Re: [PATCH v3 4/7] ifcvt/optabs: Allow using a CC comparison for emit_conditional_move.

[Jeff Law <jeffreyalaw@gmail.com>]

On 12/9/2021 10:20 AM, Robin Dapp wrote:
> Hi Jeff,
>
> thanks for looking into this.
NP.  I'd been watching this set evolve and I think it'll help our target 
as well, so it seemed natural to handle the review :-)

>
>> What if the condition has a side effect?  Doesn't this drop the side
>> effect by converting the conditional move into a simple move?
> Hmm, good point, if the condition does more than a CC compare, it might
> get tricky as we are not canonicalizing here (on purpose). Is there an
> easy way out like checking something like side_effects_p ()?
I think if side_effects_p is true, we can just emit the conditional move 
as-is without trying to collapse it to a simple move.  It should be 
exceedingly rare to have a side effect in the destination.    Checking 
side_effects_p will also reject if the destination is volatile MEM, but 
that should be OK and also exceedingly rare.
>
> Maybe we should drop this altogether and let the backend deal with it?
> It would probably not know what to do and FAIL.
I like the idea of collapsing to a simple move if the true/false arms 
are the same.    Did you see that happen in practice?  If so, I'd like 
to keep it, but just guard with the !side_effects_p check.

jeff