[Mesa-dev] [RFC PATCH] glsl: Allow mixing of GLSL 1.40 and later shader versions.

[Kenneth Graunke]

On 10/16/2013 04:56 PM, Ian Romanick wrote:
> On 10/16/2013 10:29 AM, Paul Berry wrote:
>> ---
>>
>> I'm not 100% sure this is the right way to go, and here's why:
>>
>> Taken together, all the GLSL specs except GLSL 4.30 and GLSL 4.40 tell
>> a consistent story: desktop shader versions 1.10 and 1.20 may be
>> linked together, and desktop shader versions 1.40 and above may be
>> linked together.  No other cross-version linking is allowed.
>>
>> However, cross-version linking restrictions were explicitly removed in
>> GLSL 4.30 (the change is listed under "Summary of Changes from Version
>> 4.20 as "Remove cross-version linking restrictions.").  GLSL 4.30 and
>> 4.40 state that *any* version of desktop GLSL may be linked with any
>> other version of desktop GLSL.  (Note that cross-version linking is
>> still prohibited for ES shaders).
> 
> This came from a Khronos bug that I submitted.  The problem is that no
> other driver enforces the spec mandated restriction.  On top of that,
> you can't fully enforce the restriction (without draw-time errors) with
> separate shader objects.  I *thought* the change in 4.3 was to allow
> mixed versions between stages, but mixing versions within a stage is
> still forbidden.
> 
>> This leads to a conundrum.  Normally when the GLSL spec changes from
>> one version to the next, we implement different rules depending on the
>> user-supplied "#version" directive.  But we can't do that for
>> cross-version linking rules since it's not clear which version of GLSL
>> should apply.  Should we:
>>
>> (a) always follow pre-GLSL 4.30 linking rules, since we don't support
>> GLSL 4.30 yet?  (that's what this patch implements).
>>
>> (b) always follow post-GLSL 4.30 linking rules, since they're probably
>> the clearest reflection of the Khronos board's intent?
>>
>> (c) make some kind of dynamic determination of which set of rules to
>> follow?
>>
>> FWIW, the NVIDIA proprietary driver for Linux (version 313.18) appears
>> to implement (b).
> 
> There are different cases: intrastage and interstage.  I assume they
> allow mixing interstage.  What about intrastage?

I ran a quick test on my AMD Radeon 6870, using Catalyst 13.10 (which
supports 4.30).  It allowed both kinds of linking.  Here's my test:

[require]
GL >= 2.1
GLSL >= 1.10

[vertex shader]
#version 400 compatibility

void main()
{
    gl_Position = gl_Vertex;
}

[fragment shader]
#version 330 core
uniform vec4 color;

vec4 get_color()
{
    return color;
}

[fragment shader]
#version 110
vec4 get_color();

void main()
{
    gl_FragColor = get_color();
}

[test]
uniform vec4 color 0 1 0 1
draw rect -1 -1 2 2
relative probe rgba (0.5, 0.5) (0.0, 1.0, 0.0, 1.0)