[Patch] Teach RTL ifcvt to handle multiple simple set instructions

[Bernd Schmidt]

On 09/10/2015 11:11 PM, Jeff Law wrote:
> I think that's probably what James is most interested in getting some
> ideas around -- the cost model.
>
> I think the fundamental problem is BRANCH_COST isn't actually relative
> to anything other than the default value of "1".  It doesn't directly
> correspond to COSTS_N_INSNS or anything else.  So while using
> COSTS_N_INSNS (BRANCH_COST)) would seem to make sense, it actually
> doesn't.  It's not even clear how a value of 10 relates to a value of 1
> other than it's more expensive.
>
> ifcvt (and others) comparing to magic #s is more than a bit lame.  But
> with BRANCH_COST having no meaning relative to anything else I can see
> why Richard did things that way.
>
> In an ideal world we'd find some mapping from BRANCH_COST that relates
> to CONST_N_INSNS.  I suspect a simple mapping doesn't necessarily exist
> and we'll likely regress targets with any simplistic mapping.  But maybe
> now is the time to address that fundamental problem and deal with the
> fallout.

I think the right approach if we want to fix this is a new 
branch_cost_ninsns target hook (maybe with arguments taken_percentage, 
predictability), and gradually move everything to use that instead of 
BRANCH_COST.


Bernd