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

[Eduard Zingerman <eddyz87@gmail.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":

> 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.