Re: [GCC 13][PATCH] PR101836: Add a new option -fstrict-flex-array[=n] and use it in __builtin_object_size

[Martin Sebor <msebor@gmail.com>]

On 6/28/22 13:01, Qing Zhao wrote:
> 
> 
>> On Jun 28, 2022, at 2:49 PM, Jakub Jelinek <jakub@redhat.com> wrote:
>>
>> On Tue, Jun 28, 2022 at 06:29:01PM +0000, Qing Zhao wrote:
>>>
>>>
>>>> On Jun 28, 2022, at 2:22 PM, Jakub Jelinek <jakub@redhat.com> wrote:
>>>>
>>>> On Tue, Jun 28, 2022 at 06:15:58PM +0000, Qing Zhao wrote:
>>>>>> Because the flag just tells whether some array shouldn't be treated as (poor man's)
>>>>>> flexible array member.  We still need to find out if some FIELD_DECL is to
>>>>>> be treated like a flexible array member, which is a minority of
>>>>>> COMPONENT_REFs.
>>>>>> struct S { int a; char b[0]; int c; } s;
>>>>>> struct T { int d; char e[]; };
>>>>>> struct U { int f; struct T g; int h; } u;
>>>>>> Neither s.b nor u.g.e is to be treated like flexible array member,
>>>>>> no matter what -fstrict-flex-array= option is used.
>>>>>
>>>>> Then, to resolve this issue, we might need a opposite  flag DECL_IS_FLEXARRAY in FIELD_DECL?
>>>>>
>>>>> The default is FALSE for all FIELD_DECL.
>>>>
>>>> Doesn't matter whether it is positive or negative, you still need to analyze
>>>> it.  See the above example.  If you have struct T t; and test t.e, then it
>>>> is flexarray.  But u.g.e is not, even when the COMPONENT_REF refers to the
>>>> same FIELD_DECL.  In the t.e case e is the very last field, in the latter
>>>> case u.g.e is the last field in struct T, but struct U has the h field after
>>>
>>> So, do you mean that the current FE analysis will not be able to decide whether a specific array field is at the end of the enclosing structure?
>>> Only the middle end can decide this ?
>>
>> Well, anything that analyzes it, can be in the FE or middle-end, but there
>> is no place to store it for later.
> 
> Then I am a little confused:
> 
> If the FE can decide wether an array field is at the end of the enclosing structure,  then combined with whether it’s a [0], [1] or [], and which level of -fstrict-flex-array,
> 
> The FE should be able to decide whether this array field is a flexible array member or not, then set the flag DECL_IS_FLEXARRAY (or DECL_NOT_FLEXARRAY).
> 
> The new flag is the place to store such info, right?
> Do I miss anything here?

I think the problem is that there is just one FIELD_DECL for member
M of a given type T but there can be more than one instance of that
member, one in each struct that has a subobject of T as its own
member.  Whether M is or isn't a (valid) flexible array member
varies between the two instances.

Martin