[Bug target/102024] [12 Regression] zero width bitfields and ABIs

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102024

--- Comment #13 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:e902136b310ee17d4b49eb42d9d5e487d5dcf4a1

commit r12-3324-ge902136b310ee17d4b49eb42d9d5e487d5dcf4a1
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Sep 3 09:46:32 2021 +0200

    c++, abi: Set DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD on C++ zero width
bitfields [PR102024]

    The removal of remove_zero_width_bitfields function and its call from
    C++ FE layout_class_type (which I've done in the P0466R5
    layout-compatible helper intrinsics patch, so that the FE can actually
    determine what is and isn't layout-compatible according to the spec)
    unfortunately changed the ABI on various platforms.
    The C FE has been keeping zero-width bitfields in the types, while
    the C++ FE has been removing them after structure layout, so in various
    cases when passing such structures in registers we had different ABI
    between C and C++.

    While both the C and C++ FE had some code to remove zero width bitfields
    after structure layout, in both FEs it was buggy and didn't really remove
    any.  In the C FE that code has been removed later on, while in the C++ FE
    for GCC 4.5 in PR42217 it has been actually fixed, so the C++ FE started
    to remove those bitfields.

    The following patch doesn't change anything ABI-wise, but allows the
    targets to decide what to do, emit -Wpsabi warnings etc.
    Non-C zero width bitfields will be seen by the backends as normal
    zero width bitfields, C++ zero width bitfields that used to be previously
    removed will have DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD flag set.
    I've reused the DECL_FIELD_ABI_IGNORED flag which is only used on
non-bitfield
    FIELD_DECLs right now, but the macros now check DECL_BIT_FIELD flag.

    Each backend can then decide what it wants, whether it wants to keep
    different ABI between C and C++ as in GCC 11 and older (i.e. incompatible
    with G++ <= 4.4, compatible with G++ 4.5 .. 11), for that it would
    ignore for the aggregate passing/returning decisions all
    DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD FIELD_DECLs), whether it wants to never
    ignore zero width bitfields (no changes needed for that case, except
perhaps
    -Wpsabi warning should be added and for that
DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD
    can be tested), or whether it wants to always ignore zero width bitfields
    (I think e.g. riscv in GCC 10+ does that).

    All this patch does is set the flag which the backends can then use.

    2021-09-03  Jakub Jelinek  <jakub@redhat.com>

            PR target/102024
    gcc/
            * tree.h (DECL_FIELD_ABI_IGNORED): Changed into rvalue only macro
            that is false if DECL_BIT_FIELD.
            (SET_DECL_FIELD_ABI_IGNORED, DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD,
            SET_DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD): Define.
            * tree-streamer-out.c (pack_ts_decl_common_value_fields): For
            DECL_BIT_FIELD stream DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD instead
            of DECL_FIELD_ABI_IGNORED.
            * tree-streamer-in.c (unpack_ts_decl_common_value_fields): Use
            SET_DECL_FIELD_ABI_IGNORED instead of writing to
            DECL_FIELD_ABI_IGNORED and for DECL_BIT_FIELD use
            SET_DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD instead.
            * lto-streamer-out.c (hash_tree): For DECL_BIT_FIELD hash
            DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD instead of
DECL_FIELD_ABI_IGNORED.
    gcc/cp/
            * class.c (build_base_field): Use SET_DECL_FIELD_ABI_IGNORED
            instead of writing to DECL_FIELD_ABI_IGNORED.
            (layout_class_type): Likewise.  In the place where zero-width
            bitfields used to be removed, use
            SET_DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD on those fields instead.
    gcc/lto/
            * lto-common.c (compare_tree_sccs_1): Also compare
            DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD values.