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

[Thomas Koenig <tkoenig@netcologne.de>]

On 18.09.22 11:10, Mikael Morin wrote:
> 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.

Probably, yes.

> It is unfortunate as there is some desirable behavior within reach here. 

I think some of the desired behavior can still be salvaged.  For
example, for

   subroutine foo(a,n)
     integer :: n
     integer, dimension(n), intent(in) :: n

...

   subroutine bar(a)
     integer, intent(out) :: a

...

   integer :: a(3)

   call foo(a,3)
   call foo(a(1),3)

clobbers for the whole array can still be generated, but not for

   call foo(a(2),2)

so one would have to look at the lower bound.

For this case, it would be helpful to clobber a range a(2:), but that
is a wishlist item for the future ;-)

What is unsafe, currently, is

   call bar(a(1))

Best regards

	Thomas