Re: [PATCH 2/2] tree-object-size: More consistent behaviour with flex arrays

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Dec 21, 2022 at 05:25:54PM -0500, Siddhesh Poyarekar wrote:
> The tree object size pass tries to fail when it detects a flex array in
> the struct, but it ends up doing the right thing only when the flex
> array is in the outermost struct.  For nested cases (such as arrays
> nested in a union or an inner struct), it ends up taking whatever value
> the flex array is declared with, using zero for the standard flex array,
> i.e. [].
> 
> Rework subobject size computation to make it more consistent across the
> board, honoring -fstrict-flex-arrays.  With this change, any array at
> the end of the struct will end up causing __bos to use the allocated
> value of the outer object, bailing out in the maximum case when it can't
> find it.  In the minimum case, it will return the subscript value or the
> allocated value of the outer object, whichever is larger.

I think it is way too late in the GCC 13 cycle to change behavior here
except for when -fstrict-flex-arrays is used.
Plus, am not really convinced it is a good idea to change the behavior
here except for the new options, programs in the wild took 2+ years
to acommodate for what we GCC requiring and am not sure they'd be willing
to be adjusted again.

> gcc/ChangeLog:
> 
> 	PR tree-optimization/107952
> 	* tree-object-size.cc (size_from_objects): New function.
> 	(addr_object_size): Call it.  Fully rely on
> 	array_ref_flexible_size_p call to determine flex array.

	Jakub