On 23.03.21 12:36, Rasmus Villemoes wrote:
On 23/03/2021 08.45, Oliver Hartkopp wrote:
IMO we facing a compiler problem here - and we should be very happy that
the BUILD_BUG_ON() triggered an issue after years of silence.
I do not have a good feeling about what kind of strange effects this
compiler issue might have in other code of other projects.
So I would explicitly suggest NOT to change the af_can.c code to work
around this compiler issue.
Let the gcc people fix their product and let them thank all of us for
detecting it.
I'm sure you'd be eligible for a full refund in case this was a bug in
gcc. It is not. It's a pretty clear ABI requirement for (at least some
flavors of) ARM:
https://stackoverflow.com/questions/43786747/struct-layout-in-apcs-gnu-abi
and more directly from the horse's mouth:
https://developer.arm.com/documentation/dui0067/d/arm-compiler-reference/c-and-c---implementation-details/structures--unions--enumerations--and-bitfields
Field alignment
Structures are arranged with the first-named component at the lowest
address. Fields are aligned as follows:
A field with a char type is aligned to the next available byte.
A field with a short type is aligned to the next even-addressed
byte.
Bitfield alignment depends on how the bitfield is declared. See
Bitfields in packed structures for more information.
All other types are aligned on word boundaries.
That anonymous union falls into the "All other types" bullet.
__packed is the documented and standard way to overrule the
compiler's/ABI's layout decisions.
So why is there a difference between
gcc version 10.2.0
and
gcc version 10.2.1 20210110 (Debian 10.2.1-6)
https://lore.kernel.org/linux-can/20210323073437.yo63wreqnubbeqby@xxxxxxxxxxxxxx/
??
Would this mean that either STRUCTURE_SIZE_BOUNDARY or the command line
option -mstructure_size_boundary=<n>
are set differently?
https://stackoverflow.com/questions/43786747/struct-layout-in-apcs-gnu-abi/43829053#43829053
I'm not a compiler expert but this does not seem to be consistent.
Especially as we only have byte sizes (inside and outside of the union)
and "A field with a char type is aligned to the next available byte."
The union is indeed aligned to the word boundary - but the following
byte is not aligned to the next available byte.
Regards,
Oliver
