Re: [PATCH 09/10] fortran: Support clobbering of variable subreferences [PR88364]

[Mikael Morin <morin-mikael@orange.fr>]

Le 18/09/2022 à 08:12, Richard Biener a écrit :
> On Sat, Sep 17, 2022 at 9:33 PM Mikael Morin <morin-mikael@orange.fr> wrote:
>>
>> Le 17/09/2022 à 19:03, Thomas Koenig via Fortran a écrit :
>>>
>>> I have a concern about this part, though.  My understanding at the
>>> time was that it is not possible to clobber an individual array
>>> element, but that this clobbers anything off the pointer that this
>>> is based on.
>>>
>> Well, we need the middle-end guys to give a definitive answer on this
>> topic, but I think it would be a very penalizing limitation if that was
>> the case.  I have assumed that the clobber spanned the value it was
>> applied on, neither more nor less, so just the array element in case of
>> array elements.
> 
> There is IL verification that the LHS of a CLOBBER is either
> a declaration or a pointer dereference, no array or component
> selection is allowed there.  Now, nothing should go wrong here,
> but we might eventually just drop those CLOBBERs or ICE if
> we frontend hands us an "invalid" one.
> 

Obviously I have assumed too much here; it's probably best to drop this 
patch.
It is unfortunate as there is some desirable behavior within reach here. 
  The test shows that the patch permits the elimination of a useless 
store.  And IL verification doesn't seem that upset with it by the way.
Does *(&a[1]) count as a pointer dereference?  Even in the original dump 
it is already simplified to a straight a[1].