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Subject: [Bug target/102024] [12 Regression] zero width bitfields and ABIs
Date: Wed, 27 Apr 2022 07:23:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102024

--- Comment #39 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Andreas Krebbel <krebbel@gcc.gnu.org>:

https://gcc.gnu.org/g:bc79f0d9048375e402497d5f2ef457c9500310e4

commit r12-8277-gbc79f0d9048375e402497d5f2ef457c9500310e4
Author: Andreas Krebbel <krebbel@linux.ibm.com>
Date:   Wed Apr 27 09:20:41 2022 +0200

    PR102024 - IBM Z: Add psabi diagnostics

    For IBM Z in particular there is a problem with structs like:

    struct A { float a; int :0; };

    Our ABI document allows passing a struct in an FPR only if it has
    exactly one member. On the other hand it says that structs of 1,2,4,8
    bytes are passed in a GPR. So this struct is expected to be passed in
    a GPR. Since we don't return structs in registers (regardless of the
    number of members) it is always returned in memory.

    Situation is as follows:

    All compiler versions tested return it in memory - as expected.

    gcc 11, gcc 12, g++ 12, and clang 13 pass it in a GPR - as expected.

    g++ 11 as well as clang++ 13 pass in an FPR

    For IBM Z we stick to the current GCC 12 behavior, i.e. zero-width
    bitfields are NOT ignored.  A struct as above will be passed in a
    GPR. Rational behind this is that not affecting the C ABI is more
    important here.

    A patch for clang is in progress: https://reviews.llvm.org/D122388

    In addition to the usual regression test I ran the compat and
    struct-layout-1 testsuites comparing the compiler before and after the
    patch.

    gcc/ChangeLog:
            PR target/102024
            * config/s390/s390-protos.h (s390_function_arg_vector): Remove
            prototype.
            * config/s390/s390.cc (s390_single_field_struct_p): New functio=
n.
            (s390_function_arg_vector): Invoke s390_single_field_struct_p.
            (s390_function_arg_float): Likewise.

    gcc/testsuite/ChangeLog:
            PR target/102024
            * g++.target/s390/pr102024-1.C: New test.
            * g++.target/s390/pr102024-2.C: New test.
            * g++.target/s390/pr102024-3.C: New test.
            * g++.target/s390/pr102024-4.C: New test.
            * g++.target/s390/pr102024-5.C: New test.
            * g++.target/s390/pr102024-6.C: New test.=