Re: Giving targets a say in match.pd

On 6/6/2026 6:32 AM, Georg-Johann Lay wrote:
> Am 06.06.26 um 00:17 schrieb Jeffrey Law:
>>
>> On 6/5/2026 3:25 AM, Georg-Johann Lay via Gcc wrote:
>>> In match.pd there are two kinds of patterns: Canonicalizations
>>> and optimizations.
>>>
>>> While canonicalizations simplify the compile task by reducing
>>> combinatorial complexity, optimization patterns try to improve
>>> code performance.
>>>
>>> The trouble with the optimization patterns is that they operate 
>>> blindly,
>>> not considering costs or target capabilities in any way.
>> [ ... ]
>> This would go against guiding principles in the gimple phases. 
>> Essentially we very much have avoided things like target costing in 
>> gimple.  That's part of what makes gimple transformations predictable 
>> and relatively easy to evaluate.
>
> At no point I proposed to introduce costing.
>
> All I said is that match.pd /is/ doing "optimizations" without using
> any metric, which is a correct statement as far as I know.
>
> And without a metric, you have no idea where you are heading.
> That works for trivial cases like x * 0 = 0, and when all the
> dozens of targets behave similar in this regard.
>
> There are cases though where different targets means different metrics.
You may not not explicitly mentioned costing, but the net effect is the 
same -- the targets end up making arbitrary changes to the gimple IL and 
that's what we've fundamentally wanted to avoid since the initial 
development of the tree-ssa work.

Jeff