Re: [PATCH bpf-next 2/2] selftests/bpf: Match tests against regular expression.

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> On Thu, 2024-06-06 at 10:19 -0700, Andrii Nakryiko wrote:
>
> [...]
>
>> > Some other test, would expect that struct fields would be in some
>> > particular order, while GCC decides it would benefit from reordering
>> > struct fields. For passing those tests I need to disable GCC
>> > optimization that would make this reordering.
>> > However reordering of the struct fields is a perfectly valid
>> 
>> Nope, it's not.
>> 
>> As mentioned, struct layout is effectively an ABI, so the compiler
>> cannot just reorder it. Lots and lots of things would be broken if
>> this was true for C programs.
>
> I'll chime in as well :)
> Could you please show a few examples when GCC does reordering?
> As Alexei and Andrii point out in general C language standard does not
> allow reordering for fields, e.g. here is a wording from section
> 6.7.2.1, paragraph 17 of "WG 14/N 3088, Programming languages — C":

GCC does not reorder struct fields.
The option -ftoplevel-reorder enables reordering of data declarations.

>> Within a structure object, the non-bit-field members and the units
>> in which bit-fields reside have addresses that increase in the order
>> in which they are declared. A pointer to a structure object,
>> suitably converted, points to its initial member (or if that member
>> is a bit-field, then to the unit in which it resides), and vice
>> versa. There may be unnamed padding within a structure object, but
>> not at its beginning.
>
> So, I'm curious what's happening.