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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. |
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| contrib | ||
| fixincludes | ||
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