> 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.
