Re: [PATCH] tree: Fix up save_expr [PR52339]

[Jason Merrill <jason@redhat.com>]

On 5/30/23 04:23, Jakub Jelinek wrote:
> On Tue, May 30, 2023 at 10:03:05AM +0200, Eric Botcazou wrote:
>>> We want to be able to treat such things as invariant somehow even if we
>>> can't do that for references to user data that might be changed by
>>> intervening code.
>>>
>>> That is, indicate that we know that the _REF actually refers to a const
>>> variable or is otherwise known to be unchanging.
>>>
>>> Perhaps that should be a new flag that tree_invariant_p can check
>>> instead of TREE_READONLY.
>>
>> Richard earlier suggested a langhook; given that Ada will be the main (sole?)
>> user of it, this would probably be better.
> 
> Are the DECL_INVARIANT_P FIELD_DECLs in Ada really invariant no matter how
> exactly they are accessed?  Or can Ada suffer from the same problem as
> C/C++, where the FIELD_DECL is TREE_READONLY, but could go out of scope or
> a pointer to it could change.
> I mean the p->fld cases in C/C++, where there could be free (p); or p++
> etc. in between the place where save_expr is first evaluated and later
> uses?

Note that it is fine to treat p->fld as invariant in C++ if fld is 
TREE_READONLY and p is itself invariant.  The implementation is allowed 
to assume that other code didn't destroy *p and create a new object with 
a different value of p in the same location under 
https://eel.is/c++draft/basic.memobj#basic.life-8.3

Are the Ada references to VLA bounds represented that way?

Jason