Hi, Richard,
> On Jun 28, 2022, at 3:16 AM, Richard Biener <richard.guent...@gmail.com>
> wrote:
>
> On Mon, Jun 27, 2022 at 4:20 PM Qing Zhao via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>>
>> Hi,
>>
>> Per our discussion in the bug report, I came up with the following patch:
>>
>> =======
>>
>> PR101836: Add a new option -fstrict-flex-array[=n]
>>
>> Add the new option and use it in __builtin_object_size.
>>
>> Treat the trailing array of a structure as a flexible array member in a
>> stricter way. The value of 'n' controls the level of strictness.
>> 'n'=0 is the least strict, all trailing arrays of structures are treated
>> as flexible array members; This is the default behavior of GCC without
>> specify
>> this option.
>> 'n'=3 is the strictest, only when the trailing array is declared as a
>> flexible array member per C99 standard onwards ([]), it is treated as a
>> flexible array member;
>> There are two more levels in between 0 and 3, which are provided to support
>> older codes that use GCC zero-length array extension ([0]), or one-size
>> array as
>> flexible array member ([1]):
>> When 'n' is 1, the trailing array is treated as a flexible array member
>> when it is declared as either [], [0], or [1];
>> When 'n' is 2, the trailing array is treated as a flexible array member
>> when it is declared as either [], or [0].
>>
>> There are other places in GCC that conservatively treat flexible array
>> members.
>> A follow-up patch will make -ftrict-flex-array option to control all these
>> places consistently.
>>
>> Bootstrapped and regression tested on both X86 and aarch64, no issues.
>>
>> Any comment and suggestion?
>
> Since this aims at the C or C++ frontends but the middle-end eventually
> consumes
> this it would be much nicer to encode this in the types themselves.
Yes, I agree.
Let the C/C++ FE to decide whether the [0], [1], or [] trailing array field of
a structure is a flex array member or not based on the option
-fstrict-flex-array and
then encode such info in the FIELD_DECL as flag DECL_NOT_FLEXARRAY.
Later, the middle end just check the flag DECL_NOT_FLEXARRAY of the FIELD_DECL
to decide whether the trailing array is flexible array or not.
This will eliminate all the hacks in the middle-end (as you mentioned,
array_at_struct_end_p,
and “trailing_array”, etc, and there are quite some phases use this routine to
query, and in
an in-consistent way)
> Since the least
> strict reading is the default right now it would be a flag (on the
> FIELD_DECL I suppose)
> like DECL_NOT_FLEXARRAY or DECL_FIXED_SIZE? Alternatively the flag could
> also be on the record type enclosing the trailing array member (but
> type sharing might
> make this more difficult in the end).
>
> There's also array_at_struct_end_p which is supposed to be the main
> query interface
> for this (but it seems people sneaked in more variants with eventually
> different semantics ... :/)
Yes, there are many places right now that query “array_at_struct_end_p”, I was
planning a follow-up
patchs to replace all “array_at_struct_end_p” with is_flexible_array_p. I guess
that this follow-up patch
will take quite some time to finish.
So, my next step:
1. Update this current patch per your suggestion above, i.e,
A. Add a new flag (DECL_NOT_FLEXARRAY) in FIELD_DECL, default is FALSE;
B. In C/C++ FE, for a trailing array field of a structure, decide
DECL_NOT_FLEXARRAY flag as following:
Level 1: any trailing array that is NOT [0], [1], [],
DECL_NOT_FLEXARRAY is TRUE;
Level 2: any trailing array that is NOT [0], [], DECL_NOT_FLEXARRAY is
TRUE;
Level 3: any trailing array that is not [], DECL_NOT_FLEXARRAY is TRUE
C. Use DECL_NOT_FLEXARRAY in tree-object-size.c for __builtin_object_size
to resolve bug PR101836.
2. Then replace all “array_at_struct_end_p” with using DECL_NOT_FLEXARRAY in
GCC, adding new testing cases
For different phases with different level, resolving all regressions.
I plan separate patches for 1 and 2. Commit 1 first to enable kernel work as
soon as possible. Then continue working
on 2 to make GCC consistent in gcc13.
Let me know if you have any suggestion or comment.
Thanks
Qing
>
> Richard.
>
>
>
>> Okay for commit to Gcc13?
>>
>> thanks.
>>
>> Qing
>>
>> =======================
>>
>> gcc/
>>
>> PR tree-optimization/101836
>> * common.opt (fstrict-flex-array, fstrict-flex-array=): New options.
>> * doc/invoke.texi (-fstrict-flex-array, -fstrict-flex-array=):
>> Document.
>> * tree-object-size.cc (addr_object_size): Call is_flexible_array_p to
>> check whether an array is a flexible array.
>> * tree.cc (special_array_member_type): New routine.
>> (is_flexible_array_p): New routine.
>> (component_ref_size): Call special_array_member_type to decide the
>> type of special array member.
>> * tree.h (enum struct special_array_member): Add is_vla, trail_flex.
>> (special_array_member_type): New prototype.
>> (is_flexible_array_p): New prototype.
>>
>> gcc/testsuite/
>>
>> PR tree-optimization/101836
>> * gcc.dg/pr101836.c: New test.
>> * gcc.dg/pr101836_1.c: New test.
>> * gcc.dg/pr101836_2.c: New test.
>> * gcc.dg/pr101836_3.c: New test.
>> * gcc.dg/pr101836_4.c: New test.
>> * gcc.dg/pr101836_5.c: New test.
>>
>>
>> The complete patch is:
