Re: ix86_address_cost

[Jan Hubicka <jh at suse dot cz>]

> Hello,
> 
> what is the purpose of the following code in ix86_address_cost:
> 
>   /* More complex memory references are better.  */
>   if (parts.disp && parts.disp != const0_rtx)
>      cost--;
> ?
> 
> It in effect claims that [reg] is more expensive than [reg - 4].
> This leads ivopts to generate pretty weird code; for example on code
> of type

The main idea behind this hack is to force CSE to use complex addressing
modes (ie if it uses reg-4 instead of reg2, one might hope for that
"reg2=reg-4" statement will get dead and elliminated completely).

This would ineed be interesting thing to re-benchmark so we see if it
still does any good.

Honza
> 
> for (i = 0; i < n; i++)
>   {
>     a[i] = something;
>     b[i] = something;
>     c[i] = something;
>   }
> 
> it decides that it may be better to use an induction variable with
> initial value 1:
> 
> for (ivtmp = 1; ivtmp - 1 < n; ivtmp++)
>   {
>     a[ivtmp - 1] = something;
>     b[ivtmp - 1] = something;
>     c[ivtmp - 1] = something;
>   }
> 
> Since obviously we gain 1 in cost for every use in address, and lose
> relatively little on having to express ivtmp - 1.
> 
> Zdenek