> Am 01.12.2025 um 17:32 schrieb Jeff Law <[email protected]>:
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>
>
>> On 12/1/25 9:22 AM, Jakub Jelinek wrote:
>> On Mon, Dec 01, 2025 at 09:12:29AM -0700, Jeff Law wrote:
>>>> I noticed that GCC on x86-64 also misses this optimization, which
>>>> suggests this maybe generally beneficial for targets with expensive
>>>> multiplication. Given this, would you advise investigating a generic RTL
>>>> fix (e.g., in simplify-rtx.cc)?
>>> We could try to make a case that the multiply, even though its a single op
>>> is an exception to the simplistic cost model of gimple. I'm not sure that's
>>> a great solution, but it's worth keeping in mind.
>>>
>>> You could try to see if it could be improved at expansion time, we should be
>>> able to see the expression as (a * b) == 0 due to TER and during expansion
>>> we can query target costs and adjust the initial RTL we generate.
>> If it is a win on all targets, then perhaps lower it at isel time (or if
>> only on some of them, again decide at isel or expansion time what is
>> cheaper).
> While we have traditionally thought of multiplies as expensive, 2c
> multipliers should continue to become more common, though the silicon cost
> may make duplicating them in every ALU too expensive. It's hard to see how a
> 2c multiply is going to be worse than the comparison based sequences, unless
> the multiplier is already busy.
>
> Net, I don't think we can make a blanket statement about either form being
> always better than the other.
Don’t forget AVR where I’m not sure whether one or the other version is better.
Richard
> Jeff
>
>
