[Mesa-dev] [PATCH] glsl: add support for gl_InstanceID

[Ian Romanick]

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On 03/14/2011 04:01 PM, Jakob Bornecrantz wrote:
> On Mon, Mar 14, 2011 at 11:46 PM, Ian Romanick <idr at freedesktop.org> wrote:
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>> On 03/10/2011 03:30 PM, Marcin Slusarz wrote:
>>> ...by copying support for gl_InstanceIDARB, but without "#extension" check,
>>> because EXT_draw_instanced spec does not say anything about it (as opposed
>>> to ARB_draw_instanced / gl_InstanceIDARB) and NVIDIA driver already allow it
>>
>> NAK for a couple reasons.
>>
>> This extension depends on EXT_gpu_shader4, which we don't support.
>> There's no #extension bit required for this extension because the GLSL
>> changes are implemented by EXT_gpu_shader4.  Without EXT_draw_instanced,
>> gl_InstanceID always reads 0.
>>
>> Note that EXT_gpu_shader4 says that a #extension line is required to use
>> the features of the extension, including gl_InstanceID.  If their driver
>> allows gl_InstanceID without a #extension line, it is a bug in their driver.
> 
> Oh you clearly haven't tried using new features on the nVidia blobs
> have you, bugs galore! Mind you, you do get a warning per feature used
> not #extension:ed or #version:ed in the shader log, but who bothers to
> read that as long as the shader compiles and links correctly[1] :-/

And a little googling will find a *ton* of people pissed that their
shaders work on NVIDIA's drivers and *nowhere* else.  NVIDIA's
implementation is clearly wrong here, and we should not copy their
portability bugs.

> The problem we have is that most who develops advanced 3D programs do
> it against the blobs since they have all the cool features, me
> included and yeah I'm lazy as well. Once EXT_gpu_shader4 lands in mesa
> I guess I would be motivated to revisit my shaders and code and make
> sure they are proper.

I'm not sure EXT_gpu_shader4 should ever land in Mesa.  All of the
features in EXT_gpu_shader4 are in 1.30 or 1.40 (right?).  Every piece
of hardware that currently supports EXT_gpu_shader4 already also
supports 1.30 and 1.40 (right?).  As a result, there aren't a lot of
apps that use just EXT_gpu_shader4 (and not 1.30 or 1.40), and it seems
unlikely that new ones are being developed.

But, this a debate we can have another day... since we don't have
support for /any/ of it yet.

> [1]The topping of this particular cake is that if you try to use any
> of the #versions or #extensions you get errors when you try to use
> deprecated things that are convenient to use, which promotes people to
> not do the right thing here and just ignore the #version and/or
> #extension for features.

How do you mean?  Unless you specifically request a "core" context,
everything should still work fine.  Certainly saying '#extension
GL_EXT_gpu_shader4: require' shouldn't cause anything to stop working.
Right?
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