[Bug target/94711] [8/9/10 Regression] class with empty base passed incorrectly with -std=c++17 on arm

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94711

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Matthew Malcomson <matmal01@gcc.gnu.org>:

https://gcc.gnu.org/g:a5bff8af0a68d039e1586087639c86d6931c9b81

commit r10-8013-ga5bff8af0a68d039e1586087639c86d6931c9b81
Author: Matthew Malcomson <matthew.malcomson@arm.com>
Date:   Tue Apr 28 15:38:43 2020 +0100

    [Arm] Account for C++17 artificial field determining Homogeneous Aggregates

    In C++14, an empty class deriving from an empty base is not an
    aggregate, while in C++17 it is.  In order to implement this, GCC adds
    an artificial field to such classes.

    This artificial field has no mapping to Fundamental Data Types in the
    Arm PCS ABI and hence should not count towards determining whether an
    object can be passed using the vector registers as per section
    "7.1.2 Procedure Calling" in the arm PCS
   
https://developer.arm.com/docs/ihi0042/latest?_ga=2.60211309.1506853196.1533541889-405231439.1528186050

    This patch avoids counting this artificial field in
    aapcs_vfp_sub_candidate, and hence calculates whether such objects
    should be passed in vector registers in the same manner as C++14 (where
    the artificial field does not exist).

    Before this change, the test below would pass the arguments to `f` in
    general registers.  After this change, the test passes the arguments to
    `f` using the vector registers.

    The new behaviour matches the behaviour of `armclang`, and also matches
    the GCC behaviour when run with `-std=gnu++14`.

    > gcc -std=gnu++17 -march=armv8-a+simd -mfloat-abi=hard test.cpp

    ``` test.cpp
    struct base {};

    struct pair : base
    {
      float first;
      float second;
      pair (float f, float s) : first(f), second(s) {}
    };

    void f (pair);
    int main()
    {
      f({3.14, 666});
      return 1;
    }
    ```

    We add a `-Wpsabi` warning to catch cases where this fix has changed the
ABI for
    some functions.  Unfortunately this warning is not emitted twice for
multiple
    calls to the same function, but I feel this is not much of a problem and
can be
    fixed later if needs be.

    (i.e. if `main` called `f` twice in a row we only emit a diagnostic for the
    first).

    Testing:
        Bootstrapped and regression tested on arm-linux.
        This change fixes the struct-layout-1 tests Jakub added
        https://gcc.gnu.org/pipermail/gcc-patches/2020-April/544204.html
        Regression tested on arm-none-eabi.

    gcc/ChangeLog:

    2020-04-28  Matthew Malcomson  <matthew.malcomson@arm.com>
                Jakub Jelinek  <jakub@redhat.com>

            PR target/94711
            * config/arm/arm.c (aapcs_vfp_sub_candidate): Account for C++17
empty
            base class artificial fields.
            (aapcs_vfp_is_call_or_return_candidate): Warn when PCS ABI
            decision is different after this fix.