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  <title>Shading Language</title>
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  The Mesa 3D Graphics Library
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<h1>Shading Language</h1>
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<p>
This page describes the features and status of Mesa's support for the
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<a href="https://opengl.org/documentation/glsl/">
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OpenGL Shading Language</a>.
</p>

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<p>
Contents
</p>
<ul>
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<li><a href="#envvars">Environment variables</a>
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<li><a href="#support">GLSL 1.40 support</a>
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<li><a href="#unsup">Unsupported Features</a>
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<li><a href="#notes">Implementation Notes</a>
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<li><a href="#hints">Programming Hints</a>
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<li><a href="#standalone">Stand-alone GLSL Compiler</a>
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<li><a href="#implementation">Compiler Implementation</a>
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<li><a href="#validation">Compiler Validation</a>
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</ul>


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<h2 id="envvars">Environment Variables</h2>
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<p>
The <b>MESA_GLSL</b> environment variable can be set to a comma-separated
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list of keywords to control some aspects of the GLSL compiler and shader
execution.  These are generally used for debugging.
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</p>
<ul>
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<li><b>dump</b> - print GLSL shader code to stdout at link time
<li><b>log</b> - log all GLSL shaders to files.
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    The filenames will be "shader_X.vert" or "shader_X.frag" where X
    the shader ID.
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<li><b>cache_info</b> - print debug information about shader cache
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<li><b>cache_fb</b> - force cached shaders to be ignored and do a full
    recompile via the fallback path</li>
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<li><b>uniform</b> - print message to stdout when glUniform is called
<li><b>nopvert</b> - force vertex shaders to be a simple shader that just transforms
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    the vertex position with ftransform() and passes through the color and
    texcoord[0] attributes.
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<li><b>nopfrag</b> - force fragment shader to be a simple shader that passes
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    through the color attribute.
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<li><b>useprog</b> - log glUseProgram calls to stderr
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<li><b>errors</b> - GLSL compilation and link errors will be reported to stderr.
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</ul>
<p>
Example:  export MESA_GLSL=dump,nopt
</p>

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<h3 id="replacement">Experimenting with Shader Replacements</h3>
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<p>
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Shaders can be dumped and replaced on runtime for debugging purposes. This
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feature is not currently supported by SCons build.

This is controlled via following environment variables:
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</p>
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<ul>
<li><b>MESA_SHADER_DUMP_PATH</b> - path where shader sources are dumped
<li><b>MESA_SHADER_READ_PATH</b> - path where replacement shaders are read
</ul>
Note, path set must exist before running for dumping or replacing to work. 
When both are set, these paths should be different so the dumped shaders do 
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not clobber the replacement shaders. Also, the filenames of the replacement shaders
should match the filenames of the corresponding dumped shaders.
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<h3 id="capture">Capturing Shaders</h3>

<p>
Setting <b>MESA_SHADER_CAPTURE_PATH</b> to a directory will cause the compiler
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to write <code>.shader_test</code> files for use with
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<a href="https://gitlab.freedesktop.org/mesa/shader-db">shader-db</a>, a tool
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which compiler developers can use to gather statistics about shaders
(instructions, cycles, memory accesses, and so on).
</p>
<p>
Notably, this captures linked GLSL shaders - with all stages together -
as well as ARB programs.
</p>

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<h2 id="support">GLSL Version</h2>
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<p>
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The GLSL compiler currently supports version 3.30 of the shading language.
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</p>

<p>
Several GLSL extensions are also supported:
</p>
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<ul>
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<li>GL_ARB_draw_buffers
<li>GL_ARB_fragment_coord_conventions
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<li>GL_ARB_shader_bit_encoding
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</ul>


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<h2 id="unsup">Unsupported Features</h2>
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<p>XXX update this section</p>

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<p>
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The following features of the shading language are not yet fully supported
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in Mesa:
</p>

<ul>
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<li>Linking of multiple shaders does not always work.  Currently, linking
    is implemented through shader concatenation and re-compiling.  This
    doesn't always work because of some #pragma and preprocessor issues.
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<li>The gl_Color and gl_SecondaryColor varying vars are interpolated
    without perspective correction
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</ul>

<p>
All other major features of the shading language should function.
</p>


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<h2 id="notes">Implementation Notes</h2>
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<ul>
<li>Shading language programs are compiled into low-level programs
    very similar to those of GL_ARB_vertex/fragment_program.
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<li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
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    float[4] registers.
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<li>Float constants and variables are packed so that up to four floats
    can occupy one program parameter/register.
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<li>All function calls are inlined.
<li>Shaders which use too many registers will not compile.
<li>The quality of generated code is pretty good, register usage is fair.
<li>Shader error detection and reporting of errors (InfoLog) is not
    very good yet.
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<li>The ftransform() function doesn't necessarily match the results of
    fixed-function transformation.
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</ul>

<p>
These issues will be addressed/resolved in the future.
</p>


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<h2 id="hints">Programming Hints</h2>
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<ul>
<li>Use the built-in library functions whenever possible.
    For example, instead of writing this:
<pre>
        float x = 1.0 / sqrt(y);
</pre>
    Write this:
<pre>
        float x = inversesqrt(y);
</pre>
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</li>
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</ul>


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<h2 id="standalone">Stand-alone GLSL Compiler</h2>
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<p>
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The stand-alone GLSL compiler program can be used to compile GLSL shaders
into low-level GPU code.
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</p>

<p>
This tool is useful for:
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</p>
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<ul>
<li>Inspecting GPU code to gain insight into compilation
<li>Generating initial GPU code for subsequent hand-tuning
<li>Debugging the GLSL compiler itself
</ul>

<p>
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After building Mesa, the compiler can be found at src/compiler/glsl/glsl_compiler
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</p>

<p>
Here's an example of using the compiler to compile a vertex shader and
emit GL_ARB_vertex_program-style instructions:
</p>
<pre>
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    src/compiler/glsl/glsl_compiler --version XXX --dump-ast myshader.vert
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</pre>

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Options include
<ul>
<li><b>--dump-ast</b> - dump GPU code
<li><b>--dump-hir</b> - dump high-level IR code
<li><b>--dump-lir</b> - dump low-level IR code
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<li><b>--dump-builder</b> - dump GLSL IR code
<li><b>--link</b> - link shaders
<li><b>--just-log</b> - display only shader / linker info if exist,
without any header or separator
<li><b>--version</b> - [Mandatory] define the GLSL version to use
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</ul>

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<h2 id="implementation">Compiler Implementation</h2>
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<p>
The source code for Mesa's shading language compiler is in the
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<code>src/compiler/glsl/</code> directory.
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</p>

<p>
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XXX provide some info about the compiler....
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</p>

<p>
The final vertex and fragment programs may be interpreted in software
(see prog_execute.c) or translated into a specific hardware architecture
(see drivers/dri/i915/i915_fragprog.c for example).
</p>

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<h2 id="validation">Compiler Validation</h2>
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<p>
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Developers working on the GLSL compiler should test frequently to avoid
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regressions.
</p>
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<p>
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The <a href="https://piglit.freedesktop.org/">Piglit</a> project
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has many GLSL tests.
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</p>

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<p>
The Mesa demos repository also has some good GLSL tests.
</p>
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