Re: [Qemu-devel] [PATCH] scsi: Address spurious clang warning

[Eric Blake <eblake@redhat.com>]

On 11/27/18 12:49 PM, John Snow wrote:
> Some versions of Clang prior to 6.0 (and some builds of clang after,
> such as 6.0.1-2.fc28) fail to recognize { 0 } as a valid initializer
> for a struct with subobjects when -Wmissing-braces is enabled.
> 
> https://bugs.llvm.org/show_bug.cgi?id=21689 and
> https://reviews.llvm.org/rL314499 suggests this should be fixed in 6.0,
> but it might not be the case for older versions or downstream versions.
> 
> For now, follow the precedent of ebf2a499 and replace the standard { 0 }
> with the accepted { } to silence this warning and allow the build to
> work under clang 6.0.1-2.fc28, and builds prior to 6.0.
> 
> Signed-off-by: John Snow <jsnow@redhat.com>
> 

Reviewed-by: Eric Blake <eblake@redhat.com>

I'm okay if this goes into -rc3 as a build-fix; I'm also okay if it 
slips to 4.0.

> ---
> 
> What I am actually less clear on is why this appears to be a problem
> only now; since the introduction of { 0 } was in 2.11. It might be
> a regression only in the fedora distribution of Clang 6.0.

Or even a redefinition of struct dm_ioctl in some header where you are 
just now picking up a new struct layout that tickles the Clang issue in 
relation to the previous layout (since it is possible to have two 
structs that are ABI-compatible but where only one of the two has a 
nested substruct).

> +++ b/scsi/qemu-pr-helper.c
> @@ -236,7 +236,7 @@ static void dm_init(void)
>           perror("Cannot open " CONTROL_PATH);
>           exit(1);
>       }
> -    struct dm_ioctl dm = { 0 };
> +    struct dm_ioctl dm = { };

Random thought: would it be worth having "qemu/compiler.h" define a macro:

#if ...broken clang
#define ZERO_INIT {}
#else
#define ZERO_INIT {0}
#endif

and then rewrite all our '= { 0? }' initializers into '= ZERO_INIT'?  Or 
is that aesthetically too ugly?

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