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

[Martin Sebor <msebor@gmail.com>]

On 6/28/22 01:16, Richard Biener 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.
> 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 ... :/)

The conservative array_at_struct_end_p has historically been used
for codegen.  component_ref_size was added as a separate function
with more flexible (including stricter) semantics to implement
warnings without running the risk of interfering with codegen.

Martin

> 
> 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:
>>