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

[Richard Biener <richard.guenther@gmail.com>]

> Am 30.06.2022 um 16:08 schrieb Qing Zhao via Gcc-patches <gcc-patches@gcc.gnu.org>:
> 
> 
> 
>> On Jun 29, 2022, at 5:14 PM, Martin Sebor <msebor@gmail.com> wrote:
>> 
>> 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.
> 
> Okay, I see. 
> A FIELD_DECL might be shared by multiple structure or unions, and whether 
> it’s a flexible array member varies between different enclosing structures or unions.
> Therefore FIELD_DECL cannot carry the flexible array member information accurately. 

No, that’s not true.  A FIELD_DELC is only shared for cv variants of a structure.


> Then, how about encoding the flexible array member information into the enclosing structure or union? 
> 
> 
> Another thing is:  All this complexity is caused by GNU extension which permits the flexible array 
> member not at the end of the struct. (As I mentioned in a previous email, I listed here again)
> 
> For example the following two examples:
> 
> 1. [opc@qinzhao-ol8u3-x86 trailing_array]$ cat t1.c
> struct AX
> {
>  int n;
>  short ax[];
>  int m;
> };
> 
> void warn_ax_local (struct AX *p)
> {
>  p->ax[2] = 0;   
> }
> 
> 2. [opc@qinzhao-ol8u3-x86 trailing_array]$ cat t2.c
> struct AX
> {
>  int n;
>  short ax[];
> };
> 
> struct UX
> {
>  struct AX b;
>  int m;
> };
> 
> void warn_ax_local (struct AX *p, struct UX *q)
> {
>  p->ax[2] = 0;   
>  q->b.ax[2] = 0;
> }
> 
> [opc@qinzhao-ol8u3-x86 trailing_array]$ gcc -O2 -Wall t1.c -S
> t4.c:4:9: error: flexible array member not at end of struct
>    4 |   short ax[];
> 
> [opc@qinzhao-ol8u3-x86 trailing_array]$ gcc -O2 -Wall t2.c -S
> 
> It’s clear to see that in the above t1.c,  GCC  reports error when the flexible array member is Not at the end of the structure  (AX) that immediately enclosing the field.
> However, for t2.c, when the flexible array member is Not at the end of the structure that does not immediately enclosing it (UX), then it’s accepted.   
> 
> I am very confused about t2.c, is the struct UX a correct declaration? 
> 
> Thanks.
> 
> Qing
> 
>> 
>> Martin
>