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 8:59 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> 
> On Fri, Jul 01, 2022 at 12:55:08PM +0000, Qing Zhao wrote:
>> 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?
> 
> The point is recording early what FIELD_DECLs could be vs. can't possibly be
> treated like flexible array members and just use that flag in the decisions
> in the current routines in addition to what it is doing.

Okay. 

Based on the discussion so far, I will do the following:

1. Add a new flag “DECL_NOT_FLEXARRAY” to FIELD_DECL;
2. In C/C++ FE, set the new flag “DECL_NOT_FLEXARRAY” for a FIELD_DECL based on [0], [1],
    [] and the option -fstrict-flex-array, and whether it’s the last field of the DECL_CONTEXT.
3. In Middle end,  Add a new utility routine is_flexible_array_member_p, which bases on 
    DECL_NOT_FLEXARRAY + array_at_struct_end_p to decide whether the array
    reference is a real flexible array member reference. 


Middle end currently is quite mess, array_at_struct_end_p, component_ref_size, and all the phases that
use these routines need to be updated, + new testing cases for each of the phases.


So, I still plan to separate the patch set into 2 parts:

  Part A:    the above 1 + 2 + 3,  and use these new utilities in tree-object-size.cc to resolve PR101836 first.
                 Then kernel can use __FORTIFY_SOURCE correctly;

  Part B:    update all other phases with the new utilities + new testing cases + resolving regressions.

Let me know if you have any comment and suggestion.

Thanks a lot for all your help.

Qing

> 
> 	Jakub
>