[Mesa-dev] Can we use anonymous unions?

[Jason Ekstrand]

On Tue, Aug 18, 2015 at 5:36 PM, Ian Romanick <idr at freedesktop.org> wrote:
> On 08/18/2015 10:08 AM, Connor Abbott wrote:
>> On Tue, Aug 18, 2015 at 9:56 AM, Kenneth Graunke <kenneth at whitecape.org> wrote:
>>> Hey,
>>>
>>> I was thinking about using an anonymous union.  Specifically, something
>>> like:
>>>
>>> struct shader {
>>>    ...
>>>    union {
>>>       struct geometry_shader_info gs;
>>>       struct tess_eval_shader_info tes;
>>>       ...
>>>    };
>>> };
>>>
>>> Are those acceptable in Mesa?  I don't think we've traditionally used
>>> them, but I'm not sure why.  Apparently they're part of C11, though not
>>> part of C99.  However, GCC allows them, presumably Clang, and it looks
>>> like MSVC 2005 supports them in C:
>>>
>>> https://msdn.microsoft.com/en-us/library/y9zewe0d%28v=vs.80%29.aspx
>>>
>>> We might be able to use these to avoid some of our fun
>>> thing->base.Base.base.program.Base.base shenanigans...
>>>
>>> --Ken
>>>
>>> _______________________________________________
>>> mesa-dev mailing list
>>> mesa-dev at lists.freedesktop.org
>>> http://lists.freedesktop.org/mailman/listinfo/mesa-dev
>>>
>>
>> Well, we already use anonymous unions in various places in NIR (for
>> example, the definition of nir_dest and nir_src), so they should be ok
>> for core Mesa. IIRC there was a problem with combining anonymous
>> unions and C99-style initializers, but it doesn't sound like you'll be
>> too worried about that.
>
> Hm... is that why I get thousands of uninitialized field warnings?  Or
> are NIR_SRC_INIT and NIR_DEST_INIT just missing some things?

That depends on how you look at it.  You could say that they are
missing things.  However, the correct initialization for either a
nir_src or a nir_dest is effectively a memset to 0.  Since C (and C++)
guarantee that missing fields in a structure initializer (or compound
literal) are initialized to 0 as long as one of them is, we get what
we want without having to have the entire struct layout full of zeros.
Also, since there is a union or two in there, it makes everything
easier.

> ../../../../../src/glsl/nir/nir.h: In function 'nir_src
> nir_src_for_ssa(nir_ssa_def*)':
> ../../../../../src/glsl/nir/nir.h:552:41: warning: missing initializer
> for member 'nir_src::use_link' [-Wmissing-field-initializers]
>     nir_src src = NIR_SRC_INIT;
>                                          ^
> ../../../../../src/glsl/nir/nir.h:552:41: warning: missing initializer
> for member 'nir_src::<anonymous>' [-Wmissing-field-initializers]
> ../../../../../src/glsl/nir/nir.h:552:41: warning: missing initializer
> for member 'nir_src::is_ssa' [-Wmissing-field-initializers]
> ../../../../../src/glsl/nir/nir.h: In function 'nir_src
> nir_src_for_reg(nir_register*)':
> ../../../../../src/glsl/nir/nir.h:563:41: warning: missing initializer
> for member 'nir_src::use_link' [-Wmissing-field-initializers]
>     nir_src src = NIR_SRC_INIT;
>                                          ^
> ../../../../../src/glsl/nir/nir.h:563:41: warning: missing initializer
> for member 'nir_src::<anonymous>' [-Wmissing-field-initializers]
> ../../../../../src/glsl/nir/nir.h:563:41: warning: missing initializer
> for member 'nir_src::is_ssa' [-Wmissing-field-initializers]
> ../../../../../src/glsl/nir/nir.h: In function 'nir_dest
> nir_dest_for_reg(nir_register*)':
> ../../../../../src/glsl/nir/nir.h:576:48: warning: missing initializer
> for member 'nir_reg_dest::def_link' [-Wmissing-field-initializers]
>     nir_dest dest = NIR_DEST_INIT;
>                                                 ^
> ../../../../../src/glsl/nir/nir.h:576:48: warning: missing initializer
> for member 'nir_reg_dest::reg' [-Wmissing-field-initializers]
> ../../../../../src/glsl/nir/nir.h:576:48: warning: missing initializer
> for member 'nir_reg_dest::indirect' [-Wmissing-field-initializers]
> ../../../../../src/glsl/nir/nir.h:576:48: warning: missing initializer
> for member 'nir_reg_dest::base_offset' [-Wmissing-field-initializers]
> ../../../../../src/glsl/nir/nir.h:576:48: warning: missing initializer
> for member 'nir_dest::is_ssa' [-Wmissing-field-initializers]
>
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>
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