Re: [PATCH 1/2] ivopts: Revert computation of address cost complexity.

[Jeff Law <jeffreyalaw@gmail.com>]

On 10/28/22 01:00, Richard Biener wrote:
> On Fri, Oct 28, 2022 at 8:43 AM Dimitrije Milosevic
> <Dimitrije.Milosevic@syrmia.com> wrote:
>> Hi Jeff,
>>
>>> THe part I don't understand is, if you only have BASE+OFF, why does
>>> preventing the calculation of more complex addressing modes matter?  ie,
>>> what's the point of computing the cost of something like base + off +
>>> scaled index when the target can't utilize it?
>> Well, the complexities of all addressing modes other than BASE + OFFSET are
>> equal to 0. For targets like Mips, which only has BASE + OFFSET, it would still
>> be more complex to use a candidate with BASE + INDEX << SCALE + OFFSET
>> than a candidate with BASE + INDEX, for example, as it has to compensate
>> the lack of other addressing modes somehow. If complexities for both of
>> those are equal to 0, in cases where complexities decide which candidate is
>> to be chosen, a more complex candidate may be picked.
> But something is wrong then - it shouldn't ever pick a candidate with
> an addressing
> mode that isn't supported?  So you say that the cost of expressing
> 'BASE + INDEX << SCALE + OFFSET' as 'BASE + OFFSET' is not computed
> accurately?

This is exactly what I was trying to get to.   If the addressing mode 
isn't supported, then we shouldn't be picking it as a candidate.  If it 
is, then we've probably got a problem somewhere else in this code and 
this patch is likely papering over it.


Jeff