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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/102024] [12 Regression] zero width bitfields and ABIs
Date: Wed, 30 Mar 2022 13:53:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102024

--- Comment #28 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Xi Ruoyao from comment #27)
> (In reply to Jakub Jelinek from comment #23)
>=20
> > struct A { double a; int : 0; double b; };
>=20
> For MIPS I've done some experiment with this and the result (with N64 ABI)
> is:
>=20
> With GCC trunk, G++ trunk, and GCC 11.2: argument passed via FPR $f12 and
> GPR $5,
> returned via GPR $2 and $3

Ah, indeed.
The
              for (; field; field =3D DECL_CHAIN (field))
                if (TREE_CODE (field) =3D=3D FIELD_DECL
                    && int_bit_position (field) >=3D bitpos)
                  break;
loop will simply stop on the first field with bitpos equal or greater than
bitpos, so the zero sized bit-fields (and generally any other zero sized fi=
elds
like struct S {} s; in GNU C) will be treated as being part of the next slo=
t.
So in struct B { int : 0; double a, b; }; it will go into GPR and FPR,
while struct C { double a; int : 0; double b; }; into FPR and GPR, and
struct D { double a, b; int : 0; } into FPR & FPR.

> With G++ 11.2: argument passed via FPR $f12 and $f13, returned via FPR $f0
> and $f2
>=20
> So I guess we need -Wpsabi for both mips_function_arg and
> mips_fpr_return_fields.

Is there somebody who can clarify the MIPS ABI intent?
Also, what does LLVM do?=