[Mesa-dev] [PATCH 4/8] glsl: Add new builtins required by GL_ARB_sample_shading

[Ian Romanick]

On 10/15/2013 07:50 PM, Kenneth Graunke wrote:
> On 10/15/2013 01:58 PM, Ian Romanick wrote:
>> On 10/15/2013 01:50 PM, Anuj Phogat wrote:
>>> On Tue, Oct 15, 2013 at 10:02 AM, Ian Romanick <idr at freedesktop.org> wrote:
>>>> On 10/14/2013 10:12 AM, Anuj Phogat wrote:
>>>>> @@ -789,6 +794,12 @@ builtin_variable_generator::generate_fs_special_vars()
>>>>>        if (state->AMD_shader_stencil_export_warn)
>>>>>           var->warn_extension = "GL_AMD_shader_stencil_export";
>>>>>     }
>>>>> +
>>>>> +   if (state->ARB_sample_shading_enable) {
>>>>> +      add_input(VARYING_SLOT_SAMPLE_ID, int_t, "gl_SampleID");
>>>>> +      add_input(VARYING_SLOT_SAMPLE_POS, vec2_t, "gl_SamplePosition");
>>>>> +      add_output(FRAG_RESULT_SAMPLE_MASK, array(int_t, 1), "gl_SampleMask");
>>>>
>>>> I don't see code anywhere in this patch series that correctly sizes this
>>>> array.  I thought gl_SampleMask.length() == gl_NumSamples... except you
>>>> can't use .length() on gl_SampleMask because the size can't be known at
>>>> compile-time.  We should have a test that 'gl_SampleMask.length()' fails
>>>> to compile. :)
>>>>
>>> I'll add a piglit compiler test for that.
>>>
>>>> The only other array that works like this is gl_TexCoord.  That array is
>>>> sized to 0 (line 837 of builtin_variables.cpp).  I believe gl_SampleMask
>>>> should do the same.
>>>>
>>> Yes, I noticed  gl_TexCoord is using array size of 0 and it works for
>>> gl_SampleMask as well.  But, It generates same glsl ir with array size
>>> of 0 or 1:
>>> (declare (shader_out ) (array int 1) gl_SampleMask)
>>>
>>> what's the utility of using size 0?
>>> I'll take care of rest of your comments in this patch.
>>
>> Using size 0 is like writing
>>
>> int gl_SampleMask[]; // unsized
>>
>> in the shader.  Later during compilation, usually in linking, unsized
>> arrays become sized.  Somewhere between generating the built-in variable
>> and printing the IR a size is established for it.
> 
> Right, but gl_SampleMask is not an unsized array.  It's statically sized
> based on your implementation's maximum supported number of color samples
> (ceil(samples/32)), and this is totally known at compile time...

That doesn't match what the spec says.  At least the GLSL 4.00 spec:

    "In the fragment language, built-in variables are intrinsically
    declared as:

        in vec4 gl_FragCoord;
        in bool gl_FrontFacing;
        in float gl_ClipDistance[];
        in vec2 gl_PointCoord;
        in int gl_PrimitiveID;
        in int gl_SampleID;
        in vec2 gl_SamplePosition;
        out vec4 gl_FragColor;
        out vec4 gl_FragData[gl_MaxDrawBuffers];
        out float gl_FragDepth;
        out int gl_SampleMask[];"

Note that both gl_ClipDistance and gl_SampleMask are unsized, and both
have a maximum size based on implementation limits.  Later the GLSL 4.00
spec says:

    "This array must be sized in the fragment shader either implicitly
    or explicitly to be the same size as the implementation-dependent
    maximum sample-mask (as an array of 32bit elements), determined by
    the maximum number of samples."

The maximum size of the array is ceil(max_samples/32), but initially it
is unsized.

> --Ken