Re: [PATCH 1/3] configure: Fix atomic64 test for --enable-werror on macOS

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

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On Freitag, 24. Juli 2020 16:32:18 CEST Thomas Huth wrote:
> When using --enable-werror for the macOS builders in the Cirrus-CI,
> the atomic64 test is currently failing, and config.log shows a bunch
> of error messages like this:
> 
>  config-temp/qemu-conf.c:6:7: error: implicit declaration of function
>  '__atomic_load_8' is invalid in C99
> [-Werror,-Wimplicit-function-declaration] y = __atomic_load_8(&x, 0);
>       ^
>  config-temp/qemu-conf.c:6:7: error: this function declaration is not a
>  prototype [-Werror,-Wstrict-prototypes]
> 
> Seems like these __atomic_*_8 functions are available in one of the
> libraries there, so that the test links and passes there when not
> using --enable-werror. But there does not seem to be a valid prototype
> for them in any of the header files, so that the test fails when using
> --enable-werror.
> 
> Fix it by using the "official" built-in functions instead (see e.g.
> https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html).
> We are not using the *_8 variants in QEMU anyway.
> 
> Suggested-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  configure | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/configure b/configure
> index 4bd80ed507..9eaf501f50 100755
> --- a/configure
> +++ b/configure
> @@ -5919,11 +5919,11 @@ int main(void)
>  {
>    uint64_t x = 0, y = 0;
>  #ifdef __ATOMIC_RELAXED
> -  y = __atomic_load_8(&x, 0);
> -  __atomic_store_8(&x, y, 0);
> -  __atomic_compare_exchange_8(&x, &y, x, 0, 0, 0);
> -  __atomic_exchange_8(&x, y, 0);
> -  __atomic_fetch_add_8(&x, y, 0);
> +  y = __atomic_load_n(&x, __ATOMIC_RELAXED);
> +  __atomic_store_n(&x, y, __ATOMIC_RELAXED);
> +  __atomic_compare_exchange_n(&x, &y, x, 0, __ATOMIC_RELAXED,
> __ATOMIC_RELAXED); +  __atomic_exchange_n(&x, y, __ATOMIC_RELAXED);

Ah right, there is also the __atomic_*_n() variant of these functions. I 
actually had the more generic variants in mind.

But LGTM and yes, it resolves the warnings on macOS, so ...

Reviewed-by: Christian Schoenebeck <qemu_oss@crudebyte.com>

> +  __atomic_fetch_add(&x, y, __ATOMIC_RELAXED);
>  #else
>    typedef char is_host64[sizeof(void *) >= sizeof(uint64_t) ? 1 : -1];
>    __sync_lock_test_and_set(&x, y);

Best regards,
Christian Schoenebeck

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