[rtlanal] Do a better job of costing parallel sets containing flag-setting operations.

[Richard Earnshaw (lists)]

On 19/06/17 16:09, Segher Boessenkool wrote:
> On Mon, Jun 19, 2017 at 03:45:23PM +0100, Richard Earnshaw (lists) wrote:
>>>> At present all such insns are treated as having unknown cost (ie 0) and
>>>> combine assumes that such insns are infinitely more expensive than any
>>>> other insn sequence with a non-zero cost.
>>>
>>> That's not what combine does: it optimistically assumes any combination
>>> with unknown costs is an improvement.
>>
>> Actually the logic is
>>
>>   int reject = old_cost > 0 && new_cost > old_cost;
>>
>> So reject will never be true if old cost is zero.
> 
> Yes, exactly; and neither if new_cost is zero.  If any cost is unknown
> combine just hopes for the best.
> 
> 
> Segher
> 

Yeah, and I'm not suggesting we change the logic there (sorry if the
description was misleading).  Instead I'm proposing that we handle more
cases for parallels to not return zero.

R.