Re: [PATCH v2 0/3] Inline helpers into Rust without full LTO

[Arnd Bergmann]

On Fri, Mar 27, 2026, at 08:56, Geert Uytterhoeven wrote:
>>
>> I noticed a similar issue with m68k-linux, which has a bitfield
>> alignment different from anything else on gcc, but uses the normal
>> behavior on clang.
>
> Ugh, I wasn't aware of that. Adrian, did you know?

To clarify, this illustrates what I mean here:

echo 'struct { short a : 3; short b :15; short c :14; } x; int y = sizeof(x);' | m68k-linux-gcc -xc - -S -o-

this produces '4' on m68k-linux-gcc, but '6' everywhere else. I originally
thought this was related to this 2009 change in both compilers

https://github.com/llvm/llvm-project/commit/830dfccfb2c06a1749938ce8cb78a1d6a295b942
https://gcc.gnu.org/cgit/gcc/commit/?id=2cd36c22d3d16e24f342829a989ac35f2f442656

but that only allowed bitfields to be marked as __attribute__((packed))
in order to get tightly packed fields and return '4' on all architectures,
while m68k-linux-gcc apparently has all bitfields implicitly packed unless they
are explicitly marked __attribute__((aligned(x))). This behavior is
independent of the -malign-int flag.

     Arnd