[RFC, ivopts] fix bugs in ivopts address cost computation

[Oleg Endo]

On Fri, 2012-06-08 at 22:33 -0400, Hans-Peter Nilsson wrote:
> On Tue, 5 Jun 2012, Sandra Loosemore wrote:
> 
> > (1) While the address cost computation is assuming in some situations
> > that pre/post increment/decrement addressing will be used if
> > supported by the target, it isn't actually using the target's address
> > cost for such forms -- instead, just the cost of the form that would
> > be used if autoinc weren't available/applicable.
> 
> There are lots of bugzilla entries complaining about bad
> autoinc/dec generation.  Maybe your patch solves some of them?
> 

I've tried some of the cases mentioned in 
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50749
with Sandra's patch applied.  Unfortunately it didn't help much.  There
seem to be other things going wrong with auto-inc-dec.

BTW, auto-inc-dec uses 'set_src_cost' in 'attempt_change' to determine
the address costs.  At least the SH target will not respond to that
properly.  I was thinking of adding something to sh_rtx_costs to invoke
sh_address_cost as a fix for that, but on the other hand I was wondering
why the target's address cost function isn't used in auto-inc-dec
directly ... 

Cheers,
Oleg