mesa3d/docs/shading.html

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<h1>Shading Language Support</h1>

<p>
This page describes the features and status of Mesa's support for the
<a href="http://opengl.org/documentation/glsl/" target="_parent">
OpenGL Shading Language</a>.
</p>

<p>
Contents
</p>
<ul>
<li><a href="#envvars">Environment variables</a>
<li><a href="#120">GLSL 1.20 support</a>
<li><a href="#unsup">Unsupported Features</a>
<li><a href="#notes">Implementation Notes</a>
<li><a href="#hints">Programming Hints</a>
<li><a href="#standalone">Stand-alone GLSL Compiler</a>
<li><a href="#implementation">Compiler Implementation</a>
<li><a href="#validation">Compiler Validation</a>
</ul>


<a name="envvars">
<h2>Environment Variables</h2>

<p>
The <b>MESA_GLSL</b> environment variable can be set to a comma-separated
list of keywords to control some aspects of the GLSL compiler and shader
execution.  These are generally used for debugging.
</p>
<ul>
<li><b>dump</b> - print GLSL shader code to stdout at link time
<li><b>log</b> - log all GLSL shaders to files.
    The filenames will be "shader_X.vert" or "shader_X.frag" where X
    the shader ID.
<li><b>nopt</b> - disable compiler optimizations
<li><b>opt</b> - force compiler optimizations
<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
    the vertex position with ftransform() and passes through the color and
    texcoord[0] attributes.
<li><b>nopfrag</b> - force fragment shader to be a simple shader that passes
    through the color attribute.
<li><b>useprog</b> - log glUseProgram calls to stderr
</ul>
<p>
Example:  export MESA_GLSL=dump,nopt
</p>


<a name="120">
<h2>GLSL Version</h2>

<p>
The GLSL compiler currently supports version 1.20 of the shading language.
</p>

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


<a name="unsup">
<h2>Unsupported Features</h2>

<p>XXX update this section</p>

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

<ul>
<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.
<li>gl_ClipVertex
<li>The gl_Color and gl_SecondaryColor varying vars are interpolated
    without perspective correction
</ul>

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


<a name="notes">
<h2>Implementation Notes</h2>

<ul>
<li>Shading language programs are compiled into low-level programs
    very similar to those of GL_ARB_vertex/fragment_program.
<li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
    float[4] registers.
<li>Float constants and variables are packed so that up to four floats
    can occupy one program parameter/register.
<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.
<li>The ftransform() function doesn't necessarily match the results of
    fixed-function transformation.
</ul>

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


<a name="hints">
<h2>Programming Hints</h2>

<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>
</li>
</ul>


<a name="standalone">
<h2>Stand-alone GLSL Compiler</h2>

<p>
The stand-alone GLSL compiler program can be used to compile GLSL shaders
into low-level GPU code.
</p>

<p>
This tool is useful for:
<p>
<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>
After building Mesa, the compiler can be found at src/glsl/glsl_compiler
</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>
    src/glsl/glsl_compiler --dump-ast myshader.vert
</pre>

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
<li><b>--link</b> - ???
</ul>


<a name="implementation">
<h2>Compiler Implementation</h2>

<p>
The source code for Mesa's shading language compiler is in the
<code>src/glsl/</code> directory.
</p>

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

<h3>Code Generation Options</h3>

<p>
Internally, there are several options that control the compiler's code
generation and instruction selection.
These options are seen in the gl_shader_state struct and may be set
by the device driver to indicate its preferences:

<pre>
struct gl_shader_state
{
   ...
   /** Driver-selectable options: */
   GLboolean EmitHighLevelInstructions;
   GLboolean EmitCondCodes;
   GLboolean EmitComments;
};
</pre>

<ul>
<li>EmitHighLevelInstructions
<br>
This option controls instruction selection for loops and conditionals.
If the option is set high-level IF/ELSE/ENDIF, LOOP/ENDLOOP, CONT/BRK
instructions will be emitted.
Otherwise, those constructs will be implemented with BRA instructions.
</li>

<li>EmitCondCodes
<br>
If set, condition codes (ala GL_NV_fragment_program) will be used for
branching and looping.
Otherwise, ordinary registers will be used (the IF instruction will
examine the first operand's X component and do the if-part if non-zero).
This option is only relevant if EmitHighLevelInstructions is set.
</li>

<li>EmitComments
<br>
If set, instructions will be annoted with comments to help with debugging.
Extra NOP instructions will also be inserted.
</br>

</ul>


<a name="validation">
<h2>Compiler Validation</h2>

<p>
Developers working on the GLSL compiler should test frequently to avoid
regressions.
</p>

<p>
The <a href="http://people.freedesktop.org/~nh/piglit/">Piglit</a> project
has many GLSL tests and the
<a href="http://glean.sf.net" target="_parent">Glean</a> glsl1 test
tests GLSL features.
</p>

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

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