[Mesa-dev] [PATCH 0/6] Eliminating unused built-in varyings

[Ian Romanick]

On 06/28/2013 08:42 AM, Ian Romanick wrote:
> On 06/13/2013 05:25 AM, Marek Olšák wrote:
>> Hi everyone,
>>
>> this series adds a new GLSL compiler optimization pass which
>> eliminates unused and set-but-unused built-in varyings and adds a few
>> improvements to the GLSL linker in the process.
>>
>> Before I show you how it works, I wanna say that there are patches
>> which are related to and will most probably conflict with the geometry
>> shader work, but they are necessary because the linkage of varyings is
>> largely suboptimal.
>>
>> Also, the GL_EXT_separate_shader_objects extension must be disabled
>> for this optimization to be enabled. The reason is a program object with
>> both a VS and FS can be bound partially, e.g. by
>> glUseShaderProgramEXT(GL_VERTEX_SHADER, prog), so the extension makes
>> every program object be just a set of "separate shaders". The extension
>> is not important anyway.
>
> Could you elaborate on this a bit?  The problem already exists that
> shader can be par-linked (e.g., just a vertex shader) and used with
> fixed-function.  A big part of the reason that I implemented EXT_sso
> back in the day was to generate infrastructure, etc. for better using
> shaders with fixed function.
>
>> Now, to illustrate how the optimization works, consider these 2 shader
>> IR dumps:
>>
>>
>> Vertex shader (8 varyings):
>> ...
>> (declare (shader_out ) vec4 gl_FrontColor)
>> (declare (shader_out ) vec4 gl_FrontSecondaryColor)
>> (declare (shader_out ) (array vec4 6) gl_TexCoord)
>> (function main
>>    (signature void
>>      (parameters
>>      )
>>      (
>>        ...
>>        (assign  (xyzw) (var_ref gl_FrontColor)  (var_ref gl_Color) )
>>        (assign  (xyzw) (var_ref gl_FrontSecondaryColor)  (var_ref
>> gl_SecondaryColor) )
>>        (assign  (xyzw) (array_ref (var_ref gl_TexCoord) (constant int
>> (1)) )  (var_ref gl_MultiTexCoord1) )
>>        (assign  (xyzw) (array_ref (var_ref gl_TexCoord) (constant int
>> (4)) )  (var_ref gl_MultiTexCoord4) )
>>        (assign  (xyzw) (array_ref (var_ref gl_TexCoord) (constant int
>> (5)) )  (var_ref gl_MultiTexCoord5) )
>>      ))
>> )
>>
>> Fragment shader (6 varyings):
>> ...
>> (declare (shader_in ) vec4 gl_SecondaryColor)
>> (declare (shader_in ) (array vec4 5) gl_TexCoord)
>> (function main
>>    (signature void
>>      (parameters
>>      )
>>      (
>>        (declare () vec4 r)
>>        (assign  (xyzw) (var_ref r)  ... (var_ref gl_SecondaryColor) ) )
>>        (assign  (xyzw) (var_ref r)  ... (array_ref (var_ref
>> gl_TexCoord) (constant int (1)) ) ) ) )
>>        (assign  (xyzw) (var_ref r)  ... (array_ref (var_ref
>> gl_TexCoord) (constant int (2)) ) ) ) )
>>        (assign  (xyzw) (var_ref r)  ... (array_ref (var_ref
>> gl_TexCoord) (constant int (3)) ) ) ) )
>>        (declare (temporary ) vec4 assignment_tmp)
>>        (assign  (xyzw) (var_ref assignment_tmp)  ... (array_ref
>> (var_ref gl_TexCoord) (constant int (4)) ) ) ) )
>>        ...
>>      ))
>> )
>>
>>
>> Note that only gl_TexCoord[1], gl_TexCoord[4], and gl_SecondaryColor
>> are used by both shaders. The optimization replaces all occurences of
>> varyings which are unused by the other stage by temporary variables. It
>> also breaks down the gl_TexCoord array into separate vec4 variables if
>> needed. Here's the result:
>
> This sounds similar to the way Paul's varying packing works.  Is there
> synergy there?  Also, since variables are renamed, does this interact
> with transform feedback?  The queries of GL_ARB_program_interface_query?
>   I suspect that it won't since this only affects built-in varyings, and
> built-in varyings aren't usable with those interfaces.

I take part of that back.  You can do transform feedback on built-in 
varyings.  I had that crossed with not being able to do XFB on the 
output of fixed-function.

>> Vertex shader (3 varyings instead of 8):
>> ...
>> (declare (shader_out ) vec4 gl_out_TexCoord1)
>> (declare (shader_out ) vec4 gl_out_TexCoord4)
>> (declare (temporary ) vec4 gl_out_TexCoord5_dummy)
>> (declare (temporary ) vec4 gl_out_FrontColor0_dummy)
>> (declare (shader_out ) vec4 gl_FrontSecondaryColor)
>> (function main
>>    (signature void
>>      (parameters
>>      )
>>      (
>>        ...
>>        (assign  (xyzw) (var_ref gl_out_FrontColor0_dummy)  (var_ref
>> gl_Color) )
>>        (assign  (xyzw) (var_ref gl_FrontSecondaryColor)  (var_ref
>> gl_SecondaryColor) )
>>        (assign  (xyzw) (var_ref gl_out_TexCoord1)  (var_ref
>> gl_MultiTexCoord1) )
>>        (assign  (xyzw) (var_ref gl_out_TexCoord4)  (var_ref
>> gl_MultiTexCoord4) )
>>        (assign  (xyzw) (var_ref gl_out_TexCoord5_dummy)  (var_ref
>> gl_MultiTexCoord5) )
>>      ))
>> )
>>
>> Fragment shader (3 varyings instead of 6):
>> ...
>> (declare (shader_in ) vec4 gl_in_TexCoord1)
>> (declare (temporary ) vec4 gl_in_TexCoord2_dummy)
>> (declare (temporary ) vec4 gl_in_TexCoord3_dummy)
>> (declare (shader_in ) vec4 gl_in_TexCoord4)
>> (declare (shader_in ) vec4 gl_SecondaryColor)
>> (function main
>>    (signature void
>>      (parameters
>>      )
>>      (
>>        (declare () vec4 r)
>>        (assign  (xyzw) (var_ref r)  ... (var_ref gl_SecondaryColor) ) )
>>        (assign  (xyzw) (var_ref r)  ... (var_ref gl_in_TexCoord1) ) ) )
>>        (assign  (xyzw) (var_ref r)  ... (var_ref
>> gl_in_TexCoord2_dummy) ) ) )
>>        (assign  (xyzw) (var_ref r)  ... (var_ref
>> gl_in_TexCoord3_dummy) ) ) )
>>        (declare (temporary ) vec4 assignment_tmp)
>>        (assign  (xyzw) (var_ref assignment_tmp)  ... (var_ref
>> gl_in_TexCoord4) ) ) )
>>        ...
>>      ))
>> )
>>
>> The locations of varyings remain the same. That's all. Please review.
>>
>> Marek
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