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>]

Hi, Richard,


> On Jun 28, 2022, at 3:16 AM, Richard Biener <richard.guenther@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: