[Mesa-dev] About the usage of the "flat" interpolation qualifier with input (unsigned) integers and doubles in fragment shaders

[Andres Gomez]

Hi,

I haven't had any input on this and I will welcome opinions from more
knowledgeable developers :)

Do you think my approach in this case would be the best?
      * To enforce the usage of the "flat" qualifier when in a member of
        an input block interface in a fragment shader for (unsigned)
        integers and doubles.
      * Open a bug in Khronos specs bugzilla about the ambiguity of
        this.

Br.

On Fri, 2016-03-04 at 15:43 +0200, Andres Gomez wrote:
> Hi,
> 
> as complementary work to the one done to "Add FP64 support to the i965
> shader backends" at:
> https://bugs.freedesktop.org/show_bug.cgi?id=92760
> 
> I've been working to add piglit tests that would check the new features
> added by this addition.
> 
> Checking the specification of the extension, when talking about inputs,
> the extension instructs that any fragment input being a double or
> derived type must be qualified with the interpolation qualifier "flat".
> 
> This extends the restriction that already existed for signed and
> unsigned integers to doubles too.
> 
> In following versions of the spec, when doubles are already part of it
> we can read:
> 
> "
> Fragment shader inputs that are signed or unsigned integers, integer
> vectors, or any double-precision floating-point type must be qualified
> with the interpolation qualifier flat.
> "
> 
> The specification doesn't explicitly say it, but as far as I understand,
> it is reasonable to think that the restriction applies to input
> variables in the form of arrays or members of structures of these types.
> 
> In fact, current implementation already fails on compilation if a
> non-flat integer is in a fragment input as an array or a struct member.
> The same happens with doubles (llvmpipe).
> 
> It is also reasonable to think that the restriction would also apply to
> input interface blocks holding variables of these types.
> 
> However, the compilation doesn't fail when using a non-flat integer or a
> double. On execution, the integer version just gives bogus values while,
> with doubles, it crashes.
> 
> --
> 
> The situation with the glslangValidator is (it doesn't support doubles):
>       * When using a non-flat integer as a fragment input in the form of
>         an array it fails with an error of the type:
> 
> int' : must be qualified as flat in
> 
>       * When using a non-flat integer as a fragment input in the form of
>         a member of a struct or an interface block it succeeds on compilation.
> 
> 
> The situation with nvidia proprietary driver is:
>       * When using a non-flat integer as a fragment input in the form of
>         a scalar, an array, a member of a struct or a member of an
>         interface block it doesn't fail on compilation but on linkage
>         with an error of the type:
> 
> error C5215: Integer varying <variable> must be flat 
> 
>       * When using a double as a fragment input in the form of a scalar
>         or an array it fails on compilation with an error of the type:
> 
> error C7570: 64 bit input '<variable>' should be flat 
> 
>       * When using a double as a fragment input in the form of a a
>         member of a struct or a member of an interface block it doesn't
>         fail. Actually, it seems to do some kind of interpolation.
> 
> --
> 
> Hence, based on this, my proposal would be:
>       * To enforce the usage of the "flat" qualifier when in a member of
>         an input block interface in a fragment shader for (unsigned)
>         integers and doubles.
>       * Open a bug in Khronos specs bugzilla about the ambiguity of
>         this.
> 
> 
> Opinions?
> 
> _______________________________________________
> mesa-dev mailing list
> mesa-dev at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/mesa-dev

-- 
Br,

Andres
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