[Mesa-dev] [PATCH] Linking fails when not writing gl_Position.

[Marek Olšák]

On Tue, Sep 9, 2014 at 8:26 PM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> On Tuesday, September 09, 2014 10:30:16 AM Ian Romanick wrote:
>> On 09/08/2014 01:10 AM, Tapani Pälli wrote:
>> > From: Kalyan Kondapally <kalyan.kondapally at intel.com>
>> >
>> > According to GLSL-ES Spec(i.e. 1.0, 3.0), gl_Position value is undefined
>> > after the vertex processing stage if we don't write gl_Position. However,
>> > GLSL 1.10 Spec mentions that writing to gl_Position is mandatory. In case
>> > of GLSL-ES, it's not an error and atleast the linking should pass.
>> > Currently, Mesa throws an linker error in case we dont write to gl_position
>> > and Version is less then 140(GLSL) and 300(GLSL-ES). This patch changes
>> > it so that we don't report an error in case of GLSL-ES.
>>
>> Wow.  We can add this to the list of ways OpenGL ES is just plain
>> broken.  Since there is absolutely NO WAY this shader can produce any
>> useful results, we should at least do the courtesy of generating a
>> warning.  So, keep the first if-condistion the same, but do
>> linker_warning instead of linker_error if prog->IsES.
>
> I like that idea.  But what about 1.40 or 1.50?  I'm having a hard time imagining how 1.40 would let you produce any useful results, either (so maybe we should issue a warning there too).  With 1.50, you at least have geometry shaders, so you can produce real output later...

If you are using transform feedback or store instructions with
rasterization disabled, gl_Position is useless.

Marek