Urgent GCC ABI backend maintainer ping re zero width bitfield passing (PR102024)

[Jakub Jelinek <jakub@redhat.com>]

Hi!

I'd like to ping port maintainers about
https://gcc.gnu.org/PR102024

As I wrote, the int : 0 bitfields are present early in the TYPE_FIELDS
during structure layout and intentionally affect the layout.
We had some code to remove those from TYPE_FIELDS chains in the C and C++
FEs, but for C that removal never worked correctly (never removed any)
and the non-working removal was eventually removed.  For C++ it
didn't initially work either, but for GCC 4.5 that was fixed in PR42217,
so on various backends where TYPE_FIELDS are analyzed for how to pass or
return certain aggregates starting with GCC 4.5 the C++ and C ABI diverged.
In August, I have removed that zero width bitfield removal from C++ FE
as the FE needs to take those bitfields into account later on as well.

The x86_64 backend was changed in r12-6418-g3159da6c46 to match recently
approved clarification of the x86-64 psABI and the zero width bitfields
are now ignored for both C and C++ (so an ABI change for C from 11.x and
earlier to 12.x and for C++ from GCC 4.4 and earlier to 4.5 and later)
with a -Wpsabi diagnostics about it.

The rs6000 backend was changed in r12-3843-g16e3d6b8b2 to never ignore
those bitfields (so no ABI change for C, for C++ ...-4.4 and 12+ are
ABI incompatible with 4.5 through 11.x; note, it affects I think just
ppc64le ABI, which didn't really exist before 4.8 I think) and diagnostics
has been added about the ABI change.

As I wrote in the PR, I believe most of the GCC backends are unaffected,
x86_64 and rs6000 are handled, riscv was changed already in GCC 10 to
ignore those bitfields and emit a -Wpsabi diagnostics.

I can see code-generation differences certainly on armv7hl and aarch64.
ia64, iq2000, mips, s390 and sparc are maybe affected, haven't checked.

Simple testcase could be e.g.:
struct S { float a; int : 0; float b; };

__attribute__((noipa)) struct S
foo (struct S x)
{
  return x;
}

void
bar (void)
{
  struct S s = { 0.0f, 0.0f };
  foo (s);
}
where one should look at the argument and return value passing
in GCC 11 C, GCC 11 C++, GCC trunk C, GCC trunk C++.

The FE now sets bits on the bitfields that make it possible to
differentiate between the different cases, so each port may decide to do
one of the 3 things:
1) keep ABI exactly compatible between GCC 11 and 12, which means
   C and C++ will continue to be incompatible
2) keep the G++ 4.5 through 11 ABI of ignoring zero width bitfields and
   change C ABI
3) keep the GCC < 11 C ABI of not ignoring zero width bitfields and
   change the C++ ABI (which means restoring ABI compatibility in
   this regard between G++ 4.4 and earlier with G++ 12 and later)
Furthermore, it would be very nice to emit -Wpsabi diagnostics for the
changed ABI unless 1) is decided.
One should take into account psABI as well as what other compilers do.

The current state of GCC trunk is that 3) is done except that x86_64
did 2) and riscv did 2 already for GCC 10 and all of x86_64, riscv and
rs6000 emit -Wpsabi diagnostics (though I think rs6000 doesn't guard
it with -Wpsabi).

I can help with the backend implementations if needed, but I can't
decide which possibility you want to choose for each backend.
It would be really nice to decide about this soon, because changing
the ABI in GCC 12 only to change it again in GCC 13 doesn't look much
desirable and even if 3) is the choice, it is really nice to have
some diagnostics about ABI changes.

Thanks

	Jakub