IRA costs tweaks, PR 56069

[Richard Sandiford]

Bernd Schmidt <bschmidt@redhat.com> writes:
> On 03/02/2016 10:53 PM, Vladimir Makarov wrote:
>>> 2. update_costs_from_allocno records a cost update not just for the
>>> initial allocno, but for each of the visited ones. I can sort of see
>>> an argument for doing that (let's say if you assign an allocno in the
>>> middle of a copy chain you'd want the tail end of the chain to be
>>> reset), but in practice I don't think the present algorithm can work
>>> at all. In the case of an allocno in the middle of a copy chain the
>>> restore would progress in both directions, and in any case it looks
>>> like this approach can end up double-counting things when restoring
>>> costs.
>>>
>> It is just a heuristic.  Richard Sandiford proposed this update
>> approach.  Before it we had only updates of allocnos directly connected
>> to allocno in question.  Richard's approach helped to improve code in
>> some cases.  If something works better we should use.  The bechmarking
>> is the best criterium.
>
> Ccing Richard in case he has comments.

TBH I don't remember anything about this now.  Is it:

  https://gcc.gnu.org/ml/gcc-patches/2008-09/msg00541.html

?  I think that was just tweaking the traversal order in an existing
cost update, rather than adding a new one.

You might be talking about a different patch though, sorry.

Richard