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

[John Snow <jsnow@redhat.com>]

On 11/27/18 2:02 PM, Eric Blake wrote:
> 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?
> 

Obscures perfectly legitimate C code without solving anything, IMO. As
much code as can reflect "naked" C89/C99/GNU99, the better.

--js