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 Jul 1, 2022, at 2:49 AM, Richard Biener <richard.guenther@gmail.com> wrote:
> 
> On Thu, Jun 30, 2022 at 9:30 PM Qing Zhao <qing.zhao@oracle.com> wrote:
>> 
>> 
>> 
>>> On Jun 30, 2022, at 1:03 PM, Jakub Jelinek <jakub@redhat.com> wrote:
>>> 
>>> On Thu, Jun 30, 2022 at 03:31:00PM +0000, Qing Zhao wrote:
>>>>> 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?
>>> 
>>> const/volatile qualified variants.  So
>> Okay. I see. thanks.
>>> 
>>>> 2. For the following example:
>>>> 
>>>> struct AX { int n; short ax[];};
>>> 
>>> struct AX, const struct AX, volatile const struct AX etc. types will share
>>> the FIELD_DECLs.
>> 
>> Okay.
>>> 
>>>> 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?
>>> 
>>> No, there are just n and ax FIELD_DECLs with DECL_CONTEXT of struct AX and
>>> b and m FIELD_DECLs with DECL_CONTEXT of struct UX.
>> 
>> Ah, right.
>> 
>> 
>>> 
>>> But, what is important is that when some FIELD_DECL is last in some
>>> structure and has array type, it doesn't mean it should have an
>>> unconstrained length.
>>> In the above case, when struct AX is is followed by some other member, it
>>> acts as a strict short ax[0]; field (even when that is an exception), one
>>> can tak address of &UX.b.ax[0], but can't dereference that, or &UX.b.ax[1].
>> 
>> So, is this a GNU extension. I see that CLANG gives a warning by default and GCC gives a warning when specify -pedantic:
>> [opc@qinzhao-ol8u3-x86 trailing_array]$ cat t3.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]$ clang -O2 -Wall t3.c -S
>> t3.c:9:13: warning: field 'b' with variable sized type 'struct AX' not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
>>  struct AX b;
>>            ^
>> [opc@qinzhao-ol8u3-x86 trailing_array]$ gcc -O2 -Wall t3.c -pedantic -S
>> t3.c:9:13: warning: invalid use of structure with flexible array member [-Wpedantic]
>>    9 |   struct AX b;
>>      |             ^
>> 
>> But, Yes, I agree, even though this is only a GNU extension, We still need to handle it and accept it as legal code.
>> 
>> Then, yes, I also agree that encoding the info of is_flexible_array into FIELD_DECL is not good.
> 
> Which is why I suggested to encode 'not_flexible_array'.  This way the
> FE can mark all a[1] this way in some mode
> but leave a[] as possibly flexarray (depending on context).

Then, FE marking (not_flexible_array) can not do the complete job to mark 
whether a field array is flexible array member or not,  Middle end still need to 
check the “context” (i.e, whether the array ref is at the end of a structure?)

So, only FE marking + Middle-end “context checking” together will decide a REAL flex array? 

If so, comparing to the current implemenation to have all the checking in middle-end, what’s the 
major benefit of moving part of the checking into FE, and leaving the other part in middle-end?

> 
>> How about encoding the info of “has_flexible_array” into the enclosing RECORD_TYPE or UNION_TYPE node?
> 
> But that has the same issue.  Consider
> 
> struct A { int n; int a[1]; };
> 
> where a is considered possibly a flexarray vs.
> 
> struct B { struct A a; int b; };
> 
> where B.a would be not considered to have a flexarray (again note
> 'possibly' vs. 'actually does').
> 
> Also
> 
> struct A a;
> 
> has 'a' as _not_ having a flexarray (because it's size is statically
> allocated) but
> 
> struct A *a;
> struct B *b;
> 
> a->a[n];
> 
> as possibly accessing the flexarray portion of *a while
> 
> b->a.a[n]
> 
> is not accessing a flexarray because there's a member after a in b.
> 
> For your original proposal it's really the field declaration itself
> which changes so annotating the FIELD_DECL
> seems correct to me.

Then middle-end still need to check the context, and combined 
with the “not_flexible_array” flag that is encoded in FIELD_DECL
 to make the final decision?

Thanks.

Qing
> 
>> For example, in the above example,  the RECORD_TYPE for “struct AX” will be marked as “has_flexible_array”, but that for “struct UX” will not.
>> 
>>> 
>>> I believe pedantically flexible array members in such cases don't
>>> necessarily mean zero length array, could be longer, e.g. for the usual
>>> x86_64 alignments
>>> struct BX { long long n; short o; short ax[]; };
>>> struct VX { struct BX b; int m; };
>>> I think it acts as short ax[3]; because the padding at the end of struct BX
>>> is so long that 3 short elements fit in there.
>>> While if one uses
>>> struct BX bx = { 1LL, 2, { 3, 4, 5, 6, 7, 8, 9, 10 } };
>>> (a GNU extension), then it acts as short ax[11]; - the initializer is 8
>>> elements and after short ax[8]; is padding for another 3 full elemenets.
>>> And of course:
>>> struct BX *p = malloc (offsetof (struct BX, ax) + n * sizeof (short));
>>> means short ax[n].
>>> Whether struct WX { struct BX b; };
>>> struct WX *p = malloc (offsetof (struct WX, b.ax) + n * sizeof (short));
>>> is pedantically acting as short ax[n]; is unclear to me, but we are
>>> generally allowing that and people expect it.
>> 
>> Okay, I see now.
>>> 
>>> Though, on the GCC side, I think we are only treating like flexible arrays
>>> what is really at the end of structs, not followed by other members.
>> 
>> My understanding is, Permitting flexible array to be followed by other members is a GNU extension.  (Actually, it’s not allowed by standard?).
>> 
>> Thanks a lot for your patience and help.
>> 
>> Qing
>>> 
>>>      Jakub