[Bug target/94383] [8/9/10 Regression] class with empty base passed incorrectly with -std=c++17 on aarch64

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94383

--- Comment #14 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:e73a32d6d47ec7c5fb5a5fe7eb896c0e1258ea68

commit r10-7914-ge73a32d6d47ec7c5fb5a5fe7eb896c0e1258ea68
Author: Matthew Malcomson <matthew.malcomson@arm.com>
Date:   Thu Apr 23 15:33:55 2020 +0100

    [AArch64] (PR94383) Avoid C++17 empty base field checking for HVA/HFA

    In C++17, an empty class deriving from an empty base is not an
    aggregate, while in C++14 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
    AArch64 PCS ABI and hence should not count towards determining whether an
    object can be passed using the vector registers as per section
    "6.4.2 Parameter Passing Rules" in the AArch64 PCS.
   
https://github.com/ARM-software/abi-aa/blob/master/aapcs64/aapcs64.rst#the-base-procedure-call-standard

    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 behaviour when run with `-std=gnu++14`.

    > gcc -std=gnu++17 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:
        Bootstrap and regression test on aarch64-linux.
        All struct-layout-1 tests now pass.

    gcc/ChangeLog:

    2020-04-23  Matthew Malcomson  <matthew.malcomson@arm.com>
                Jakub Jelinek  <jakub@redhat.com>

            PR target/94383
            * config/aarch64/aarch64.c (aapcs_vfp_sub_candidate): Account for
C++17
            empty base class artificial fields.
            (aarch64_vfp_is_call_or_return_candidate): Warn when ABI PCS
decision is
            different after this fix.