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

[Qing Zhao <qing.zhao@oracle.com>]

> 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?

Qing
> 
> 	Jakub