[Mesa-dev] [PATCH 10/17] glsl: Define a gl_LastFragData built-in for GLSL versions that have gl_FragData.

[Francisco Jerez]

Kenneth Graunke <kenneth at whitecape.org> writes:

> On Wednesday, July 20, 2016 9:49:40 PM PDT Francisco Jerez wrote:
>> The EXT_shader_framebuffer_fetch extension defines alternative
>> language for GLES2 shaders where user-defined fragment outputs are not
>> allowed.  Instead of using inout user-defined fragment outputs the
>> shader is expected to read from the gl_LastFragData built-in array.
>> In addition this allows using the same language on desktop GLSL
>> versions prior to 4.2 that support the deprecated gl_FragData built-in
>> in preparation for the MESA_shader_framebuffer_fetch desktop GL
>> extension.
>> 
>> Both legacy and user-defined inout outputs have a common
>> representation at the GLSL IR level, so it shouldn't make any
>> difference for optimization passes and back-ends whether the
>> application is using gl_LastFragData or user-defined outputs, all
>> they'll see is a variable dereference of a fragment output at a
>> certain interface location with the fb_fetch_output bit set to one.
>> ---
>>  src/compiler/glsl/builtin_variables.cpp | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
>> 
>> diff --git a/src/compiler/glsl/builtin_variables.cpp b/src/compiler/glsl/builtin_variables.cpp
>> index f63dc3a..6a756ed 100644
>> --- a/src/compiler/glsl/builtin_variables.cpp
>> +++ b/src/compiler/glsl/builtin_variables.cpp
>> @@ -1136,6 +1136,16 @@ builtin_variable_generator::generate_fs_special_vars()
>>                   array(vec4_t, state->Const.MaxDrawBuffers), "gl_FragData");
>>     }
>>  
>> +   if (state->has_framebuffer_fetch() && !state->is_version(420, 300)) {
>> +      ir_variable *const var =
>> +         add_output(FRAG_RESULT_DATA0,
>> +                    array(vec4_t, state->Const.MaxDrawBuffers),
>> +                    "gl_LastFragData");
>> +      var->data.precision = GLSL_PRECISION_MEDIUM;
>> +      var->data.read_only = 1;
>> +      var->data.fb_fetch_output = 1;
>> +   }
>> +
>
> Personally, I'd only create gl_LastFragData in desktop 1.10/1.20,
> and not 1.30+ where it's deprecated.  Sure, you /can/ use it, but
> you can also do the 'inout' syntax that's the preferred way going
> forward, so we may as well just require shader authors to do so.
>

My thinking here was that since we need to drop the gl_LastFragData
built-in at some point, it would be the least confusing if we did it at
the same point that Khronos dropped gl_FragData.  It doesn't seem like
it will make much of a difference in practice though, because this
extension has never been available in desktop GL, so backwards
compatibility is not really a concern -- I've changed this series
locally to check for GLSL 1.3 instead of GLSL 4.2 in the code above and
in PATCH 12, and updated my draft spec text so that gl_LastFragData is
not required to be present on GLSL 1.3 and above.

>>     if (state->es_shader && state->language_version == 100 && state->EXT_blend_func_extended_enable) {
>>        /* We make an assumption here that there will only ever be one dual-source draw buffer
>>         * In case this assumption is ever proven to be false, make sure to assert here
>> 
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