on Loop Invariant on Arithmetic operations on RTL

Steven Bosscher
Tue Feb 17 10:44:00 GMT 2015

On Tue, Feb 17, 2015 at 9:45 AM, Ajit Kumar Agarwal wrote:
> Hello All:
> I can see the Loop invariant pass in the GCC on RTL considering the register pressure and the cost manipulation
> With respect to SET destination node in RTL.
> The Loop invariant takes care of only address arithmetic candidates of Loop invariance.
> In the function get_inv_cost, I can see the following check for the updation of comp_cost and the comp_cost
> Gets incremented with respect all the dependence on the def node of the invariant variable.
> If (!inv->cheap_address || inv-def->n_addr_uses < inv->def->n_uses)
>   (*comp_cost += inv->cost + inv->eqno
> Is there any specific reasons of addr_uses  less than the actual uses check for the address arithmetic candidate
> Of Loop invariant. I think we should be aggressive enough and can do the cost calculation without this check.
> One more point the above cost calculation should not be considering the register pressure_costs.
> Thoughts Please ?

When all is said and done, it's just a matter of what works best for
performance of the produced code. Heuristics and theory are often not
on the same page, even if the heuristics are "rationalized". You
should just try and see what the effects are if you change something.


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