On Tue, Mar 05, 2019 at 05:33:45PM -0500, Jason Merrill wrote:
> The x86_64 psABI says that an empty class isn't passed or returned in memory
> or registers, so we shouldn't set %eax in this function. This seems like a
> missed case from the GCC 8 TYPE_EMPTY_P changes.
>
> Shall I apply this for GCC 9, or hold it for stage 1?
>
> * function.c (expand_function_end): Don't copy a TYPE_EMPTY_P
> result.
I'd go for it now. See below for a testsuite nit:
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/opt/empty3.C
> @@ -0,0 +1,8 @@
> +// PR c++/88529
> +// { dg-do compile { target c++11 } }
> +// { dg-final { scan-assembler-not mov { target x86_64-*-* } } }
But this probably needs to be { target { { i?86-*-* x86_64-*-* } && { ! { ia32
} } } }
or similar instead, so that it is done for x86_64 -m64 and -mx32, but not
-m32.
> +// The x86_64 psABI says that f() doesn't put the return value anywhere.
> +
> +class A{};
> +
> +A f() { return {}; }
Jakub
