Re: [Qemu-devel] [PATCH] hw/display/qxl: Suppress clang-7 warning about misaligned atomic operation

[Eric Blake <eblake@redhat.com>]

On 9/27/18 10:55 AM, Peter Maydell wrote:

> Newer Spice versions (with Spice commit
> beda5ec7a6848be20c0cac2a9a8ef2a41e8069c1) will fix the bug;
> for older Spice versions, work around it by telling the compiler
> explicitly that the alignment is OK using __builtin_assume_aligned().
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>   include/qemu/compiler.h |  9 +++++++++
>   hw/display/qxl.c        | 26 +++++++++++++++++++++++++-
>   2 files changed, 34 insertions(+), 1 deletion(-)
> 

> +#ifdef HAS_ASSUME_ALIGNED
> +#define ALIGNED_UINT32_PTR(P) ((uint32_t *)__builtin_assume_aligned(P, 4))
> +#else
> +#define ALIGNED_UINT32_PTR(P) ((uint32_t *)P)
> +#endif
> +
> +    old_pending = atomic_fetch_or(ALIGNED_UINT32_PTR(&d->ram->int_pending),

The only client of your macro is not impacted, but if you want to be 
technically robust against all future uses of the macro, the fallback is 
underparenthesized, and would be safer as:

#define ALIGNED_UINT32_PTR(P) ((uint32_t *)(P))

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org