> On Jun 30, 2022, at 10:24 AM, Richard Biener <[email protected]>
> wrote:
>
>
>
>> Am 30.06.2022 um 16:08 schrieb Qing Zhao via Gcc-patches
>> <[email protected]>:
>>
>>
>>
>>> On Jun 29, 2022, at 5:14 PM, Martin Sebor <[email protected]> wrote:
>>>
>>> On 6/28/22 13:01, Qing Zhao wrote:
>>>>> On Jun 28, 2022, at 2:49 PM, Jakub Jelinek <[email protected]> wrote:
>>>>>
>>>>> On Tue, Jun 28, 2022 at 06:29:01PM +0000, Qing Zhao wrote:
>>>>>>
>>>>>>
>>>>>>> On Jun 28, 2022, at 2:22 PM, Jakub Jelinek <[email protected]> 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
>>
