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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html lang="en">
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8">
<title>Shading Language Support</title>
<link rel="stylesheet" type="text/css" href="mesa.css">
</head>
<body>
<div class="header">
<h1>The Mesa 3D Graphics Library</h1>
</div>
<iframe src="contents.html"></iframe>
<div class="content">
<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/">
OpenGL Shading Language</a>.
</p>
<p>
Contents
</p>
<ul>
<li><a href="#envvars">Environment variables</a>
<li><a href="#support">GLSL 1.40 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>
<h2 id="envvars">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>
<p>
Shaders can be dumped and replaced on runtime for debugging purposes. Mesa
needs to be configured with '--with-sha1' to enable this functionality. This
feature is not currently supported by SCons build.
This is controlled via following environment variables:
<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
not clobber the replacement shaders.
</p>
<h2 id="support">GLSL Version</h2>
<p>
The GLSL compiler currently supports version 3.30 of the shading language.
</p>
<p>
Several GLSL extensions are also supported:
</p>
<ul>
<li>GL_ARB_draw_buffers
<li>GL_ARB_fragment_coord_conventions
<li>GL_ARB_shader_bit_encoding
</ul>
<h2 id="unsup">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>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>
<h2 id="notes">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>
<h2 id="hints">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>
<h2 id="standalone">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>
<h2 id="implementation">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>
<dl>
<dt>EmitHighLevelInstructions</dt>
<dd>
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.
</dd>
<dt>EmitCondCodes</dt>
<dd>
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.
</dd>
<dt>EmitComments</dt>
<dd>
If set, instructions will be annotated with comments to help with debugging.
Extra NOP instructions will also be inserted.
</dd>
</dl>
<h2 id="validation">Compiler Validation</h2>
<p>
Developers working on the GLSL compiler should test frequently to avoid
regressions.
</p>
<p>
The <a href="http://piglit.freedesktop.org/">Piglit</a> project
has many GLSL tests.
</p>
<p>
The Mesa demos repository also has some good GLSL tests.
</p>
</div>
</body>
</html>
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