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 30, 2022, at 10:24 AM, Richard Biener <richard.guenther@gmail.com> wrote:
> 
> 
> 
>> 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.

Sorry for my dump questions: 

1. What do you mean by “cv variants” of a structure?
2. For the following example:

struct AX { int n; short ax[];};
struct UX {struct AX b; int m;};

Are there two different FIELD_DECLs in the IR, one for AX.ax, the other one is for UX.b.ax?

Qing

> 
> 
>> 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
>>