[Mesa-dev] [PATCH] [rfc] glsl: allow invariant on fragment shader outputs.

[Andres Gomez]

Hei Dave,

recently, khronos bug 16140 was opened to address my comments regarding
the "#pragma STDGL invariant(all)" sentence in GLSL specs since 1.30.

The action to be taken will be removing such sentence. In other words,
the "invariant" keyword should be also legal for FS outputs.

Therefore, Dave, please, go ahead and land your patch.

Reviewed-by: Andres Gomez <agomez at igalia.com>

Br.

On Wed, 2016-08-10 at 16:26 +0300, Andres Gomez wrote:
> Hi,
> 
> On Mon, 2016-05-23 at 14:18 +1000, Dave Airlie wrote:
> > From: Dave Airlie <airlied at redhat.com>
> > 
> > A CTS test manages to generate this:
> > GL45-CTS.shading_language_420pack.qualifier_order
> > 
> > I cannot find definitive evidence in the spec that it isn't
> > allowed. The specs mentions some things can't be used on
> > fragment shader outputs, but never specifies invariant as
> > one of them. Chris found GLSL1.20 was more explicit about this
> > but the language was changed/removed in GLSL 1.30.
> > 
> > This might be a spec bug, I'm not sure.
> 
> 
> Specs are ambiguous, so I think it can be considered a bug in the
> specs, since they are not that explicit to ban the qualifier in the FS
> outputs.
> 
> However, my understanding is that using the qualifier is not valid and,
> hence, this patch should be dropped.
> 
> From page 27 (page 33 of the PDF) of the GLSL 1.20 spec:
>     
>   " Only variables output from a vertex shader can be candidates for
>     invariance."
>     
> But this later changes to:
>     
> From page 37 (page 43 of the PDF) of the GLSL 1.30 spec:
>     
>   " Only variables output from a shader can be candidates for
>     invariance."
>     
> However, there is little sense on allowing this qualifier for the FS
> outputs and, actually, we can also read, in this direction:
>     
> From page 37 (page 43 of the PDF) of the GLSL 1.30 spec:
>     
>   " Initially, by default, all output variables are allowed to be
>     variant. To force all output variables to be invariant, use the
>     pragma
>     
>       #pragma STDGL invariant(all)
>     
>     before all declarations in a shader. If this pragma is used after
>     the declaration of any variables or functions, then the set of
>     outputs that behave as invariant is undefined. It is an error to
>     use this pragma in a fragment shader."
>     
> Then, I think it is safe to assume that the individual use of
> "invariant" is also forbidden in FS outputs.
> 
> However, would the conclusion be that FS outputs are actually allowed
> to be qualified as invariant, this patch LGTM, then.
> 
> 
-- 
Br,

Andres